WO2006000015A1

WO2006000015A1 - Stimulus generation

Info

Publication number: WO2006000015A1
Application number: PCT/AU2005/000825
Authority: WO
Inventors: Alexander Raymond Ward; Peter Ian Ampt
Original assignee: La Trobe University
Priority date: 2004-06-24
Filing date: 2005-06-10
Publication date: 2006-01-05

Abstract

A method for treatment of a body by applying electrical stimuli to the body, characterised in that the electrical stimuli comprise at least one train (211) of a plurality of bursts (201), each burst in the train having a frequency between 0.5kHz and 2kHz or a frequency between 3kHz and 6kHz and a duration between 1 millisecond and 6 milliseconds.

Description

STIMULUS GENERATION

Field of the Invention

This invention relates to a method and apparatus for generating controlled electrical stimuli. It is particularly but not exclusively directed to the generation of electrical stimuli to treat the nerves and muscles of human patients in particular although it may also be suitable for animals as well.

Background of the Invention

Physiotherapists and other health practitioners traditionally use electrical stimulation for patient treatment. Conventional stimulation devices "clinical stimulators" are normally mains powered devices although some battery powered devices have been available for home use. In most countries they are restricted to use in the clinic by a qualified health practitioner as the output provided by the stimulator could potentially be harmful if applied inappropriately and/or at an excessive intensity.

There are three forms of electrical stimulation which are commonly used by physiotherapists. These are known as conventional TENS, Russian currents and Interferential currents. Each are well established clinical treatment modalities. Interferential and Russian current stimulators are the "heavy guns" stimulators. They are normally mains-powered and capable of producing potentially hazardous levels of stimulation. The output is alternating current (AC) in the kHz frequency range (2.5IcHz for Russian currents and 4 or 5 kHz for the interferential currents). Their use is restricted to clinicians trained in the safe use of such apparatus because of the risks of electrocution or tissue injury. Conventional TENS stimulators are often small, portable, battery-operated units which are relatively safe for take-home use under indirect medical supervision. However their effectiveness is limited because of the nature of the electrical stimuli associated with TENS. Interferential and Russian current stimulators enjoy a privileged therapeutic status which seems to be based more upon tradition than evidence. They are claimed to be better than conventional TENS for pain control (interferential) and muscle contraction (Russian currents) but their high output and high energy consumption restricts their use to mains-powered clinical stimulators. Furthermore, their effectiveness for muscle stimulation and pain control has limitations.

It is known that many electrical stimulation treatment regimens are most effective when the stimulation is applied several times daily. This is normally impractical if each treatment requires a clinical consultation. Thus, there is a need for an approach which allows a patient to be subjected to electrical stimulation treatment several times daily without requiring a clinical consultation and without being subjected to the risks normally associated with electrical stimulation treatment methods not actually carried out by a qualified health practitioner. -There is also a need to develop regimes for electrical stimulation which can improve upon or provide an alternative to the current ones.

Disclosure of the Invention

The invention provides in one aspect a method for treatment of a body by applying electrical stimuli to the body, characterised in that the electrical stimuli comprise at least one train of a plurality of bursts, each burst in the train having a frequency between 0.5kHz and 2kHz or a frequency between 3kHz and 6kHz and a duration between 1 millisecond and 6 milliseconds.

hi another aspect the invention provides a method of generating controlled electrical stimuli comprising, setting a master electrical stimulus machine to produce electrical stimuli according to a range of parameters appropriate for a particular patient, downloading the parameters in electronically encoded form into an electronic memory, and generating electrical stimuli according to the range of parameters defined above from the electronically encoded parameters in the electronic memory in a slave electrical stimulus machine separate from the master electrical stimulus machine,

By carrying out a method in this fashion, it can be seen that a qualified health practitioner may be able to set the parameters on the master electrical stimulus machine during a normal consultation. In this way, the patient can be assured that the parameters which are set are carefully controlled and safe. Having set the parameters, the health practitioner can then download the parameters into the slave machine and allow the patient to take home the slave machine whose parameters have been set in accordance with the parameters set on the master electrical stimulus machine, hi this way, a patient can safely use the slave stimulus machine at home with the knowledge that the parameters which have been set are appropriate for that patient's treatment.

The intensity as set by the voltage of the electrical stimulation will generally be less than 500 volts, more preferable less than 250 volts. Typically, it may lie between 10 and 100 volts.

The stimulus delivered by the machine may be delivered in short duration trains of bursts. Bursts may be adjustable from one cycle to continuous. The stimulus may be AC, DC or combinations thereof. More suitably, it will be an AC signal which will typically be a square, triangular or sine wave.

Suitably the frequency of the train of bursts may typically be adjustable between 1 Hz and 200 Hz, more preferably between 1 Hz and 150 Hz. The train of bursts may be part of a structured signal which varies in intensity over time. The structured signal may comprise a combination of "rise time", "on time" and "fall time". These terms are explained in more detail below.

Rise time relates to a period over which the intensity of the train of bursts may be increased. Typically, this may be adjustable between 0 and 20 seconds. The bursts making up the train of bursts will themselves be separated by short periods of zero signal.

After the intensity has been ramped up to its maximum amount, it may be maintained over a particular period. For example, it may typically be maintained between 0 and 60 seconds. This is the "on time".

Similarly, the bursts may be reduced in intensity over time after the maintenance of the maximum intensity signal over a period of time. The reduction in intensity known as "fall time" may typically be over a period of 0 to 20 seconds.

There may be an off time between trains of bursts, the off time typically being settable for a period of between 0 to 60 seconds. Thus, the electrical stimulus signal may comprise a series of trains of bursts which vary in intensity, having a defined rise time, on time and fall time, each series of trains of bursts being separated by an off time period.

hi order to conserve energy, particularly if the slave machine is battery powered and to obtain enhanced therapeutic effect, the actual time period of a burst signal may typically be 1 to 6 milliseconds, more preferably 2 to 4 milliseconds. In this way, the electrical stimulus making up the structured signal is only being produced for a small fraction of the treatment time. This conserves energy without compromising treatment effectiveness.

In another aspect, the invention provides a master electrical stimulus machine comprising, an electrical stimulus generator, a microcontroller incorporating an alterable electronic memory arranged to control electrical stimuli produced by the electrical stimulus generator in accordance with electronically encoded parameters in the alterable electronic memory, a control panel for setting a range of patient parameters appropriate for a particular patient whereby the patient parameters are electronically encoded by the control panel into the alterable electronic memory, and an electronic interface for downloading the electronically encoded parameters into a separate slave electronic stimulus machine, characterized in that the stimuli are in accordance with the parameters hereinbefore defined.

The invention also provides a separate slave electronic stimulus machine for receiving patient parameters electronically encoded by the master electrical stimulus machine in a non- volatile alterable memory provided in the slave machine and applying electrical stimulus in accordance with those parameters.

Preferred aspects of the invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 shows a plan view of a control panel for a master electrical stimulus machine according to the invention; Figure 2 shows a block diagram of a circuit for the master electrical stimulus machine of Figure 1; Figure 3 shows a block diagram of a circuit for a slave unit for use in association with the master machine of Figures 1 and 2; Figure 4 shows graphs of voltage vs time for a typical TENS current stimulus; Figure 5 shows graphs of voltage vs time for typical Russian and Interferential current stimuli; and Figure 6 shows graphs of voltage vs time for a typical structured current stimulus according to the present invention. Detailed Description of the Preferred Embodiments

The various elements identified by numerals in the drawings are listed in the following integer list.

Integer List

I Control panel 3 Timing display block 5 Stimulus burst display block 7 Program setting/display block 9 Output/run/stop block I 1 Setting block 13 Time display 15 Treatment time indicator 17 Bar graph 19 Surge time indicator 21 Rise time indicator 22 On time indicator 23 Fall time indicator 24 Off time indicator 26 Stimulus burst display 28 Frequency indicator 29 Duration indicator 30 Duration indicator 31 Duration indicator 32 Duration indicator 33 AC frequency indicator 34 AC frequency indicator 36 AC frequency indicator 38 Voltage output bar graph 39 On indicator 41 Voltage adjustment knob 43 Run/stop switch 45 Display 47 Recall mode indicator 48 Download mode indicator 49 Save mode indicator 51 Program selection switch 53 Setting advance button 55 Up button 56 Down button 60 Switch module 61 Display driver 62 Display module 64 Microcontroller 65 Stimulus generator 66 Potentiometer 67 Beeper 69 Fixed DC power 70 Programmable DC power 71 24 V fixed power 72 Mains power 73 Patient 78 Battery power 79 Display 80 Potentiometer 140 Graph of TENS stimulus 141 Current spikes 142 Stimulation period 143 Stimulation plus no stimulation period 145 Graph of TENS stimulus over longer time 146 Trains of spikes 147 Rising intensity 148 Maximum intensity 149 Falling intensity 150 Zero stimulus 160 Graph of Russian or interferential stimulus 161 AC stimulus burst 162 No stimulation period 165 Graph 160 taken over longer period 166 Trains of bursts 170 Graph 165 over shorter time with alternative waveform 172 AC burst 200 Graph of stimulus according to the invention 201 AC stimulus burst 202 Burst stimulation period 203 No stimulation 204 Single cycle period 210 Graph 200 taken over longer period 211 Train of bursts 212 No stimulation 213 Rising intensity 214 Maximum intensity 215 Falling intensity

Referring to Figure 1, there is shown a control panel generally designated 1 for a master electrical stimulus machine of the type which may be used by a qualified health practitioner such as a physiotherapist for patient treatment by electrical stimulation.

The control panel is set up as a number of discrete blocks for setting the various parameters of the electrical output. Thus, the control panel includes a timing display block 3, a stimulus burst display block 5, a program setting/display block 7, an output/run/stop block 9 and a setting block 11. The timing display block includes a time display 13. This may comprise an electronic light emitting diode (LED) display for displaying the time set for each of the time parameters for a particular signal. The particular times set for the different aspects of the electrical signal include the surge time. In this regard, there is a surge time indicator 19 which may typically comprise a LED to indicate the surge time is being set in association with the display on the time display 13. Similarly, the rise time indicator 21, on time indicator 22, full time indicator 23 and off time indicator 24 may all comprise LEDs for indicating that the particular times are being set according to the time displayed on the time display 13. Typically, the times indicated for the surge time, rise time, on time, full time and off time will be indicated in seconds in the time display 13.

Similarly, a treatment time indicator LED 15 to indicate time in minutes on the time display 13 is also provided.

The timing display block includes a bar graph 17 for showing the variation in stimulus intensity when surging is used. Typically the bar graph may comprise a linear array of LEDs as is known in the art.

The stimulus burst display block 5 is arranged along similar lines to the timing display block in that it includes a stimulus burst display 26 for displaying values of parameters set in relation to the various items on the display block on an electronic screen. The stimulus burst display 26 may typically comprise an LCD or LED electronic display panel.

As in the case of the timing display block, the stimulus burst display block 5 includes a number of indicators. These may typically comprise an LED to indicate the activation of a particular parameter being set and being displayed on the stimulus burst display 26. Thus, the stimulus burst display block includes indicators in the form of LEDs comprising the frequency indicator 28 for indicating the display of frequency in Hz, the duration indicator 29 for displaying a duration of two milliseconds for a burst, the duration indicator 30 for displaying a time duration of four milliseconds for a burst, the duration indicator 31 for showing a duration of 50% duty cycle for bursts and the duration indicator 32 for showing a duration of 100% duty cycle ie. a duration of 50% indicates that the stimulus burst is applied for 50% of the burst on plus off time.

Similarly, the stimulus burst display block 5 includes AC frequency indicators 33, 34, 35 and 36 to indicate AC frequencies in kHz of 1, 2.5, 4 and 10 kHz respectively.

The output/run/stop block 9 includes an electronic voltage output bar graph 38, an on indicator 39, a voltage adjustment knob 41 and a run/stop switch 43.

The program setting/display block includes an electronic display 45 for displaying the program number selected and a series of indicators 47, 48 and 49 for showing a particular mode being programmed. Again, as in the previous cases, the indicators may typically comprise light emitting diodes. The various indicators comprise a recall mode indicator 47, a download mode indicator 48 and a save mode indicator 49. The program selection switch 51 is used to activate the particular mode chosen.

The setting block 11 used for setting the various parameters in the different blocks simply includes up and down buttons 55 and 56 respectively together with a setting advance button 53 for advancing to the next parameter in a series of parameters after the previous parameter has been set.

Referring to the circuit block diagram shown in Figure 2 for the master electrical stimulus unit of Figure 1, the core part of the master unit comprises a microcontroller 64 incorporating program memory, alterable or non-volatile memory, RAM, timers analogue to digital converters and serial data interfaces.

A switch module 60 containing the up and down switches 55 and 56, set mode 53, run mode 43, and program mode 51 interface with the microcontroller to set the various parameters for the electrical stimulation. A dual motor potentiometer 66, controlled by the knob 41 is used to adjust the voltage settings via the microcontroller and the programmable DC power supply 70.

The power supply includes a fixed DC power supply 69, a 24V volt fixed power supply 71 and an electricity supply mains 72. The programmable DC power supply feeds into the stimulus generator 65 and the fixed DC power supply powers the microcontroller programmable DC power supply and display driver 61. In turn, the display driver drives the display module 62 which incorporates the numeric, bar graph and single light emitting diode displays of treatment time, surge time, stimulus burst frequency, duration and AC frequency, output voltage and program number and save/download/recall status. A beeper 67 connected directly to the microcontroller is used to provide audible confirmation of activation of the various settings.

The slave electrical stimulus machine shown in Figure 3 is much simpler than the master unit shown in Figure 2 although it is to be appreciated that the master unit could also be configured to work as a slave unit as well.

The slave unit in Figure 3 incorporates a microcontroller 64 which may incorporate the same range of features as that of the master unit. In particular, it will include program, memory, alterable non-volatile memory, RAM, timers, analogue to digital converters and serial data interfaces. However, rather than being set by switches, the alterable non- volatile memory is configured by downloading from the master unit via a serial data interface.

The displays 79 are far simpler in that they comprise a single light emitting diode display for each of the operating status and battery status of the machine. Because of its simplicity, it does not require a display driver as is required in the case of the master machine.

The arrangement of the stimulus generator 65 for stimulating a patient 73 is again similar to that for the master unit but the power supply, in being provided by standard alkaline cells 78, is the preferred supply rather than the electricity main supply 72 of the master unit. As in the case of the master unit, the battery power supply feeds into a fixed DC power supply 69 and a programmable DC power supply 70. A dual switch potentiometer 80 is used to set the on/off mode of the machine. A beeper 67 is also provided.

Preferred Emulation

The master unit/clinical stimulator

The preferred emulation of the master electrical stimulator is best described with reference to Figure 1, which shows the front panel.

When plugged-in to the electricity mains, the unit is activated by pressing the SET button. If the unit is plugged-in but has not been used for more than 30 minutes, it will go into sleep mode and the displays will turn off. Pressing the SET button will reactivate the unit. The displays will illuminate and show the previously used settings. When first plugged-in, the unit defaults to sleep mode and is activated by pressing the SET button. When the unit is activated the output current is always zero and the displays show the previous treatment settings.

If the treatment settings are those required, the RUN/STOP button can be pressed and the output current can be set by rotating the output control knob.

If different settings are required, that are not pre-programmed, the SET and UP/DOWN buttons can be pressed to make changes. The first adjustment is the treatment time, set to 20 minutes initially. It can be increased or decreased using the UP/DOWN buttons. If the SET button is pressed again, surging of the output current can be selected. The default is no surging but if surging is required, either of the UP/DOWN buttons can be pressed. If an UP or DOWN button is pressed, surging will be switched on. hi this case, the rise time indicator will be illuminated and the rise time can be adjusted with the UP/DOWN buttons. Pressing SET will cycle through the surging options and allow setting of each. Thus the rise-time, on-time, fall-time and off-time can be adjusted in turn. If surging is not switched-on, or all surging settings have been set, pressing SET will allow stimulus burst settings to be selected.

The first burst setting to be adjusted is the frequency. The FREQUENCY indicator will be illuminated and the frequency can be adjusted in the range 1 to 150 Hz using the UP/DOWN buttons. The second setting to be adjusted is the burst duration. When the SET button is pressed, selection of the burst duration (either 2ms, 4ms, 50% duty cycle or continuous can be made using the UP/DOWN buttons.

When the SET button is again pressed, the desired AC frequency (IkHz, 2.5kHz, 4 kHz or 10 kHz) can be chosen (using the UP/DOWN buttons).

Once the above described treatment settings have been made, the RUN/STOP button can be pressed to apply the selected current waveform to the patient. The RUN/STOP button is pressed and the intensity control is rotated to the desired output.

If the treatment settings are to be saved or downloaded the PROGRAM button is pressed. Repeated pressing of .the PROGRAM button allows selection of recall, download or save.

When save is pressed a number appears on the program indictor. Pressing the SET button will store the setting as that program. The program number can be adjusted using the UP/DOWN buttons. If a program is already stored in this location, the program number will flash to indicate a pre-existing program. The program will be over-written if the SET button is pressed. To store the settings as a different program, the UP/DOWN buttons are used.

When "download" is selected, pressing the SET button allows the maximum output intensity of the slave unit to be set. A bar appears on the output meter and the output control knob can be rotated to set the maximum. Once this is done, pressing the SET button again will cause the settings to be downloaded to the slave unit. The Slave Unit

The slave unit is both a take-home treatment device and a clinical stimulator. It differs from the master unit in that the treatment settings can not be changed except by downloading new settings from the master unit. The slave unit has no external control other than on on-off switch and an intensity control. It is battery-powered and self-contained. It has a socket for downloading of treatment settings and sockets for connecting the output to the patient via skin-mounted electrodes.

An advantageous feature of the slave unit is the ability to administer treatments which duplicate those of a clinical stimulator. This is made possible, without unduly compromising battery life, by using stimulus waveform bursts in the form of trains of bursts which apply electrical stimuli for only a fraction of the total treatment time.

Stimuli Structure

In order to understand the nature of the invention more fully, it is useful to compare the structure of the electrical stimuli according to the invention with the conventional stimuli, namely TENS, Russian and Interferential by reference to Figures 4 to 6.

The graph of voltage vs time for a TENS waveform shown in Figure 4, includes the graph of a TENS stimulus 140 taken over a particular period of time and a further graph 145 of the TENS stimulus shown in graph 40 taken over a longer period of time.

As shown more clearly in graph 140, the TENS stimulus comprises a plurality of current spikes 141 which are applied as a short burst of DC current for a stimulation period 142 (typically between 50 microseconds and 1 millisecond) followed by a zero stimulation period, the total stimulation plus no stimulation period 143 typically being between 5 milliseconds and 1 second. Turning to graph 145, it can be seen that the TENS stimulus can be applied as trains of spikes 146 beginning with a period 147 of rising intensity from zero to a period of maximum intensity 148 followed by a period of falling intensity 149 and a period of zero stimulus 150 between trains of spikes.

Because the actual total period of electrical stimulation compared with the total period over which there is no electrical stimulation is very small, the TENS stimulators are suitable for use with battery power sources. However, they suffer from the disadvantage that the stimulus they produce is considered to be uncomfortable by a number of users.

Referring to Figure 5, and in particular graph 160 of Figure 5, there is shown a series of AC stimulus bursts 161 separated by no stimulation periods 162. Depending on their frequency, they can be considered to be either Russian current stimulus or Interferential current stimulus.

The Russian current stimulus is characterized by having a frequency of 2.5kHz with equal periods (10 milliseconds) of on and off stimulus ie. the length of the AC signal burst 161 and the no stimulation period 162 are the same. Thus Russian current can be said to have a 50% duty cycle.

By comparison, if the frequency of the burst 161 is 4kHz or 5kHz, then the stimulus would be considered to be an Interferential stimulus.

Looking at graph 165 which effectively takes graph 160 over a long period, it can be seen that either the Russian or Interferential stimulus are typically applied as trains of bursts 166 having a period where intensity arises from zero to maximum, a period at which maximum intensity is applied and a period over which the intensity is gradually reduced to zero. Typically, the length of time for a train of bursts may be 1 to 10 seconds with an off period of 0 to 50 seconds between trains of bursts. It should be noted that graph 165 for ease of illustration is not done to scale. Referring to graph 170, there is shown an alternative waveform known as premodulated Interferential current with a frequency of 4kHz or 5kHz. In this type of stimulation, the trains of bursts are continuous and are generally set to last up to 20 seconds with a period of up to 50 seconds between trains of bursts.

It can be seen that the actual period of electrical stimulation of the Russian and Interferential type shown in Figure 5 is typically greater than that shown for the TENS stimulation of Figure 4. As a result this generally requires more electrical power and is hence more suitable for being operated via mains power rather than by battery. The type of stimulation is more effective for muscle stimulation rather than pain control for which a TENS stimulation is usually used.

Referring to Figure 6, and in particular graph 200, it can be seen that the electrical stimulus according to this particular embodiment of the invention comprises a series of AC stimulus bursts 201 for a burst stimulation period 202 lasting between 1 millisecond and 6 milliseconds. The frequency of the AC burst falls between 0.5kHz and 2kHz or between 3kHz and 6kHz depending on the nature of treatment being given ie. a frequency of 0.5kHz to 2kHz will typically be used to strengthen muscles, the duration of each burst being between 1 millisecond and 3 milliseconds whereas a burst frequency between 3kHz and 6kHz for a duration of between 2 milliseconds and 6 milliseconds is typically used to ameliorate pain.

The no stimulation period 203 between bursts is variable although it will generally be less than 1 second. Thus, a single cycle period 204 will generally be about 1 second or less.

Referring to graph 210 which is taken over a longer period than graph 200, it can be seen that the stimulation is applied as a train of bursts 211 separated by periods of no stimulation 212. Each of the train of bursts has a period of rising intensity 213 followed by a period of maximum intensity 214 and a period of falling intensity 215. Each of the trains of bursts may be applied for anything up to about 20 seconds and the periods of no stimulation 212 may typically have a length of up to 50 seconds. It can be seen that the time of electrical stimulation compared with no electrical stimulation using the electrical stimulus waveform according to the invention can be adjusted so that it requires very little drain of electrical power making it suitable for use with battery operated machines. Applicants have found that this narrow band of parameters for electrical stimulation have proven to be effective for muscular stimulation and pain control.

Whilst the above description includes the preferred embodiments of the invention, it is to be understood that many variations, alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the essential features or the spirit or ambit of the invention.

It will be also understood that where the word "comprise", and variations such as "comprises" and "comprising", are used in this specification, unless the context requires otherwise such use is intended to imply the inclusion of a stated feature or features but is not to be taken as excluding the presence of other feature or features.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge in Australia.

Claims

1. A method for treatment of a body by applying electrical stimuli to the body, characterised in that the electrical stimuli comprise at least one train of a plurality of bursts, each burst in the train having a frequency between 0.5kHz and 2kHz or a frequency between 3kHz and 6kHz and a duration between 1 millisecond and 6 milliseconds.

2. The method according to claim 1 wherein the frequency of the train of bursts is between IHz and 150Hz.

3. The method according to claim 2 wherein the intensity of the train of bursts is raised gradually from zero to maximum intensity over a period of up to 20 seconds, is maintained at maximum intensity over a period of up to 60 seconds, and is then gradually reduced to zero over a period of up to 20 seconds.

4. The method according to claim 1 wherein the electrical stimuli are applied to ameliorate pain, the burst frequency is between 3kHz and 6kHz and the duration of each burst is between 2 milliseconds and 6 milliseconds.

5. The method according to claim 1 wherein the electrical stimuli are applied for muscle strengthening, the burst frequency is between 0.5kHz and 2kHz and the duration of each burst is between 1 millisecond and 3 milliseconds.

6. The method according to claim 2 wherein the electrical stimuli comprise alternating current.

7. A method of generating controlled electrical stimuli comprising, setting a master electrical stimulus machine to produce electrical stimuli according to a range of parameters appropriate for a particular patient, downloading the parameters in electronically encoded form into an electronic memory, and generating electrical stimuli according to the range of parameters from the electronically encoded parameters in the electronic memory in a slave electrical stimulus machine separate from the master electrical stimulus machine, characterised in that the range of parameters sets the slave machine to generate electrical stimuli as defined in claim 1.

8. The method of claim 7 wherein the parameters are set to generate stimuli as defined in claim 2.

9. Apparatus for generation of electrical stimuli comprising, a microcontroller, an electronic memory, and a stimulus generator responsive to electronic signals from the microcontroller, characterised in that the electronic memory is operatively connected to the microcontroller and is encoded with data which sets the microcontroller to cause the stimulus generator to generate electrical stimuli comprising at least one train of bursts, each burst in the train having a frequency between 0.5kHz and 2kHz or a frequency between 3kHz and 6kHz and a duration between 1 millisecond and 6 milliseconds.

10. The apparatus according to claim 9 wherein frequency of the train of bursts is between IHz and 150Hz.

11. A method according to claim 1 substantially as hereinbefore described.

12. A method according to claim 7 substantially as hereinbefore described.

13. Apparatus according to claim 9 substantially as hereinbefore described.