Method and apparatus for measurement of evoked neural response

Abstract

The invention provides a method of electrical artefact compensation in measurement of a neural response. The neural response is evoked by a first stimulus, after which a compensatory stimulus is applied in order to counteract a stimulus artefact caused by the first stimulus. The invention also provides for short circuiting the stimulating electrode subsequent to the first stimulus. A system for implementing such steps is also provided. The invention may be of application in measurement of physiological responses, including neural responses and in particular a neural response of the auditory nerve.

1 . A method of electrical artefact compensation in measurement of a neural response, the neural response evoked by a first stimulus, the method comprising the step of: 
 subsequent to the first stimulus, applying a compensatory stimulus in order to counteract a stimulus artefact caused by the first stimulus.    
   
   
       2 . The method of  claim 1  wherein the first stimulus comprises: 
 a first phase during which an electrical stimulus of first polarity is applied; and    a second phase, subsequent to the first phase, during which an electrical stimulus of second polarity opposite to the first polarity is applied.    
   
   
       3 . The method of  claim 2  wherein the first phase and second phase are charge balanced.  
   
   
       4 . The method of  claim 2  or  claim 3  wherein the compensatory stimulus is of the first polarity.  
   
   
       5 . The method of  claim 1  wherein the compensatory stimulus is of controlled profile and duration.  
   
   
       6 . The method of  claim 1  wherein the compensatory stimulus is adaptive, whereby characteristics of the compensatory stimulus are chosen in order to optimise the extent of cancellation of the artefact which exists following the first stimulus.  
   
   
       7 . The method of  claim 6  further comprising the step of: 
 determining characteristics of the compensatory stimulus from a measured effectiveness of a previously applied compensatory stimulus.    
   
   
       8 . The method of  claim 7  wherein the step of determining characteristics of the compensatory stimulus is performed in an iterative manner, to provide an ongoing optimisation of the compensatory stimulus, based on the effectiveness of one or more previously applied compensatory stimuli.  
   
   
       9 . The method of  claim 7  or  claim 8  wherein the measured effectiveness of the previously applied compensatory stimulus is determined by: 
 obtaining at least one neural response measurement of an actual performance of the previously applied compensatory stimulus; and    comparing the actual performance against a target performance, in order to determine an error between the actual performance and the target performance.    
   
   
       10 . The method of  claim 9  wherein the step of measuring the actual performance of the previously applied compensatory stimulus comprises: 
 (i) applying the first stimulus and the previously applied compensatory stimulus, and subsequently obtaining a measurement comprising a first plurality of temporally spaced neural samples from a sensor.    
   
   
       11 . The method of  claim 10  wherein the step of measuring the actual performance of the previously applied compensatory stimulus further comprises: 
 (ii) repeating step (i) in order to obtain a second plurality of measurements.    
   
   
       12 . The method of  claim 11  wherein the step of measuring the actual performance of the previously applied compensatory stimulus further comprises: 
 (iii) discarding an initial number of said second plurality of measurements to allow the sensor and other measurement components to settle.    
   
   
       13 . The method of  claim 12  wherein the step of measuring the actual performance of the previously applied compensatory stimulus further comprises: 
 (iv) taking an average of the remaining non-discarded measurements of the second plurality of measurements, to obtain an averaged measurement.    
   
   
       14 . The method of  claim 13  wherein the step of measuring the actual performance of the previously applied compensatory stimulus further comprises: 
 (v) determining a stimulus artefact from the averaged measurement, and assessing the performance of the previously applied compensatory stimulus with reference to the determined stimulus artefact.    
   
   
       15 . The method of  claim 10  wherein step (i) comprises obtaining substantially 64 neural samples at substantially 48 μs intervals.  
   
   
       16 . The method of  claim 11  wherein step (ii) comprises obtaining substantially 20 measurements.  
   
   
       17 . The method of  claim 16  wherein step (iii) comprises discarding substantially the first 10 of said measurements.  
   
   
       18 . The method of  claim 14  wherein the step of determining the stimulus artefact from the averaged measurement comprises determining a deviation of the averaged measurement from a desired response.  
   
   
       19 . The method of  claim 7  wherein the step of determining characteristics of the compensatory stimulus from the measured effectiveness of the previously applied compensatory stimulus is performed by: 
 determining an incremental change to be made to characteristics of the previously applied compensatory stimulus in order to reduce the error between the actual performance and the target performance; and    deriving the characteristics of the compensatory stimulus by altering the characteristics of the previously applied compensatory stimulus in accordance with the incremental change.    
   
   
       20 . The method of  claim 19  wherein the incremental change is determined so as to maximise a rate of convergence of the actual performance to the target performance.  
   
   
       21 . The method of  claim 19  or  claim 20  wherein the incremental change is determined so as to minimise oscillation or overshoot of the actual performance relative to the target performance.  
   
   
       22 . The method of  claim 1  wherein the compensatory stimulus comprises a substantially rectangular pulse, whereby the amplitude and duration of the pulse define an amount of charge to be inserted by the pulse.  
   
   
       23 . A method of electrical artefact compensation in measurement of a neural response, the neural response evoked by a first stimulus, the method comprising: 
 subsequent to the first stimulus, applying a compensatory stimulus in order to counteract a stimulus artefact caused by the first stimulus;    the compensatory stimulus comprising a substantially rectangular pulse, whereby the amplitude and duration of the pulse define an amount of charge to be inserted by the pulse;    determining characteristics of the compensatory stimulus from the measured effectiveness of the previously applied compensatory stimulus by:    determining an incremental change to be made to characteristics of the previously applied compensatory stimulus in order to reduce the error between an actual performance and the target performance; and    deriving the characteristics of the compensatory stimulus by altering the characteristics of a previously applied compensatory stimulus in accordance with the incremental change; and    wherein the incremental change defines a change in the amount of charge to be inserted by the pulse which is required in order to improve the actual performance of the compensatory stimulus compared to the target performance.    
   
   
       24 . The method of  claim 23  wherein, for a given charge to be applied by the compensatory stimulus, the incremental change further defines whether a relatively narrow pulse of relatively large amplitude should be applied or whether a relatively broad pulse of relatively small amplitude should be applied in delivering the required amount of charge by the compensatory stimulus.  
   
   
       25 . The method of  claim 24  wherein the compensatory stimulus has a fixed duration and a variable amplitude.  
   
   
       26 . The method of  claim 1  wherein the compensatory stimulus is completed prior to an expected time of commencement of an electrically evoked compound action potential.  
   
   
       27 . The method of  claim 1  wherein the neural response is the auditory nerve neural response.  
   
   
       28 . The method of  claim 27  wherein the first stimulus is applied by an auditory prosthesis.  
   
   
       29 . The method of  claim 28  wherein the auditory prosthesis comprises a cochlear implant having an intra-cochlear electrode array including electrodes used as stimulus electrodes and/or as sense electrodes.  
   
   
       30 . The method of  claim 29  wherein the first stimulus is applied by one or more stimulus electrodes of the array.  
   
   
       31 . The method of  claim 30  wherein the compensatory stimulus is applied by the one or more stimulus electrodes.  
   
   
       32 . The method of  claim 30  wherein the compensatory stimulus is applied by one or more electrodes in the array other than the stimulus electrodes.  
   
   
       33 . The method of  claim 32  wherein additional compensatory stimuli are applied by electrodes in close physical proximity to the stimulating electrodes, substantially simultaneously with application of the compensatory stimulus.  
   
   
       34 . A method of electrical artefact compensation in measurement of a neural response, the neural response evoked by a first stimulus applied by at least one stimulating electrode, the method comprising the step of: 
 subsequent to the first stimulus, short circuiting at least each stimulating electrode to an electrode reference voltage.    
   
   
       35 . The method of  claim 34  wherein the at least one stimulating electrode is an electrode of an auditory prosthesis for stimulus of the auditory nerve.  
   
   
       36 . The method of  claim 35  wherein the auditory prosthesis comprises an electrode array having a plurality of electrodes.  
   
   
       37 . The method of  claim 36  wherein the electrode array comprises between 22 and 30 intra-cochlear electrodes.  
   
   
       38 . The method of  claim 36  or  claim 37  wherein the method further comprises the step of shorting electrodes in the array other than the stimulating electrodes.  
   
   
       39 . The method of  claim 38  wherein the step of shorting electrodes in the array other than the stimulating electrodes comprises shorting electrodes in physical proximity to the stimulating electrodes.  
   
   
       40 . (canceled)  
   
   
       41 . A neural stimulus system operable to apply a first stimulus in order to evoke a neural response, the neural stimulus system comprising means for electrical artefact compensation in measurement of the neural response, the means for electrical artefact compensation being operable to apply a compensatory stimulus in order to counteract a stimulus artefact caused by the first stimulus.  
   
   
       42 . The neural stimulus system of  claim 41  wherein measurement of the neural response is performed by detection of a signal present on designated sense electrodes.  
   
   
       43 . The neural stimulus system of  claim 42  wherein the sense electrodes are distinct from the stimulus electrodes.  
   
   
       44 . The neural stimulus system of any one of  claims 41  to  43  wherein the neural stimulus system is an auditory prosthesis.  
   
   
       45 . The neural stimulus system of  claim 44  wherein the auditory prosthesis is a cochlear implant.  
   
   
       46 . The neural stimulus system of  claim 44  wherein the response measured is the evoked compound action potential of the auditory nerve.  
   
   
       47 . The neural stimulus system of  claim 44  wherein the auditory prosthesis comprises an array of intra-cochlear and extra-cochlear electrodes.  
   
   
       48 . The neural stimulus system of  claim 41  wherein the neural stimulus system is operable to apply the first stimulus such that the first stimulus comprises: 
 a first phase during which an electrical stimulus of first polarity is applied; and    a second phase, subsequent to the first phase, during which an electrical stimulus of second polarity opposite to the first polarity is applied.    
   
   
       49 . The neural stimulus system of  claim 48  wherein the first phase and second phase are charge balanced.  
   
   
       50 . The neural stimulus system of  claim 48  or  claim 49  wherein the means for electrical artefact compensation is operable to apply the compensatory stimulus such that the compensatory stimulus is of the first polarity.  
   
   
       51 . The neural stimulus system of  claim 41  wherein the means for electrical artefact compensation is operable to control a profile and a duration of the compensatory stimulus.  
   
   
       52 . The neural stimulus system of  claim 41  wherein the means for electrical artefact compensation is operable to control characteristics of the compensatory stimulus in order to optimise the extent of cancellation of the stimulus artefact.  
   
   
       53 . The neural stimulus system of any one  claim 41  further comprising means for measuring an effectiveness of a previously applied compensatory stimulus.  
   
   
       54 . The neural stimulus system of  claim 53  wherein the means for measuring an effectiveness of a previously applied compensatory stimulus comprises means to obtain at least one neural response measurement of an actual performance of the previously applied compensatory stimulus, and comprises means to compare the actual performance against a target performance, in order to determine an error between the actual performance and the target performance.  
   
   
       55 . The neural stimulus system of  claim 54  wherein the means to obtain at least one neural response measurement of an actual performance of the previously applied compensatory stimulus is operable to obtain a measurement comprising a first plurality of temporally spaced neural samples from a sensor.  
   
   
       56 . The neural stimulus system of  claim 55  wherein the means to obtain at least one neural response measurement of an actual performance of the previously applied compensatory stimulus is operable to obtain a measurement comprising substantially 64 neural samples spaced at substantially 48 μs intervals.  
   
   
       57 . The neural stimulus system of  claim 41  wherein the means for electrical artefact stimulation is operable to apply the compensatory stimulus such that the compensatory stimulus comprises a substantially rectangular pulse.  
   
   
       58 . The neural stimulus system of  claim 57  wherein the means for electrical artefact stimulation is operable to apply the compensatory stimulus such that the compensatory stimulus comprises a substantially rectangular pulse having a relatively narrow width and a relatively large amplitude, and is also operable to apply the compensatory stimulus such that the compensatory stimulus comprises a substantially rectangular pulse having a relatively broad width and a relatively small amplitude.  
   
   
       59 . The neural stimulus system of  claim 41  wherein the neural response is the auditory nerve neural response.  
   
   
       60 . The neural stimulus system of  claim 41  wherein the neural stimulus system comprises an auditory prosthesis.  
   
   
       61 . The neural stimulus system of  claim 60  wherein the auditory prosthesis comprises a cochlear implant having an intra-cochlear electrode array including electrodes used as stimulus electrodes and/or as sense electrodes.  
   
   
       62 . The neural stimulus system of  claim 61  wherein the first stimulus is applied by the one or more stimulus electrodes.  
   
   
       63 . The neural stimulus system of  claim 62  wherein the compensatory stimulus is applied by the one or more stimulus electrodes.  
   
   
       64 . The neural stimulus system of  claim 62  wherein the compensatory stimulus is applied by one or more electrodes in the array other than the stimulus electrodes.  
   
   
       65 . The neural stimulus system of  claim 41  further operable to apply one or more additional compensatory stimuli.  
   
   
       66 . A neural stimulus system comprising at least one stimulating electrode operable to apply a first stimulus in order to evoke a neural response, the neural stimulus system being operable to compensate for electrical stimulus artefacts by short circuiting at least each stimulating electrode to an electrode reference voltage.  
   
   
       67 . The neural stimulus system of  claim 66  wherein the neural stimulus system is an auditory prosthesis for stimulus of the auditory nerve.  
   
   
       68 . The neural stimulus system of  claim 67  wherein the auditory prosthesis comprises an electrode array having a plurality of electrodes.  
   
   
       69 . The neural stimulus system of  claim 68  wherein the electrode array comprises between 22 and 30 intra-cochlear electrodes.  
   
   
       70 . The neural stimulus system of  claim 68  or  claim 69  wherein the neural stimulus system is further operable to short electrodes in the array other than the stimulating electrodes.  
   
   
       71 . (canceled)  
   
   
       72 . A method of electrical artefact compensation in measurement of a physiological response, the physiological response evoked by a first stimulus, the method comprising the step of: 
 subsequent to the first stimulus, applying a compensatory stimulus in order to counteract a stimulus artefact caused by the first stimulus.    
   
   
       73 . The method of  claim 34  wherein the step of short circuiting occurs subsequent to the step of: 
 delivering a compensatory stimulus in order to counteract a stimulus artefact caused by the first stimulus.    
   
   
       74 . The method of  claim 73  wherein the step of short circuiting extends for a relatively short time period immediately following the compensatory stimulus.  
   
   
       75 . The method of  claim 74  wherein the step of short circuiting extends for a period of substantially 1 μs.  
   
   
       76 . The neural stimulus system of  claim 66  wherein the system is further operable to apply a compensatory stimulus in order to counteract a stimulus artefact caused by the first stimulus.  
   
   
       77 . The neural stimulus system of  claim 76  wherein the system is operable to apply the compensatory stimulus after application of the compensatory stimulus.  
   
   
       78 . The neural stimulus system of any one of claims  66 ,  76  or  77  wherein the system is operable to short circuit at least each stimulating electrode to the electrode reference voltage for a relatively short period of time.  
   
   
       79 . The neural stimulus system of  claim 78  wherein the system is operable to short circuit at least each stimulating electrode to the electrode reference voltage for substantially 1 μs.