WO1990006082A1

WO1990006082A1 - Analysis of posture responses

Info

Publication number: WO1990006082A1
Application number: PCT/US1989/005415
Authority: WO
Inventors: Lewis M. Nashner
Original assignee: Neurocom International, Inc.
Priority date: 1988-11-30
Filing date: 1989-11-30
Publication date: 1990-06-14

Abstract

A method and apparatus for assessing the stability of a subject who is actively maintaining a position in equilibrium on a support surface (12). A quantity relating to the amplitude of the subject's displacement from the assumed equilibrium position is measured (13) as a function of the rate of displacement from that position. A quantity relating to the maximum amplitude of displacement at which the assumed position in equilibrium can be maintained is calculated (18) for each rate of displacement. The difference between this measured quantity and this calculated quantity is compared (18) to a database defining clinically normal responses.

Description

ANALYSIS OF POSTURE RESPONSES

This application is a continuation-in-part of serial number 0007,294, filed on January 27, 1987 which is a continuation-in part of serial number 873,125, filed on June 11, 1986, which issued on April 19, 1988 as a patent number 4,738,269, which is continuation of serial number 408,184, which was filed on August 16, 1982, now abandoned, for an invention of Lewis M. Nashner. Such applications are incorporated herein by reference.

DESCRIPTION

Technical Field

This invention relates generally to methods and devices for the detection of balance disorders, and specifically to methods and devices for numerical and graphical analysis of postural movement response data obtained from subjects while they maintain positions in equilibrium under a variety of conditions.

Background Art

The present experimental and clinical art includes a large number of methods and devices for measuring the forces and body segment motions associated with the postural movements produced by subjects as they maintain assumed positions in equilibrium. Measurement methods include the incorporation of force transducing devices into the surface supporting an assumed posture (see, for example, Nashner, "A Model Describing Vestibular Detection of Body Sway Motion," Acta Otolarynσ Stockh. 72:429-436, 1971; Mauritz, Dichgans, Hufschmidt, "Quantitative Analysis of Stance in Late Cortical ^' Cerebellar Atrophy of the Anterior Lobe and Other Forms of Cerebellar Ataxia " Brain 102:461-482, 197-9), the attachment of position transducing devices to specific points on the body (for examples _r Nashner, A model describing vestibular detection of body sway motion, Acta Otolarvnσ Stockh. 72:429- 436, 1971; Mauritz, Dichgans, Hufschmidt, Quantitative analysis of stance in late cotical cerebellar atrophy of the anterior lobe and other forms of cerebellar ataxia, Brain 102:461-482, 1979), the fixing of inertial motion transducers to specific points on the body (Nashner, Shupert, Horak, Head- trunk coordination in standing posture, In: Vestibulospinal Control of Posture and Locomotion, Progress in Brain Research, Vol. 76, pp. 243-51, Elsevier, Amsterdam, 1988) , and the optical recoding of body motions using various video and film based technologies (for examples, Pedotti, "A study of Motor Coordination and Neuromuscular Activities in Human Locomotion," Biological Cybernetics 26:53-62_r 1977; Forssberg, "Ontogeny of Human Locomotor Control — I. Infant Stepping, Supported Locomotion and Transition to Independent Locomotion, "Experimental Brain Research 57:480-493, 1955).

The methods for analyzing the resulting force and motion data available in the present experimental and clinical art vary with the type of measurement used. When force measuring surfaces are used, the distribution of forces on the surface can be analyzed to determine the locations of the centers of force for each foot separately and for the two feet together, and to relate these quantities to the position of the center of gravity of the body above the surface (Gurfinkel, "Physical Foundations of the Stabilography, "Agressologie 14G:9-14). When potentiometric and optical methods are used to measure the position changes of specific points on the body, geometric calculations are used to determine the linear motions of body parts and the angular orientations of body parts relative to one another and to the gravitational vertical. The analysis of information from inertial motion transducers is similar to that used with the position measuring devices, except that an additional step is necessary to numerically integrate the velocity and acceleration information to determine the position of specific body parts (for example, Nashner, Shupert, Horak, "Head-trunk Coordination in the Standing Posture," in "Vestibulospinal Control of Posture and Locomotion," Progress in Brain Research, Vol 76, pp. 243-51, Elsevier, Amsterdam, 1988). The present art contains descriptions of additional methods and devices for analyzing body movement responses with the purpose of understanding the underlying physiological mechanisms of posture and equilibrium control. For example, Fourier transforms of center of force position and body center of mass position information are used to determine the amplitude of power contained in the body movement signals as a function of the frequency of the body movements (for example, Bensel, Dzendolet, "Power Spectral Density Analysis of the Standing of Males," Perception and Psychophysics 4:285- 87, 1968; Yoneda, Tokumasu, "Frequency Analysis of Body Sway in the Upright Posture, Acta Otolaryngol. Stockh. 102:87-92, 1986) . differences in the power spectra of sway with changes in task conditions and disease states are thought to reflect changes in the underlying physiological control mechanisms. Other methods to identify differences in the underlying mechanisms of control use various signal averaging techniques to determine the root mean square movement amplitudes, the root mean square movement velocities, and the means and standard deviations of movements (for example, Hufschmidt, Dichgans, Mauritz, Hufschmidt, "Some Methods and Parameters of Body Sway Quantification and Their Neurological Applications," Archives of Psychiat. Nervenkr. 228:135-150, 1980) . Still other methods employ peak to peak analysis of movement amplitudes to understand changes in the underlying posture control mechanisms (for example, the EquiTest system manufactured by Neurocom International, Inc. of Portland, Oregon, USA) . Theoretical approaches to understanding the underlying mechanisms of posture control are also described in the prior art. A number of reports described detailed mathematical formulations of the physics of body motions (for examples, Gurfinkel, Osevets, "Dynamics of the Vertical Posture in Man," Biophysics 17:496-506, 1972; Hema i, Golliday, "The Inverted Pendulum and Biped Stability," Mathematical Biosciences 34:95- 107, 1977; Hemami, Wall III, Black, Golliday, "Single Inverted Pendulum Biped Experiments," J. of Interdisciplinary Modeling and Simulation 2:211-227, 1979; Nashner, "Analysis of Stance Posture in Humans," in Handbook of Behavioral Neurobiology, Vol. 5, Plenum Publishing, New York, 1981; refs.). These formulations are analyzed either mathematically and with the assistance of computer simulations to predict the patterns of muscular forces, joint forces, and joint accelerations underlying body movements, and the patterns of incoming sensory inputs controlling these activities.

There is a body of published knowledge describing the function characteristics of the sensory receptors which detect body motions during the maintenance of equilibrium postures. The semicircular canals and utricular otolith organs of the vestibular system are the best described of the body motion sensors in this regard. A number of theoretical and physiological studies document the second order dynamic and the threshold characteristics by which the semicircular canals and utricular otoliths transduce head accelerations into neural signals of head rotational and linear positions and velocities (for examples, Van Egmond, Groen, Jongkees, "The Mechanics of the Semicircular Canal, J. Physiology Lond. 1:13, 1949; Meiry, The Vestibular System and Human Dynamic Space Orientation, NASA-CR-628, 1966; Doty, "Effect of Duration of Stimulus Parameters on the Angular Acceleration Threshold, J. EXP. Psychology 80:317-321, 1969; Young, Meiry "A Revised Dynamic Otolith Model," Aerospace Medicine 39:606-611, 19- 68) . Using the results of these studies, I (Nashner, " A Model Describing Vestibular Detection of Body Sway Motion, "Acta Otolarvngol. Stockh. 72:429-436, 1971) developed a theoretical model which predict the threshold values of body sway at which the semicircular canals and utricular otoliths would first detect motion.

The role of vision in detecting body motions during posture control is described by a number of individuals (for examples, "Lestienne, Soechting, Berthoz," Postural Readjustments Induced by Linear Motion of Visual Scenes," Exp. Brain Research 28:363-384, 1977; Lee, Lishman, "Visual Proprioceptive Control of Stance, J. Human Movement Studies 1:87-95, 1975; Nashner, Berthoz, "Visual Contribution to Rapid Motor Responses During Posture Control, Brain Research 150:403-407, 1978; Dichgans, Held, Young, Brandt, "The Moving Visual Scenes Influence the Apparent Direction of Gravity," Science 178:1217-19, 1972). These studies identify minimum thresholds for visual detection of body sway motions and show that vision is used primarily to control lower frequencies of body motion. Finally, investigations demonstrate that the somatosensory and proprioceptive inputs derived from the forces and motions of contact with the support surface (hereinafter termed support surface inputs) are among the most sensitive and fast acting of the postural input systems (for example, Diener, Dichgans, Guschlbaure, Mau, "The Significance of Proprioception on Postural Stabilization as Assessed by Ischemia," Brain Research 296:103-109, 1984). No investigators, however, have developed quantitative mathematical models of visual and support surface detection of body motions equivalent to the detailed dynamic and threshold models described for the vestibular system.

In a series of publications, which are not necessarily herein admitted to be prior art, by authors including the inventor (Nashner, Black, Wall III, 'Adaption to Altered Support Surface and Visual Conditions During Stance: Patients with Vestibular Deficits," J. Neuroscience 2:536-544, 1982; Black, Nashner, "Vestibulospinal Control Differs in Patients with Reduced versus Distorted Vestibular Function, Acta Otolaryngol. Stockh. 406:110-114, 1984), the importance of sensory selection in the sensory control the posture is demonstrated. These studies employ a technique and device for testing the organization of the senses in posture control in which the subject is exposed to a series of six different sensory conditions. During individual sensory conditions information from either one or both of the visual and support surface senses is made inaccurate by moving the surface in assumed equilibrium position. For this method and device I have been granted United States patent number 4,783,269. A system for performing diagnostic assessments of patients with vestibular, neurological, and orthopedic balance and movement problems using these patented methods and devices is currently being manufactured and sold by the NeuroCom International Corporation of Portland Oregon.

A colleague and I (Nashner, McCollum, "The Organization of Human Postural Movements: A Formal Basis and Experimental Synthesis, "Behavioral and Brain Sciences 9:135-172, 1985) show that biomechanical and geometric constraints on the ability of the body to move play an important role in shaping the coordinated patterns of muscular activities responsible for generating the postural movements which allow subjects to maintain assumed positions in equilibrium. Another colleague and I (Horak, Nashner, "Central Programming of Postural Movements: Adaptation to Altered Support Surface Configurations, J. Neuroph siology 55:1369-81, 1986) experimentally demonstrate the effects of these constraints.

In the present state of the art, however, no investigators have described methods and devices by which the sensory control of posture and equilibrium can be understood in terms of the mechanical and sensory constraints and their interactions. By analyzing the interactions between these two types of constraints, I have discovered several new analytical and graphical methods for analyzing the postural movement responses of normal subjects and patients with sensory and motor abnormalities affecting posture as they maintain positions in equilibrium under a variety of visual and support surface conditions.

There are numerous statistical methods described in the prior art for performing a measurement protocol on a group of subjects judged to be normal by standard clinical means, determining the normal range of scores for that measurement, and then using the normal data base to identify individuals with abnormal scores for that measurement. For example, angular sway displacement were measured in a group of normal subjects and then analyzed statistically to determine the probability that any given score would occur in a normal population (Black, Wall III, Rockett, Kitch, Normal subject postural sway during the Romberg tests, American J. Otolaryngol. 3:309-18, 1982). The sway displacement scores of a group of patients with dizziness were then compared to the normal data base of scores to determine the effectiveness of a postural stability test distinguishing between normals and patients with dizziness (Wall, Black, Postural stability and rotational tests: their effectiveness for screening dizzy patients, Acta Otolaryngol. Stockh. 95:235-46, 1983) .

The methods and devices described in this application can be used in conjunction with the EquiTest system manufactured by NeuroCom International, Inc. to provide additional information regarding the relative ability of a patient to sense angular motions of the body with the semicircular canals and linear motions with the utricular otoliths. In the present state of the art, there are no simple objective methods for assessing the ability to sense the linear input component, even though current clinical theories suggest that abnormal linear motion sensations are a major factor in some balance disorders (for example, Schuknecht, Patholoogy of the Ear, pp.475-78, Harvard University Press, Cambridge, 1974). Thus, methods and devices described in my present invention will provide useful new clinical information to clinician.

DISCLOSURE OF INVENTION

This present invention provides a method for assessing the stability of a subject who is actively maintaining a position in equilibrium on a support surface. In a preferred embodiment of a method according to the present invention, the subject is placed on a support surface, and assumes a position in equilibrium thereon. This is followed by measuring a ^' quantity related to the amplitude of the subject's displacement from the assumed equilibrium position as a function of the rate of displacement from that position, the "displacement function." For each rate of displacement there is calculated a quantity related to the maximum amplitude of displacement at which the assumed position in equilibrium can be maintained without loss of equilibrium or additional external support, the "stability limit function." There is then calculated for each rate of displacement the difference between the subject's measured displacement function and the calculated stability limit function, the "stability margin function." By referring to a data base defining the range of clinically normal displacement functions and clinically normal stability margin functions, the "normal displacement function" and the "normal stability margin function," respectively, the subject is identified as unstable at those rates of displacement at which the subject's stability margin function is smaller than the normal margin function described.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic block diagram of the components of one possible embodiment of an apparatus in accordance with the present invention.

Figure 2 shows the single inverted pendulum mathematical model used for calculating the limits of stability for the erect standing position.

Figure 3A and 3B show example two-dimensional plots of a stability limit function and a displacement function of a possible embodiment of a display according the present invention. DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention is designed to provide sensitive and reliable information regarding the extent of possible balance disorder and of the possible involvement of individual sensory input systems. One specific application of the invention is to determine the relative contribution of vestibular system utricular otolith and semicircular canal abnormalities to the balance disorder. The methods and devices described in this invention can be used to provide medical practitioners with assessment information helpful in diagnosing patients with balance and movement deficits, and with complaints of dysequilibrium, vertigo, and/or motion sickness.

Scope of the Present Invention: The present invention provides new methods and devices for analyzing and displaying postural movement response data obtained during the assessment of a subject's ability to maintain an assumed position in equilibrium using test means available in the present art.

In preferred embodiments, the methods and devices provided by my present invention perform the following functions: (1) analyze movement response data in terms of the biomechanical constraints on a subject's ability to move the body while maintaining a position in equilibrium and in terms of the sensory constraints on a subject's ability to sense displacements from an assumed equilibrium position, (2) compare the results of analysis to a normal data base of analyzed results, and (3) display the results in a format that facilitates interpretation by clinicians.

In accordance with a preferred embodiment of the first of two methods of the present invention, the subject to be tested performs an active posture control task by assuming a position in equilibrium on a support surface. First, a quantity related to the displacement of the subject from the assumed equilibrium position is measured over an interval of time I call a trial. Second, a quantity I call the displacement function is calculated which characterizes the amplitude of the subject's displacement as a function of the frequency of displacement during the trial. Third, a quantity I call the stability limit function is calculated which characterizes, as a function of the displacement frequency, the maximum displacement of the body from the assumed equilibrium position that is possible without the subject losing balance or requiring additional external support. The stability limit function is determined using a mathematical description of the body biomechanics as described in the prior art and knowledge of the subject's height, body mass, and foot length. Fourth, a quantity I call the stability margin function is calculated which characterizes, as a function of the displacement frequency, the difference between the measured displacement and the stability limit. Finally, the equilibrium of the subject being tested is stability margin differs from a "normal stability margin function" according to a statistical test of difference. The normal stability margin function is defined by referring to a data base which, for example, can be determined by repeating the first four steps on a group of other subjects judged normal by accepted clinical methods and then calculating on a statistical basis the normal distribution of displacement and stability margin functions. In accordance with a preferred embodiment of the second of two methods of the present invention, the subject to be tested performs an active posture control task by assuming a position in equilibrium on a support surface. First, a quantity I call the displacement function is calculated which characterizes the amplitude of the subject's displacement as a function of the frequency of displacement during the trial. Second, information from all orientation senses but those to be assessed in the test is either eliminated or disrupted. (Methods for elimination of vision include instructing the subject to close the eyes. Methods for disrupting orientationally accurate information from vision and somatosensory support surface inputs are described in detail in United States patent 4,738,269, for an invention of mine, and patent application serial number 007,294, filed on January 27, 1987 for a second invention of mine. These methods include moving the visual surround and/or the support surface in functional relation to the measured displacement of the subject from the assumed equilibrium position and placing the subject on a support surface compliant to the forces exerted by the feet. While there are presently no known clinically practical methods for eliminating orientationally accurate information from the inner ear vestibular system while a subject maintains a position in equilibrium, this is generally not a problem since the vestibular senses are most often the ones tested in patients with dizziness and balance disorders.) Third, a quantity related to the displacement of the subject from the assumed equilibrium position is measured over an interval of time I call a trial. Fourth, a quantity I call the displacement function is calculated which characterizes the amplitude of the subject's displacement as a function of the frequency of displacement during the trial. Fifth, a quantity I call the designated sense threshold function is calculated for each of the senses to be assessed. Each designated sense threshold function characterizes, as a function of the displacement frequency, the smallest amplitude of displacement from the assumed equilibrium position that can be detected by the sense. The designated sense threshold function is calculated from knowledge of the threshold and dynamic properties of the sense as described in the prior experimental art. Fifth, a quantity I call the designated sense margin function is calculated by determining, for each displacement frequency, the difference between the displacement function and the designated sense margin function. Finally, a designated sense is judged abnormal at those frequencies of displacement at which the designated sense margin function differs from a "normal designated sense margin function" for that sense according to a statistical test of difference. It is contemplated that this normal designated sense margin function is defined by referring to a data base which, for example, could be generated by repeating the first five steps on a group of other subjects judged normal by accepted clinical methods and then calculating on a statistical basis normal distributions for the displacement and designated sense margin functions. Assessing Stability of Eguilibrium Positions: In a preferred embodiment of my present invention, the EquiTest system manufactured by NeuroCom international Inc. of Portland Oregon creates the test environments in which a subject is placed in a position in eguilibrium, measures the subjects displacements from the assumed equilibrium position, and stores the resulting data for later analysis.

The principal components of the EquiTest system are illustrated in Figure 1. A subject 11 assumes an erect standing position in equilibrium on a support surface 12, within which are imbedded measurement means for measuring the distribution of surface reaction forces between the subject's feet and the support surface. Displacement measurement means 13 measures displacements of the subject from the assumed equilibrium position. Support surface movement means 14 moves the support surface in accordance with movement commands from the computing/electronic interface means 18. A visual enclosure 15 surrounds the subject's field of view. Visual enclosure moving means 16 moves the visual enclosure in accordance with movement commands from the computing/electronic interface means. The system includes a data entry means 17 by which an operator can enter data related to the characteristics of body motions and sensory input systems. The computing/electronic interface means 18 executes a trial in accordance with a predetermined protocol, calculating functional quantities related to measurement data from the displacement measure means 13, issuing movement commands to the support surface and visual enclosure movement means 14 and 16, calculating functional quantities related to the designated sense threshold, designated sense margin, stability limit, stability margin, calculating normal distributions of these functions, and generating displays of these functions to be transmitted to the display means 19. Display means 19 displays the displacement, designated sense threshold, stability limit, and other functional quantities calculated by the computing means.

The first EquiTest system consists of two force measuring support surfaces (forceplates) for measuring the forces exerted by each of the subject's feet and a visual enclosure surrounding the subject's field of view. The forceplate support surface is rotatable about an axis approximately 2 inches above the surface. The visual enclosure is rotatable about an axis also locates above the support surface. Rotational positions of the support surface and enclosure are controlled by separate electrical position servomotors. An electronic interface transmits rotational position commands from a computer to the support surface and visual enclosure servomotors. An electronic interface transmits rotational position commands from a computer to the support surface and visual enclosure servomotors and transmits force response data from the forceplate to the computer. A computer receives and stores force data from the electronic interface, performs mathematical calculations of the force data, and calculates rotational position commands for transmission to the electronic interface. A computer program implements the test protocols, controls the actions of the three position servomotors, controls the actions of the three position servomotors, and analyzes the resulting force transducer data in terms of prescribed methods.

Eliminating Accurate Orientation Information: To perform an EquiTest assessment, the subject assumes an erect standing position of the support surface. In a preferred protocol of the EquiTest system, the subject's ability to maintain stability in the antero-posterior plane of motion is assessed by aligning the subject's ankle joints, so as to be co-linear with the support surface rotation axis. During standing, force signals from the forceplate are transmitted to the computer. The computer uses programmed mathematical algorithms to calculate on a continuous basis, among other quantities, a quantity related to the antero-posterior angular displacements of the subject's body center of mass in relation to the center of support and then stores the resulting data in computer memory for later analysis.

In accordance with a preferred assessment protocol termed the sensory organization test, the subject's anetero-posterior angular displacements from the erect standing position are monitored during a series of six 20-second test trials. The six trials consist of the following conditions: (1) fixing the support surface and visual enclosure with subject's eyes open allow accurate visual and support surface somatosensory orientation information, (2) fixing the support surface with the subject's eyes closed allow accurate support surface somatosensory orientation information while eliminating all visual information, (3) fixing the support surface allows accurate support surface somatosensory orientation information while rotating the visual enclosure in direct relation to the measured antero-posterior angular displacements with the subject's eyes open eliminates orientationally accurate visual information, (4) rotating the support surface in direct relation to the measured antero- posterior angular displacements of the subject eliminates orientationally accurate support surface somatosensory information while fixing the visual enclosure with the subject's eyes open allows accurate visual orientation information, (5) rotating the support surface in direct relation to the measure antero-posterior angular displacements of the subject eliminates oreintationally accurate support surface somatosensory information while instructing the subject to close the eyes eliminates all visual orientation information, and (6) rotating both the support surface and the visual enclosure in direct relation to the measured antero=posterior angular displacements of the subject eliminates orientationally accurate support surface somatosensory and visual information.

The sensory organization test can also be administered to assess the lateral plane angular displacements of the subject's body center of mass and to eliminate orientationally accurate support surface somatosensory and visual orientation information in the lateral plane of motion. In this protocol, the subject assuming an erect standing position on the EquiTest forceplate surface so that a line passing through the two ankle joints is aligned perpendicular to the surface and enclosure rotation axes. The computer uses programmed mathematical algorithms to calculate on a continuous basis, among other quantities, a quantity related to the lateral plane angular displacements of the subject's body center of mass in relation to the center of support and then stores the resulting data in computer memory for later analysis. The 6 20-second trials described for the antero-posterior assessment protocol are repeated, except now the support surface and visual enclosure are rotated in direct relation to the measure lateral plane angular displacements of the subject's body center of mass.

In another preferred embodiment of the present invention, the NeuroCom International, Inc. EquiTest chair accessory is used to assess a subject's ability to maintain stability in the antero-posterior plane of motion while assuming a seated position in equilibrium. The subject assumes a seated position in equilibrium on a chair surface which is rotatable about an axis approximately co-linear with the hip joints. The subject's field of view is surrounded by an enclosure rotatable about a second axis also approximately co-linear with the subjects hip joints. An angular rate sensory attached to the upper back measures the antero-posterior angular velocity of displacement of the subject's body. Numerical integration of the angular velocity signal is used to determine the angular displacement of the upper body.

In accordance with a programmed assessment protocol termed the seated sensory organization test, the EquiTest and chair accessory system performs a series of six 20-second test trials identical to those described for the erect standing sensory organization test. During the seated sensory organization test, the support surface of the chair and visual enclosure are rotated in direct relation to the measure antero-posterior plane angular displacements of the subject's upper body.

A second protocol of the seated sensory organization test assesses the ability of the subject to maintain stability in the lateral plane of motion is assessed while assuming a seated position in equilibrium. The subject assumes a seated position in equilibrium on the EquiTest chair accessory with the subject positioned such that a line passing through the two hip joints lies perpendicular to the support surface and visual enclosure rotation axes. An angular rate sensory attached to the upper back measures the lateral plane angular velocity of displacement of the subject's upper body. The series of six 20 second trials identical to those described for the erect standing protocol is repeated, except now the support surface of the chair and visual enclosure are rotated in direct relation to the measured lateral plan angular displacements of the subject's upper body.

In other preferred embodiments of the present invention, orientationally accurate information from support surface somatosensory inputs can be selectively reduced or eliminated by placing the subject on a surface which is complient to the support surface. Accurate orientation information from support surface somatosensory inputs in both antero-posterior and lateral planes of motions can be simultaneously reduced or eliminated by placing the subject on complient foam rubber surface or on a flexible container such as a large rubber bag filled with a complient fluid such as air or water. With the fluid-filled containers, altering the fluid pressure within the container provides additional means for varying the extent of compliance.

Support surface somatosensory information in the antero- posterior plane of motion can be selectively reduced or eliminated by placing the subject on a support surface rotatable about an axis co-linear with the ankle joints and making the surface rotationally complient to the antero- posterior plane torsional forces exerted about the ankle joints. Similarly, support surface somatosensory information in the lateral plane of motion can be selectively reduced or eliminated by placing the subject on a support surface rotatable about an axis perpendicular to a line passing through the two ankle joints and making the surface rotationally complient to the lateral plane torsional forces exerted about the ankle joints.

Calculating the Displacement Function: Several analytical and computational methods are described within the prior art to calculate information regarding the amplitude of subject's angular displacement from an assumed equilibrium position relative to the rate of angular displacement using the measurement data provided by the EquiTest sensory organization test protocols. A functional quantity relating the amplitude of a subject's angular displacement from an assumed equilibrium position to the frequency of the subject's angular displacement can be calculated by performing a Fourier transform on the displacement data received during each 20- second test. A second method would be to perform a time derivative of the angular displacement data received during each 20-second test to determine the velocity of sway displacement as a function of the sway amplitude. Computer programs to perform Fourier transforms and time derivatives are commercially available; for example, in the "Assist" program sold by MacMillan, Inc. of New York City, New York.

Determining Eguilibrium Position Stability Limits: Functional quantities related to the maximum angular displacement at which a position in equilibrium or need for additional external support can be calculated using one of several mathematical descriptions or computer models of the biomechanics of body motions available in the prior art. For the erect standing position in equilibrium, stability limits in the antero-posterior plane of motion can be derived by describing body motions in terms of a single degree of freedom inverted pendulum rotating about the ankle joints as illustrated in Figure 2, in which:

Torque = IA" + MGH sinA T(max) = MGL where

I = moment body inertia A = displacement angle M — body mass G = gravity acceleration H = height of center of mass L = one-half foot length Using this inverted pendulum description, the stability limits for angular displacement of the body center of mass about the ankle joints are determined by imposing a mechanical constraint on the maximum ankle torque available to resist the destabilizing affects of gravity and body angular momentum about the ankle joints. The mechanical constraint on the maximum ankle torque in the antero-posterior length of the contact surface between the feet and the support surface. The mechanical constraint on the maximum ankle torque in the lateral plane of motion is determined from knowledge of the total body mass, the height of the center of body mass above the ankle joints, and the lateral plane width of the contact surface between the feet and the support surface.

If the assumption is made that the body center of mass is displaced in a sinusoidal manner about the ankles, then one can readily determine for each displacement frequency the angular displacement amplitude at which the peak ankle torque reaches the maximum defined by the mechanical constraint. This angular displacement amplitude defines the stability limit for that frequency of displacement. By repeating this calculation for all possible angular displacement frequencies, a stability limit functional quantity is determined which relates to the frequency of angular displacement. By repeating these calculations for antero-posterior and lateral planes of motion, antero-posterior and lateral plane stability limit functions can be determined.

If the assumption is made that the body center of mass is displacing at a constant angular velocity about the ankles, then one can readily determine for each angular displacement velocity the angular displacement amplitude at which application of the maximum possible ankle torque is required to arrest the angular displacement without loss of balance. This angular displacement amplitude defines the stability limit for that velocity of displacement. By repeating this calculation for all possible angular displacement velocities, a stability limit functional quantity is determined which relates the angular displacement limit to the velocity of angular displacement. By repeating these calculations for antero-posterior and lateral planes or motion, antero- posterior and lateral stability limit functions can be determined.

It should be understood that the configuration of the support surface affects the stability limit function for a given assumed position in equilibrium. On a support surface shorter than the feet are long, for example, the maximum possible ankle torque in the antero-posterior plane of motion is reduced. Similarly, on support surfaces narrower than the normal lateral placement of the two feet, the maximum possible ankle torque in the lateral plane of motion is reduced.

It should also be understood that other biomechanical descriptions of body motions can be used to determine the stability limit function. The combined affects of angular displacements of the ankles and hips can both be taken into account to calculate the maximum angular displacement of the center of body mass by using a double inverted pendulum model. With this description, ankle torque constraints are similar to those described for the single inverted pendulum model. Hip torque constraints can be based either on empirical observations of maximum hip joint accelerations in relation to trunk length and mass or on theoretical calculations of maximum hip muscle strengths and contractile speeds in relation to trunk length and mass. Mathematical methods similar to those described for the erect standing position in eguilibrium can be used to determine the stability limits for angular displacements of the body from assumed equilibrium positions other than erect standing. In the seated position in equilibrium, for example, antero-posterior angular displacements of the upper body about the hip joints can be described in terms of a single inverted pendulum model. The maximum torque that can be exerted about the hip joints is derived from knowledge of upper body total mass, height of upper body center of mass above the hip joints, and the antero-posterior length or lateral width of the contact surface between the buttocks and upper legs and the support surface. Using methods similar to those described for erect standing, maximum antero-posterior and lateral plane hip torques are then used to calculate the maximum antero- posterior and lateral plane angular displacement amplitudes of the upper body as functions of the upper body angular displacement frequencies and velocities.

Determining Sensory Threshold Function: In a previous publication (Nashner, 1971) , I described a mathematical method which incorporated experimentally measured dynamic and threshold response characteristics to calculate the minimum antero-posterior angular center of body mass displacements that can be detected by the semicircular canals and utricular otoliths as functions of the angular displacement frequency. When performing these calculations, I assumed that the body rotated as a rigid mass about the ankle joints and that the head was rigidly fixed to the trunk, such that the angular accelerations of the body center of mass and the vertical semicircular canals were identical. To determine the linear acceleration inputs, to the utricular otoliths, I assumed that the tilt angle of the otoliths and the angular displacement of the body center of mass were also identical. In a preferred embodiment of the present invention, the identical method is used to calculate the erect standing antero-posterior plane canal and otolith thresholds as functions of the frequency of antero-posterior plane displacements. The mathematical formulation developed by me in my 1971 publication can also be used to determine the angular displacement thresholds for the canals and otoliths as functions of the velocity of angular displacements. To determine the angular velocity threshold functions, it is assumed that the body begins displacement at a constant angular velocity from the erect standing position. Using the single inverted pendulum model, angular accelerations to the canals are identical to those of the center of body mass, while the angular tilt of the head is identical to that of the center of mass. The dynamical and threshold models of the canals and otoliths will then predict the angular displacement of the body center of mass at which the motion will first be detected. By repeating this calculation for all possible angular displacement velocities, a separate functional relation for each of the canals and otoliths is defined which describes the angular displacement amplitude at which motion is first detected as a function of the angular displacement velocity.

It should be appreciated that in other preferred embodiments of the present invention, otolith and canal threshold functions for lateral displacement from the erect standing position can be determined with methods similar to those used for the antero-posterior plane of motion. The same mathematical formulations for describing canal and otolith detection can be used for antero-posterior and lateral planes of motion, because the experimental observations used to develop the dynamic and threshold models of the canals and otoliths found that detection characteristics were similar in both antero-posterior and lateral planes of motion.

Other preferred embodiments of the present invention may use more complex biomechanical models to calculate the canal and otolith threshold functions for sensing displacements of the body center of mass from the erect standing position. The combined effects of ankle and hip joint motions on angular and linear acceleration inputs to the canals and otoliths, for example, can be taken into account using a double inverted pendulum model of body motion. If the combined effects of ankle, hip and neck rotations on the canal and otolith threshold functions is desired, a triple inverted pendulum model using the ankle, hip and neck joints may be used.

In still other preferred embodiments of the present invention, a single inverted pendulum model for describing angular displacements of the upper body about the hip joints can be used to determine the antero-posterior and lateral plane angular threshold functions for canal and otolith detection of displacements from the seated eguilibrium position. With the embodiment, I assume that the rotational motion inputs to the canals are identical to the angular tilt displacement of the upper body. In all other respects, calculations are identical to those described for the erect standing position.

Display, Displacement. Stability. and Threshold Functions: There are a number of technological means in the prior art for displaying two-dimensional functional relations between two variables. In one preferred embodiment of the present invention, the displacement, stability limit, stability margin,and threshold functions are calculated by computer and then displayed on a computer monitor. An example of such a two-dimensional plot is shown in Figures 3A and 3B.

Figure 3A shows an example of two-dimensional plots of a stability limit function and a displacement function of a possible embodiment of a display according to the present invention. The vertical axis of the plot shows the displacement angle. The horizontal axis of the plot shows the displacement angle. The horizontal axis shows the displacement frequency. Trace 31a shows the stability limit angle as a function of frequency for the erect standing position. Trace 32a shows a possible displacement function of a patient who is unstable at frequencies between 0.4 and 0.6 Hz. Fig. 3B uses the same graphical format. Trace 31b again shows the stability limit angle as a frequency for the erect standing position. Trace 32b shows a possible designated sense threshold displacement angle as a function of the displacement frequency. In this example, the displacement sense margin function determined by calculating the difference between the displacement function and the designated sense threshold function is large for frequencies between 0.4 and 0.6 Hz, indicating that the designated sense is abnormal in this frequency range.

A number of computer programs are commercially available for generating two-dimensional plots of the functional relation between two variables from either one of a table of numbers relating the two variables or a' mathematical equation relating the two variables. The "Assist" program sold by MacMillian Co of New York is one such computer program.

In other preferred embodiments of the present invention, the two-dimensional plots may be generated by computer and then printed in hard copy form by a computer-driven printing or plotting device.

In still other preferred embodiments of the present invention, the functional relations may be displayed in terms of numerical data.

Claims

What is claimed is:

1. A method for assessing the stability of a subject who is actively maintaining a position in equilibrium on a support surface, such method comprising: A. placing the subject on a support surface and having the subject assume a position in equilibrium thereon,

B. measuring a quantity related to the amplitude of the subject's displacement from the assumed equilibrium position as a function of the rate of displacement from that position, hereinafter termed the "displacement function,"

C. calculating for each rate of displacement a quantity related to the maximum amplitude of displacement at which the assumed position in equilibrium can be maintained without loss of equilibrium or additional external support, hereinafter termed the "stability limit function,"

D. calculating for each rate of displacement the difference between the subject's measured displacement function and the calculated stability limit function, hereinafter termed the "stability margin function," E. referring to a data base defining the range of clinically normal displacement functions and clinically normal stability margin functions, hereinafter termed the "normal displacement function" and the "normal stability margin function," respectively, F. identifying the subject as unstable at those rates of displacement at which the subject's stability margin function described in step D is smaller than the normal stability margin function described in step E.

2. A method according to claim 1 for graphically assessing the stability of a subject who is actively maintaining a position in equilibrium on a support surface, wherein step B includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement frequency, step C includes the additional step of displaying the stability limit function on the two-dimensional plot described in step B, step E includes the additional step of displaying the normal displacement function on the two-dimensional plot described in step B, and step F includes additional step of identifying the subject as unstable at those displacement frequencies at which' the display of the subject's displacement function lies above the display of the normal displacement function.

3. A method according to claim 1 for graphically assessing the stability of a subject who is actively maintaining a position in equilibrium on a support surface, wherein step B includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement velocity, step C includes the additional step of displaying the normal displacement function on the two-dimensional plot described in step B, and step F includes the additional step of identifying the subject as unstable at those displacement velocities at which the display of the subject's displacement function lies above the display of the normal displacement function.

4. A method for assessing the stability of a subject who is actively maintaining an erect standing position in equilibrium on a support surface, such method comprising:

A. placing the subject on a support surface and having the subject assume an erect standing position in equilibrium thereon, B. measuring a displacement function quantity related to the displacement of the body from the erect standing position, hereinafter termed the "standing displacement function,"

C. calculating a stability limit function quantity related to the limits of stability of the body in the erect standing position, hereinafter termed the "standing stability limit function,"

D. calculating a stability margin function quantity related to the margin of stability with the body assuming an erect standing position in equilibrium on a supporting surface, hereinafter termed the "standing stability margin function," E. referring to a data base defining a range of quantities related to the normal standing displacement function and the normal standing stability margin function, hereinafter termed "the normal standing margin function," F. identifying the subject as unstable at those rates of displacement of the body's center of gravity at which the standing stability margin function described in step D is smaller than the normal standing stability margin function described in step E.

5. A method according to claim 4 for assessing the antero-posterior (hereinafter abbreviated AP) plane stability of a subject who is actively maintaining an erect standing position in eguilibrium on a support surface, wherein step B includes the additional step of measuring a displacement function quantity related to the AP plane displacement of the subject from the assumed erect standing equilibrium position, hereinafter termed the "AP plane standing displacement function," and step C includes the additional step of calculating a stability limit function quantity related to the limits of stability of the body in the AP plane from the assumed erect standing position in equilibrium, hereinafter termed the "AP plane standing stability limit function."

6. A method according to claim 4 for assessing the AP plane stability of a subject who is actively maintaining an erect standing position in equilibrium on a support surface which is shorter than the length of the feet, wherein step A includes the additional step of having the subject assume an erect standing position in equilibrium on a supporting surface which is shorter than the feet are long, step B includes the additional step of measuring a quantity related to the AP plane standing displacement function, and step C includes the additional step of calculating an AP plane standing stability limit function quantity related to the limits of stability of the body in the AP plane while assuming an erect standing position in equilibrium on a support surface shorter than the feet are long, hereinafter termed the "AP plane standing short surface stability limit function."

7. A method according to claim 4 for assessing the lateral (hereinafter abbreviated LR) plane stability of a subject who is actively maintaining an erect standing position^' in eguilibrium on a support surface, wherein step B includes the additional step of measuring a displacement function quantity related to the LR displacement of the subject from the assumed erect standing equilibrium position, hereinafter termed the "LR plane standing displacement function," and step C includes the additional step of calculating a stability limit function quantity related to the limits of stability of the body in the LR plane while assuming an erect standing position in equilibrium, hereinafter termed the "LR plane standing stability limit function."

8. A method according to claim 4 for assessing the LR plane standing position in equilibrium on a support surface which is narrower than the combined widths of the two feet, wherein step A includes the additional step of having the subject assume an erect standing position in equilibrium on a supporting surface which is narrower than the combined widths of the two feet, step B includes the additional step of measuring a quantity related to the LR plane standing displacement function, and step C includes the additional step of calculating a stability limit function quantity related to the LR plane limits of stability while assuming an erect standing position in eguilibrium on a support surface narrower than the combined widths of the two feet, hereinafter the "LR plane standing narrow support stability limit function."

9. A method for assessing the stability of a subject who is actively maintaining an erect seated position in equilibrium on a support surface, such method comprising:

A. placing the subject on a support surface and having the subject assume an erect seated position in equilibrium thereon,

B. measuring over a defined interval of time a displacement function quantity related to displacement of the subject's upper body from the erect seated position, hereinafter termed the "seated displacement function," C. calculating a stability limit function quantity related to the limits of stability of the upper body while assuming a seated position in equilibrium, hereinafter termed the "seated stability limit function," D. calculating a stability margin function quantity related to the stability margin of the upper body while assuming a seated position in equilibrium, hereinafter termed the "seated stability margin function,"

E. referring to a data base defining the normal seated displacement function and the normal seated stability margin function,

F. identifying the subject as unstable at those upper body displacement rates at which the subject's seated stability margin function described in step D is smaller than the normal seated stability margin function described in step E.

10. A method for assessing the ability of a subject to control the stability of an assumed position in equilibrium with input from a designated feedback sense, such method comprising:

A. placing the subject on a support surface and having the subject assume a position in equilibrium thereon,

B. providing the subject with orientation information from the designated feedback sense and excluding orientation information from other potentially useful feedback senses,

C. measuring a quantity related to the displacement function,

D. calculating a quantity related to the minimum displacement amplitude from the assumed equilibrium position that can be detected by the designated feedback sense as a function of the rate of displacement from the assumed equilibrium position, hereinafter termed the "designated sense margin function,"

F. referring to a data base defining the range of clinically normal sense margin functions for the designated sense, hereinafter termed the "normal designated sense margin function." G. identifying the designated feedback sense as abnormal at those rates of displacement at which the designated sense margin function described in step E is larger than the normal^' designated sense margin function described in step F.

11. A method according to claim 10 for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium with input from a designated feedback sense, wherein step C includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement frequency, step D includes the additional step of displaying the designated sense threshold function on the two- dimensional plot described in step C, step F includes the additional step of displaying the normal displacement function on the two-dimensional plot described in step C, and step G includes the additional step of identifying the designated feedback sense as abnormal at those rates of displacement at which the display of the displacement function lies above the display of the normal displacement function and above the display of the designated sense threshold function.

12. A method according to claim 10 for graphically assessing the ability of a subject to control the stability of an assumed position in eguilibrium with input from a designated feedback sense, wherein step C includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement velocity, step D includes the additional step of displaying the designated sense threshold function on the two- dimensional plot described in step C, step F includes the additional step of displaying the normal displacement function on the two-dimensional plot described in step C, and step G includes the additional step of identifying the designated feedback sense as abnormal at those rates of displacement at which the display of the displacement function lies above the display of the normal displacement function and above the display of the designated sense threshold function.

13. A method for assessing the ability of a subject to control the stability of the head with input from a designated feedback sense while assuming a position in equilibrium, such method comprising:

B. providing the subject with head orientation information from the designated feedback sense and excluding orientation information from other potentially useful feedback senses, C. measuring a quantity related to the displacement of the head from the assumed equilibrium position, hereinafter termed the "head displacement function,"

D. calculating a quantity related to the minimum head displacement amplitude from the assumed equilibrium position that can be detected by the designated feedback sense as a function of the rate of head displacement from the assumed equilibrium position, hereinafter termed the "designated sense head threshold function,"

E. calculating the difference between the head displacement function and the designated senses head threshold function, hereinafter termed the "designated sense head margin function,"

F. referring to a data base defining the range of clinically normal sense head margin functions for the designated sense, hereinafter termed the "normal designated sense head margin function,"

G. identifying the designated feedback sense as abnormal at those rates of head displacement at which the designated sense head margin function described in step E is larger than the normal designated sense head margin function described in step F.

14. A method for assessing the ability of a subject to control the stability of an assumed position in equilibrium using orientation input from a two or more designated feedback senses, such method comprising:

A. placing the subject on a support surface and having the subject assume a position in equilibrium thereon, B. providing the subject with orientation information from the designated feedback senses and excluding orientation information from other potentially useful feedback senses,

C. measuring a quantity related to the displacement function

D. calculating for each of the designated feedback senses a separate sense margin function,

E. calculating for each of the designated feedback senses a separate sense margin function, F. referring to a data base defining the normal displacement function, and then determining for each of the designated feedback senses a separate normal sense margin function,

G. identifying each of the designated feedback senses as abnormal at those rates of body displacement at which the sense margin function for that sense described in step E is larger than the normal sense margin function for that sense described in step F.

15. A method for assessing the ability of a subject to control the stability of the head using orientation input from a two or more designated feedback senses while assuming a position in equilibrium, such method comprising:

A. placing the subject on a support surface and having the subject assume a position in equilibrium thereon, B. providing the subject with head orientation information from the designated feedback senses and excluding orientation information from other potentially useful feedback senses,

C. measuring a quantity related to the head displacement function,

D. calculating for each of the designated feedback senses a separate quantity related to the sense head threshold function,

E. calculating for each of the designated feedback senses a separate sense head margin function,

F. referring to a data base defining the normal head displacement function, and then determining for each of the designated feedback senses a separate normal sense head margin function,

G. identifying each of the designated feedback senses as^' abnormal at those rates of head displacement at which the sense head margin function for that sense described in step E is larger than the normal sense head margin function for that sense described in step F.

16. A method for assessing the ability of a subject to control the stability of a position in equilibrium using orientation inputs from the vestibular semicircular canals, hereinafter termed "canals." and the vestibular utricular otoliths, hereinafter termed "otoliths," such method comprising:

B. eliminating useful visual orientation information,

C. eliminating useful support surface orientation information,

D. measuring a quantity related to the displacement function,

E. calculating a sense threshold function quantity related to the ability of the canal to detect displacements from the assumed equilibrium position, hereinafter termed the "canal threshold function," and calculating a sense threshold function quantity related to the ability of the otolith to detect displacements from the the assumed equilibrium position, hereinafter termed the "otolith threshold function,"

F. calculating the canal margin function and the otolith margin function,

G. referring to a data base defining the normal displacement function, and then determining the normal canal margin function and a normal otolith margin function, H. identifying the canals as abnormal at those rates of body displacement at which the canal margin function described in step F is larger than the normal otolith margin function described in step G.

17. A method according to claim 16 for assessing the ability of a subject to control the stability of an erect standing position in equilibrium using orientation inputs from the canals and otoliths, wherein step A includes the additional step of having the subject assume an erect standing position in equilibrium, step B includes the additional step of having the subject close the eyes, step C includes the additional step of causing the support surface to rotate in functional relation to displacements from the standing position about a point along a line passing through the two ankle joints and one half the distance between the two ankles, step D includes the additional step of measuring a quantity related to the standing displacement function, and step E includes the additional step of calculating a canal threshold function quantity related to the ability of the canal to detect displacement of the body from the assumed erect standing position, hereinafter termed the "canal standing threshold function," and calculating an otolith threshold function quantity related to the ability of the otolith to detect displacement of the body from the assumed erect standing position, hereinafter termed the "otolith standing threshold function."

18. A method according to claim 16 for assessing the ability of the subject to control the stability of a seated position in equilibrium using orientation inputs from the canals and otoliths, wherein step A includes the additional step of having the subject assume a seated position in equilibrium, step B includes the additional step of having the subject close the eyes, step C includes the additional set of causing the support surface to rotate in functional relation to displacements from the seated position about a point along a line passing through the two hip joints and one half the distance between the two hip joints, step D includes the additional step of measuring the seated displacement function, and step E includes the additional step of calculating a canal threshold function quantity related to the ability of the canal to detect displacement of the upper body from the assumed seated position, thereinafter termed the "canal seated threshold function," and calculating an otolith threshold function quantity related to the ability of the otolith to detect displacement of the upper body from the assumed seated position, hereinafter termed the "otolith seated threshold function."

19. A method according to claim 16 for assessing the ability of a subject to control the AP plane stability of an erect standing position in equilibrium using orientation inputs from the canals and otoliths, wherein step A includes the additional step of having the subject assume an erect standing position in equilibrium, step B includes the additional step of having the subject close the eyes, step C includes the additional set of causing the support surface to rotate in functional relation to AP plane displacements from the standing position about an axis passing through the two ankle joints, step D includes the additional step of measuring the AP plane standing displacement function, and step E includes the additional step of calculating a canal threshold function quantity related to the ability of the canal to detect AP plane displacement of the body from the assumed erect standing position, hereinafter termed the "canal AP plane standing threshold function," and calculating an otolith threshold function quantity related to the ability of the otolith to detect AP plane displacement of the body from the assumed erect standing position, hereinafter termed the "otolith AP plane standing threshold function."

20. A method according to claim 16 for assessing the ability of a subject to control the LR plane stability of an erect standing position in equilibrium using orientation inputs from the canals and otoliths, wherein step A includes the additional step of having the subject assume an erect standing position in equilibrium, step B includes the additional step of having the subject close the eyes, step C includes the additional set of causing the support surface to rotate in functional relation to LR plane displacements from the standing position about an axis perpendicular to the line passing through the two ankle joints and parallel to the support surface, step D includes the additional step of measuring the LR plane standing displacement function, and step E includes the additional step of calculating a canal threshold function quantity related to the ability of the canal to detect LR plane displacement of the body from the assumed erect standing position, hereinafter termed the "canal LR plane standing threshold function," and calculating an otolith threshold function quantity related to the ability of the otolith to detect LR plane displacement of the body from the assumed erect standing position, hereinafter termed the "otolith LR plane standing threshold function.

21. A method for assessing the ability of a subject to control the stability of the head using orientation inputs from the canals and otoliths while assuming a position in equilibrium, such method comprising:

A. placing the subject on a support surface and having the subject assume a position in equilibrium thereon, B. eliminating useful visual orientation information,

C. measuring a quantity related to the head displacement function,

D. calculating a sense head threshold function quantity related to the ability of the canal to detect head displacements from the assumed equilibrium position, hereinafter termed the "canal head threshold function," and calculating a sense head threshold function quantity related to the ability of the otolith to detect displacements from the assumed equilibrium position, hereinafter termed the "otolith head threshold function,"

E. calculating the canal head margin function and the otolith head margin function,

F. referring to a data base defining the normal head displacement function, and then determining the normal canal head margin function and a normal otolith head margin function,

H. identifying the canals as abnormal at those rates of head displacement at which the canal head margin function described in step E is larger than the normal canal head margin function described in step F, and identifying the otoliths as abnormal when the otolith head margin function described in step E is larger than the normal otolith head margin function described in step F.

22. A method according to claim 21 for assessing the ability of a subject to control the stability of the head using orientation inputs from the canals and otoliths while assuming an erect standing position in equilibrium, wherein step A includes the additional step of having the subject assume an erect standing position in equilibrium, step B includes the additional step of having the subject close the eyes, step C includes the additional step of measuring a quantity related to the displacement of the head from the erect standing position, hereinafter termed the "head standing displacement function," and step D includes the additional step of calculating a canal threshold function quantity related to the ability of the canal to detect displacement of the head from the assumed erect standing position, hereinafter termed the "canal standing head threshold function," and calculating an otolith threshold function quantity related to the ability of the otolith to detect displacement of the head from the assume erect standing position, hereinafter termed the "otolith standing head threshold function."

23. A method according to claim 21 for assessing the ability of the subject to control the stability of the head using orientation inputs from the canals and otoliths while assuming a seated position in equilibrium, wherein step A includes the additional step of having the subject assume a seated position in equilibrium, step B includes the additional step of having the subject close the eyes, step C includes the additional step of measuring a quantity related to the displacement of the head from the seated position, hereinafter termed the "seated head displacement function," and step D includes the additional step of calculating a canal threshold function quantity related to the ability of the canal to detect displacement of the head from the assumed seated position, hereinafter termed the "canal seated head threshold function," and calculating an otolith threshold function quantity related to the ability of the otolith to detect displacement of the head from the assumed seated position, hereinafter termed the "otolith seated head threshold function."

24. A method according to claim 21 for assessing the ability of a subject to control the AP plane stability of the head using orientation inputs from the canals and otoliths while assuming an erect standing position in equilibrium, wherein step A includes the additional step of having the subject assume an erect standing position in eguilibrium, step B includes the additional step of having the subject close the eyes, step C includes the additional step of measuring a quantity related to the AP plane displacement of the head from the assumed erect standing position, hereinafter termed the "AP plane standing head displacement function," and step D includes the additional step of calculating a canal standing threshold function quantity related to the ability of the canal to detect AP plane displacement of the head from the assumed erect standing position, hereinafter termed the "canal AP plane standing head threshold function," and calculating an otolith threshold function quantity related to the ability of the otolith to detect AP plane displacement of the body from the assumed erect standing position, hereinafter termed the "otolith AP plan standing head threshold f nction."

25. A method according to claim 21 for assessing the ability of a subject to control the LR plane stability of the head using orientation inputs from the canals and otoliths while assuming an erect standing position in equilibrium, wherein step A includes the additional step of having the subject assume an erect standing position in equilibrium, step B includes the additional step of having the subject close the eyes, step C includes the additional step of measuring a quantity related to the LR plane displacement of the head from the standing position, hereinafter termed the "LR plane standing head displacement function," and step D includes the additional step of calculating a canal standing threshold function quantity related to the ability of the canal to detect LR plane displacement of the head from the assumed erect standing position, hereinafter termed the "canal LR plane standing head threshold function," and calculating an otolith standing threshold function quantity related to the ability of the otolith to detect LR plane displacement of the head from the assumed erect standing position, hereinafter termed the "otolith LR plane standing head threshold function."

26. A method for graphically assessing the ability of a subject to control the stability of an assumed position in eguilibrium using orientation inputs from a designated feedback sense, such method comprising:

B. providing the subject with orientation information from the designated feedback sense and excluding orientation information rom other potentially useful feedback senses,

C. measuring a quantity related to the displacement function, D. displaying the displacement function on a two- dimensional plot of displacement amplitude versus the rate of displacement, E. calculating a quantity related to the designated sense threshold function,

F. displaying the designated sense threshold function described in step E on the two-dimensional plot described in step D,

G. repeating steps A through D with a group of subjects judged clinically normal by accepted medical methods to determine and then display the normal displacement function, H. identifying the designated feedback sense as abnormal when the display of the displacement function lies above the display of the normal displacement function and above the display of the designated sense threshold function.

27. A method according to claim 26 for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium using orientation inputs from a designated feedback sense, wherein step D includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement velocity.

28. A method according to claim 26 for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium using orientation inputs from a designated feedback sense, wherein step D includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement velocity.

29. A method for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium using orientation inputs from two or more designated feedback senses, such method comprising:

B. providing the subject with orientation information from each of the designated feedback senses and excluding orientation information from other potentially useful feedback senses,

C. measuring a quantity related to the displacement function,

D. displaying the displacement function on a two- dimensional plot of displacement amplitude versus the rate of displacement,

E. calculating for each of the designated feedback senses a separate quantity related to the sense threshold function,

F. displaying each of the sense threshold functions described in step E on the two-dimensional plot described in step D, G. repeating steps A through D with a group of subjects judged clinically normal by accepted medical methods to determine and then display the normal displacement function,

H. identifying each of the designated feedback senses as abnormal when the display of the displacement function for that sense lies above the display of the normal displacement function and above the display of the threshold function for that sense.

30. A method for according to claim 28 for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium using orientation inputs from two or more designated feedback senses, wherein step D includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement frequency.

31. A method for according to claim 28 for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium using orientation inputs from two or more designated feedback senses, wherein step D includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement velocity.

32. A method for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium using orientation inputs from the canals and otoliths, such method comprising:

A. placing the subject on a support surface and having the subject assume a position in equilibrium thereon, B. eliminating useful visual and support surface orientation information,

C. measuring a quantity related to the displacement function,

E. calculating quantities related to the canal threshold function and otolith threshold function,

F. displaying each of the canal and otolith threshold functions described in step E on the two- dimensional plot described in step D,

G. repeating steps A through D with a group of subjects judged clinically normal by accepted medical methods to determine and then display the normal displacement function,

H. identifying the canal sense as abnormal when the display of the displacement function lies above the display of the normal displacement function and above the display of the canal threshold function, I. identifying the otolith sense as abnormal when the display of the displacement function lies above the display of the normal displacement function and above the display of the otolith threshold function.

33. A method according to claim 32 for graphically assessing the ability of a subject to control the stability of a position in equilibrium using orientation inputs from the canals and otoliths, wherein step D includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement frequency.

34. A method for according to claim 32 for graphically assessing the ability of a subject to control the stability of a position in equilibrium using orientation inputs from the canals and otoliths, wherein step D includes the additional step of displaying the displacement function on a two-dimensional plot of displacement amplitude versus displacement velocity.

35. A method according to claim 32 for graphically assessing the ability of a subject to control the stability of an erect standing position in equilibrium using orientation inputs from the canals and otoliths, wherein step A includes the additional step of having the subject assume an erect standing position in eguilibrium, step B includes the additional step of having the subject close the eyes and causing the support surface to rotate in functional relation to the displacement of the body from the erect position about a point along a line passing through the two ankle joints and one half way between the ankles, step C includes the additional step of measuring a quantity related to the standing displacement function, and step E includes the additional step of calculating quantities related to the canal standing threshold function and the otolith standing threshold function.

36. A method according to claim 32 for graphically assessing the ability of a subject to control the stability of an erect seated position in equilibrium using orientation inputs from the canals and otoliths, wherein step A includes the additional step of having the subject assume a seated position in equilibrium, step B includes the additional step of having the subject close the eyes and causing the support surface to rotate in functional relation to displacement of the upper body from the erect seated position about a point along a line passing through the two hip joints and one half way between the two hips, step C includes the additional step of measuring a quantity related to the seated displacement function, and step E includes the additional step of calculating quantities related to the canal seated threshold function and the otolith seated threshold function.

37. An apparatus for assessing the stability of a subject who is actively maintaining a position in equilibrium on a support surface comprising:

A. a support surface on which the subject may assume a position in equilibrium,

B. a measuring means for measuring a quantity related to the displacement of the subject from the assumed equilibrium position,

C. a program means, in communication with the measuring means, for calculating the displacement function,

D. a program means for calculating the stability limit function,

E. a program means for calculating the stability margin function,

F. a program means for performing statistical tests on groups of displacement function and stabilijty margin function data to determine the normal displacement function and normal stability margin function, G. a program means for performing calculations to determine the difference between the subject's sjtability margin functin and the normal stability margin function.

38. An apparatus according to claim 37 for graphically assessing the stability of a subject who is actively maintaining a position in equilibrium, wherein step C includes an additional display means to display the displacement function in a two-dimensional plot of displacement amplitude versus displacement frequency, step D includes an additional display means to display the stability limit function in a two-dimensional plot of displacement amplitude versus displacement frequency, and step F includes the additional display means to display the normal displacement function in a two-dimensional plot of displacement amplitude versus displacement frequency.

39. An apparatus according to claim 37 for graphically assessing the stability of a subject who is actively maintaining an erect standing position in equilibrium, wherein step B includes an additional measuring means for measuring a quantity related to the displacement of the subject from the erect standing equilibrium position.

40. An apparatus according to claim 37 for graphically assessing the AP plane stability of a subject who is actively maintaining an erect standing position in equilibrium, wherein step B includes an additional measuring means for measuring a quantity related to the AP plane displacement of the subject from the erect standing equilibrium position.

41. An apparatus according to claim 37 for graphically assessing the LR plane stability of a subject who is actively maintaining an erect standing position in equilibrium, wherein step B includes an additional measuring means for measuring a quantity related to the LR plane displacement of the subject from the erect standing equilibrium position.

42. An apparatus according to claim 37 for graphically assessing the stability of a subject who is actively maintaining a seated position in equilibrium, wherein step B includes an additional measuring means for measuring a quantity related to the displacement of the subject from the seated equilibrium position.

43. An apparatus for assessing the ability of a subject to control the stability of an assumed position in equilibrium with input from a designated feedback sense comprising,

A. a support surface on which a subject may assume a position in equilibrium,

B. a means for excluding information to the subject about orientation in relation to the assumed equilibrium position from senses other than that designated for assessment,

C. measuring means for measuring the subject's displacement from the assumed position in equilibrium, D. a program means for calculating the displacement function,

E. a program means for calculating the designated sense threshold function,

F. a program means for calculating the designated sense margin function,

G. a program means for performing statistical tests on groups of displacement function and designated sense margin function data to determine the normal displacement function and normal designated sense margin function, H. a program means for performing calculations to determine the difference between the subject's designated sense margin function and the normal designated sense margin function.

44. An apparatus according to claim 43 for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium with input from a designated feedback sense, wherein step D includes an additional display means to display the displacement function on a two-dimensional plot of displacement amplitude versus displacement frequency, step E includes an additional display means for displaying the sense threshold function on a two-dimensional plot of displacement amplitude display means for displaying the normal displacement function on a two-dimensional plot of displacement amplitude versus displacement frequency.

45. An apparatus according to claim 43 for graphically assessing the ability of a subject to control the stability of an assumed position in equilibrium with input from a designated feedback sense, wherein step D includes an additional display means to display the displacement function on a two-dimensional plot of displacement amplitude versus displacement velocity, step E includes an additional display means or displaying the sense threshold function on a two-dimensional plot of displacement amplitude versus displacement velocity, step G includes an additional display means for displaying the normal displacement function on a two-dimensional plot of displacement amplitude versus displacement velocity.

46. An apparatus for assessing the ability of a subject to control the stability of an erect standing position in equilibrium with input from the canals and otoliths comprising:

A. a support surface on which a subject may assume a position in equilibrium is movable,

B. a means for covering the subject's eyes to eliminate visual orientation information, C. a measuring means for measuring the subject's displacement from the assumed position in equilibrium,

D. a moving means, in connection with the measuring means, for moving the support surface in functional relation to the measured displacement quantity, the value of the function exhibiting some dependency on the value of the subject's displacement from equilibrium, so that the support surface is moved by the moving means in response to displacement of the subject from equilibrium,

E. a program means for calculating the displacement function,

F. a programs means for calculating the canal and otolith threshold functions,

G. a program means for calculating the canal and otolith margin functions, H. a program means for performing statistical tests on groups of displacement function, canal margin function, and otolith margin function data to determine the normal displacement function, normal canal margin function, and normal otolith margin function, I. a program means for performing calculations to determine the difference between the subject's canal and otolith margin functions and the normal canal and otolith margin functions.

47. An apparatus according to claim 46 for assessing the ability of a subject to control the AP plane stability of an erect standing position in equilibrium with input from the canals and otoliths, wherein step A includes a support surface which is rotatable about an axis passing through two ankle joints, step C includes a measuring means for measuring the AP plane angular displacement of the subject's center of gravity from the assumed position in equilibrium, step D includes a rotating means, in connection with the measuring means, for rotating the support surface in functional relation to the measured angular displacement quantity, the value of the function exhibiting some dependency on the value of the subject's displacement from equilibrium, so that the support surface is rotated by the rotating means in response to angular displacement of the subject from equilibrium.

48. An apparatus according to claim 46 for assessing the ability of a subject to control the LR plane stability of an erect standing position in equilibrium with input from the canals and otoliths, wherein step A includes a support surface which is rotatable about an axis perpendicular to a line passing through the two ankle joints and parallel to the support surface, step C includes a measuring means for measuring the LR plane angular displacement of the subject's center of gravity from the assumed position in equilibrium, step D includes a rotating means, in connection with the measuring means, for rotating the support surface in functional relation to the measured angular displacement quantity, the value of the function exhibiting some dependency on the value of the subject's displacement from equilibrium, so that the support surface is rotated by the rotating means in response to angular displacement of the subject from equilibrium.