Abstract:
A way of reducing channel interaction is provided in a multichannel neural implant that has the ability to stimulate multiple tissue or nerve sites simultaneously. Channel interaction is minimized through measuring or estimating what the channel interaction is or will likely be, and then using that measured or estimated channel interaction to adjust the intensity of the applied stimuli so that, with the channel interaction, the actual stimuli applied to the tissue or nerves is of a desired intensity level. In one embodiment, the measured or estimated channel interaction is collected or compiled and saved as a channel interaction matrix. The channel interaction matrix is created during a fitting procedure by stimulating one channel at a time while measuring the effects of the stimulation on the neighboring channels. The superposition principal is used, as needed, to determine all the terms of the channel interaction matrix. The channel interaction matrix, once obtained, provides all of the information needed to eliminate or minimize channel interaction.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to multi-channel neural stimulators, and more particularly to a multi-channel neural stimulator that includes the ability to minimize channel interaction. 
     A neural stimulator provides electrical stimulation pulses to selected body tissue or nerves through one or more electrodes for the purpose of providing a desired therapeutic effect, e.g., to relive pain, to treat urinary incontinence, to aid a profoundly deaf person to hear, to treat a disease, or the like. A multichannel neural stimulator provides the capability of stimulating multiple tissue sites simultaneously with different levels of stimulation, e.g., with different current pulse amplitudes, applied to each stimulation site. 
     Representative neural stimulation systems are disclosed in U.S. Pat. Nos. 4,400,590; 5,324,316; 5,571,148; 5,603,726; 5,824,022; 6,308,101; and in International Publication WO 02/09808 A1, which patents and publication are incorporated herein by reference. 
     When multiple electrodes are stimulated simultaneously, electrical interactions can arise. See, e.g., Stickney et al., “Channel Interaction and Speech Processing Strategies for Cochlear Implants”, Univ. of Texas at Dallas, House Ear Institute and Insonus Medical, Inc. Joint Presentation Report (Poster Presentation); Stickney et al. “Electrode Interaction and Speech Intelligibility in Mutlichannel Cochlear Implants”, UC Irvine Department of Otolaryngology (Poster Presentation); and Boex, et al., “Electrode interactions in Clarion Subjects”, University Hospitals, Geneva, Switzerland (Poster Presentation), which poster presentations are incorporated herein by reference. 
     It is generally known that channel interaction can be reduced by decreasing the current levels delivered to each electrode, improving electrode positioning and design, or using signal processing strategies or approaches that stimulate electrodes sequentially. The present invention provides another approach for minimizing channel interaction in a multichannel neural stimulator, such as a cochlear implant. 
     SUMMARY OF THE INVENTION 
     The present invention provides a way of reducing channel interaction in a multichannel neural implant having the ability to stimulate multiple tissue or nerve sites simultaneously. 
     Channel interaction is minimized, in accordance with the teachings of the present invention, through measuring or estimating what the channel interaction is or will likely be, and then using that measured or estimated channel interaction to adjust the intensity of the applied stimuli so that, with the channel interaction, the actual stimuli applied to the tissue or nerves is of a desired intensity level. 
     In one embodiment, the measured or estimated channel interaction is compiled and saved as a channel interaction matrix. The channel interaction matrix is created during a fitting procedure by stimulating one channel at a time while measuring the effects of the stimulation on the neighboring channels. The superposition principal is used, as needed, to determine all the terms of the channel interaction matrix. 
     The channel interaction matrix, once obtained, advantageously provides all of the information needed to eliminate or minimize channel interaction. In a preferred technique, this is done by using the diagonal terms of the channel interaction matrix [CIM] and a desired stimulation current matrix [I D ] to compute a voltage matrix [V] that calculates what the voltage at each stimulation site would be if the desired stimulation currents were applied at each stimulation site. Then, the inverse of the channel interaction matrix, [CIM] −1 , and the voltage matrix [V] are used to determine what the required stimulation currents [I R ] at each stimulation site need to be in order to produce, with the occurrence of channel interaction, the desired stimulation currents [I D ]. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1  shows a generalized bock diagram of a neural stimulation system, including an input transducer, a signal processor, an implantable neural stimulator and an electrode array; 
         FIG. 2  schematically illustrates two types of electrode arrays that may be used with an implantable neural stimulator, whereby by way of example each electrode array has sixteen individual electrode contacts, and wherein a common or reference electrode contact is provided on the case of the implantable neural stimulator; 
         FIG. 3A  shows a simplified neural stimulator having three channels through which stimulation waveforms may be applied simultaneously; 
         FIG. 3B  shows a block diagram of the simplified neural stimulator of  FIG. 3A ; 
         FIG. 4  illustrates the concept of channel interaction relative to the simplified neural stimulator of  FIGS. 3A and 3B ; 
         FIGS. 5A ,  5 B and  5 C depict how the present invention uses a channel interaction matrix (CIM) to minimize channel interaction in a neural stimulation system having eight channels; 
         FIG. 6A  is a flow chart that illustrates the main steps associated with minimizing channel interaction in accordance with the present invention; 
         FIG. 6B  details the steps that may be followed, in one embodiment, in order to carry out step  82  of the process illustrated in  FIG. 6A ; and 
         FIG. 7  is a graph depicting the inter-electrode nulling obtained using the invention. 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
     Neural stimulation systems may take many forms. One form is a cochlear stimulation system. A cochlear stimulation system is shown, by way of example, in  FIG. 1 . The cochlear stimulation system includes an external speech processor portion  10  and an implantable neural stimulator (INS) portion  12 . The speech processor portion  10  includes a speech processor (SP)  16  and a microphone  18 . The microphone  18  may be connected directly to the SP  16 , or may be coupled to the SP  16  through an appropriate communication link  24 . The neural stimulation portion  12  includes an implantable neural stimulator (INS)  21 , and an electrode array  48 . The electrode array  48  is adapted to be inserted within the cochlea of a patient. The array  48  includes a multiplicity of electrodes, e.g., sixteen electrodes, spaced along its length that are selectively connected to the INS  21 . The electrode array  48  may be substantially as shown and described in U.S. Pat. No. 4,819,647, incorporated herein by reference, or U.S. Pat. No. 6,129,753, also incorporated herein by reference. Electronic circuitry within the INS  21  allows a specified stimulation current to be applied to selected pairs or groups of the individual electrodes included within the electrode array  48  in accordance with a specified stimulation pattern, defined by the SP  16 . (Here, it should be noted, that the “implantable neural stimulator” terminology is generic to any neural stimulator system. Where, as here, the nerves stimulated by the INS are auditory nerves, the INS may be referred to more particularly as an “implantable cochlear stimulator”, or ICS.) 
     The INS  21  and the SP  14  are linked together electronically through a suitable communications link  14  that allows power and control signals to be sent from the SP  16  to the INS  21 , and that (in some embodiments) allows data and status signals to be sent from the INS  21  to the SP  16 . The details of such communication link  14  are not important for purposes of the present invention. In some embodiments, i.e., where the INS  21  and electrode array  48  are implanted within the patient, and the SP  16  and microphone  18  are carried externally (non-implanted) by the patient, the link  14  may be realized by an antenna coil in the INS and an external antenna coil coupled to the SP. In such embodiment, i.e., when the INS has been implanted, the external antenna is positioned so as to be aligned over the location where the INS is implanted, allowing such coils to be inductively coupled to each other, thereby allowing information (e.g., the magnitude and polarity of a stimulation current) and power to be transmitted from the speech processor  16  to the INS  21 . In other embodiments, i.e., where both the SP  16  and the INS  21  are implanted within the patient, the link  14  may be a direct wired connection, or other suitable link, as described, e.g., in U.S. Pat. No. 6,308,101, also incorporated herein by reference. 
     The microphone  18  senses acoustic signals and converts such sensed signals to corresponding electrical signals. The electrical signals are sent to the SP  16  over a suitable electrical or other link  24 . The SP  16  processes these converted acoustic signals in accordance with a selected speech processing strategy in order to generate appropriate control signals for controlling the INS  21 . Such control signals specify or define the polarity, magnitude, location (which electrode pair receives the stimulation current), and timing (when the stimulation current is applied to the electrode pair) of the stimulation current that is generated by the INS  21 . 
     It is common in the cochlear stimulator art, as well as in most neural stimulation art, to condition the magnitude and polarity of the stimulation current applied to the implanted electrodes of the electrode array  48  in accordance with a specified stimulation strategy. Such stimulation strategy involves defining a pattern of stimulation waveforms that are to be applied to the electrodes as controlled electrical currents. If multiple electrode pairs exist, as is the case with a multichannel neural stimulator of the type used with the present invention, then the types of stimulation patterns applied to the multiple channels may be conveniently categorized as: (1) simultaneous stimulation patterns, and (2) non-simultaneous stimulation patterns. Simultaneous stimulation patterns may be “fully” simultaneous or partially simultaneous. A fully simultaneous stimulation pattern is one wherein stimulation currents, either analog or pulsatile, are applied to the electrodes of all of the available channels at the same time. A partially simultaneous stimulation pattern is one wherein stimulation currents, either analog or pulsatile, are applied to the electrodes of two or more channels, but not necessarily all of the channels, at the same time. Examples of each type are given below. 
     Analog waveforms used in analog stimulation patterns are typically reconstructed by the generation of continuous short monophasic pulses (samples). The sampling rate is selected to be fast enough to allow for proper reconstruction of the temporal details of the signal. 
     Current pulses applied in pulsatile stimulation patterns are generally biphasic pulses applied to the electrodes of each channel. The biphasic pulse has a magnitude (e.g., amplitude and/or duration) that varies as a function of the sensed acoustic signal. (A “biphasic” pulse is generally considered as two pulses: a first pulse of one polarity having a specified magnitude, followed immediately, or after a very short delay, by a second pulse of the opposite polarity having the same total charge, which charge is the product of stimulus current times duration of each pulse or phase.) For multichannel cochlear stimulators, it is common to sample the acoustic signal at a rapid rate, and apply a biphasic stimulation pulse in sequence (i.e., non-simultaneously) to each of the pairs of electrodes of each channel in accordance with a specified pattern and cycle time, with the magnitude of the stimulation current being a function of information contained within the sensed acoustic signal at a given (e.g., the most recent) sample time. An example of such sequential, non-simultaneous stimulation pattern is a continuous interleaved sampler (CIS) strategy. 
     It is important to recognize that in between the two extremes of fully simultaneous stimulation patterns (wherein analog stimulation currents are continuously applied to all channels, e.g., using the Simultaneous Analog Strategy (SAS)) and non-simultaneous pulsatile patterns (wherein biphasic pules are applied in a specified sequence to all channels without time overlap, e.g., using the CIS strategy), there are a great number of other stimulation patterns may be formulated. Such other simulation patterns may prove more efficacious for a given patient than either of the SAS or CIS extremes. 
     It is also important to realize that the present invention finds primary applicability for use with neural stimulation systems having multiple independent current sources that can be used to independently generate the electrical stimuli applied through the electrodes of the various channels. That is, each output channel of a neural stimulator used with the present invention should be capable of being programmed to provide an electrical stimulation pattern at the same time, i.e., simultaneously with, the application of other electrical stimulation patterns to other channels. It is the presence of the simultaneous electrical stimulation patterns that gives rise to the interaction between the various channels, and it is this interaction (or the minimization of this interaction) that is the subject of the present invention. 
     Turning next to  FIG. 2 , there is shown a schematic representation of two types of electrode arrays  48  and  48 ′ that may be used with a neural stimulation system. Either type of electrode array  48  or  48 ′ has its proximal end (not shown) connected to an INS  21 . This connection enables the current generators within the INS for each channel to apply a current stimulus of a desired amplitude, polarity, duration and repetition rate to be applied to the paired electrode contacts for that channel. 
     In one embodiment, the electrode array  48  includes sixteen electrode contacts, identified in  FIG. 2  as M 1 , L 1 , M 2 , L 2 , M 3 , L 3 , . . . M 8 , L 8 . As seen in  FIG. 2 , the electrodes identified as M 1 , M 2 , M 3 , . . . M 8  are on the medial side of the electrode; and the electrodes L 1 , L 2 , L 3 , . . . L 8  reside on a lateral side of the electrode. The output circuitry within the INS  21  may be configured so as to connect selected pairs of these electrodes to provide bipolar stimulation. One form of bipolar stimulation, referred to as radial bipolar, pairs electrode contacts L 1  with M 1 , L 2  with M 2 , and so on, through L 8  with M 8 . Another form of bipolar stimulation, referred to as enhanced bipolar, pairs electrode contacts M 1  with L 2 , M 2  with L 3 , M 3  with L 4 , M 4  with L 5 , M 5  with L 6 , M 6  with L 7 , M 7  with L 8 , and M 8  with L 1 . Using bipolar electrode coupling, i.e., applying the stimulating electrical pulses through paired electrode contacts that are adjacent each other on the electrode array  48 , provides a more focused stimulation that may be more suitable for certain types of simultaneous stimulation. 
     Additionally, a reference or common electrode  58  may comprise an exposed portion of a band on the case of the INS  21 . Through use of such reference electrode  58 , it is thus also possible to configure the system for monopolar stimulation. In monopolar stimulation, a first channel may pair electrode contact M 1  or L 1  with the reference electrode, a second channel may pair electrode M 2  or L 2  with the reference electrode, and so on, with the eighth channel pairing electrode M 8  or L 8  with the reference electrode. Using monopolar electrode coupling, i.e., applying the stimulating electrical pulses through paired electrode contacts that include the distant reference electrode, provides a less focused stimulation, but one which may be more suitable for other types of stimulation. Disadvantageously, monopolar stimulation also gives rise to significant channel interaction. Therefore, the present invention is particularly useful for neural stimulation systems where monopolar stimulation is employed. 
     Additionally, as is known in the art, virtual coupling may also be employed through appropriate mapping of the SP analysis channels with the INS stimulation channels in order to fine tune the position or location where a stimulus excites neural tissue. 
     Monopolar, bipolar or virtual coupling may also be used with an electrode array  48 ′, also shown in  FIG. 2 . The array  48 ′ has all of its electrode contacts along a medical side of the array. A typical bipolar coupling will pair electrodes  1  and  2 ,  3  and  4 ,  5  and  6 , and so on, through  15  and  16 . A typical monopolar coupling for the array  48 ′ will pair electrodes  1 ,  3 ,  5 , . . .  15  with the reference electrode  58 ; or electrodes  2 ,  4 ,  6 , . . .  16  with the reference electrode  58 . 
     With the foregoing as background, the main feature of the present invention—relating to the manner in which channel interaction is minimized—will next be explained in connection with the description of  FIGS. 3A ,  3 B and  4 . 
       FIG. 3A  shows a simple implantable neural stimulator  21 ′ having three channels. This simplified three-channel neural stimulator  21 ′ will be used to teach the basic principles of the invention. It is to be understood however, that these same principles apply to an implantable neural stimulator (INS) having any number of channels, e.g., two to sixteen channels or more. Most multichannel nuerostimulators have at least four channels, and may have as many as thirty-two channels. 
     Each channel of the INS  21 ′ is connected through suitable conductors carried in lead  56  to respective electrodes E 1 , E 2  or E 3  near the distal end of the lead  56 . A functional block diagram that illustrates the circuits contained within the INS  21 ′ is shown in  FIG. 3B . As seen in  FIG. 3B , each electrode E 1 , E 2 , E 3  is connected to a respective current source amplifier A 1 , A 2 , or A 3 . These current source amplifiers A 1 , A 2  and A 3  are controlled, in turn, by a control processor  60 . A reference electrode  58 , e.g., an electrode located on the case of the INS  21 ′, is also connected to the control processor  60 . 
     The control processor  60  has suitable memory circuitry  64  connected thereto. Input/output (I/O) circuitry  62  also is coupled to the control processor  60 . Such I/O circuitry  62  allows control parameters, programs, or data to be programmed into the processor  60 , or into the memory  64  for use by the processor  60 , so that a desired stimulation function may be carried out by the INS  21 ′. The I/O circuitry  62  may take many forms, as is known in the art, in order to form a transcutaneous link through which data may be sent to, or received from, the INS  21 ′ from a location external to the INS  21 ′. Examples of such transcutaneous links include radio frequency (RF) links, inductive links, magnetic links, optical links, or direct wire links. 
     The INS  21 ′ further includes a suitable power source  66  that provides operating power to all of its circuits, as needed. Such power source  66  may take many forms, as is known in the art, such as a primary battery, a rechargeable battery, a supercapacitor, or a power supply driven continuously by an external RF power source. 
     In operation, a suitable electrical stimulus is generated by one or more of the amplifiers A 1 , A 2 , or A 3  as controlled by the processor  60 . In  FIG. 3B , a biphasic stimulus pulse  67  is shown as being present at electrode E 2 . A return path for such stimulus pulse  67  may be provided through electrode E 1  or E 3 , or through the reference electrode  58 . 
     Turning next to  FIG. 4 , the type of channel interaction that is minimized by the present invention is illustrated.  FIG. 4  assumes a three channel system, C 1 , C 2 , C 3 . In order to determine the channel interaction, a stimulus of a known intensity or magnitude is applied to one channel, and the interaction appearing at the other channels as a result of the application of the stimulus is measured on the other channels. 
     For example, assume a stimulus of intensity level  3  is applied to the first channel C 1 . As this stimulus is applied to C 1 , the magnitude of any pulses appearing on channels C 2  or C 3  is measured. As represented in the upper left corner of  FIG. 4 , a stimulus applied to C 1  of intensity level  3  causes a corresponding stimulus of intensity level  2  to appear on non-stimulated channel C 2 , and another stimulus of intensity level  1  to appear on non-stimulated channel C 3 . Thus, as seen in the example of  FIG. 4 , a C 2  stimulus equal to ⅔ of the applied stimulus C 1  appears on non-stimulated channel  2  when a stimulus is applied to C 1 , and a C 3  stimulus equal to ⅓ of the applied stimulus C 1  appears on non-stimulated channel  3  when a stimulus is applied to C 1 . The stimulus pulses that appear on the non-stimulated channels in response to a stimulus applied to a stimulated channel represent the type of channel interaction that the present invention seeks to minimize. 
     Similarly, as shown in the example of  FIG. 4 , a stimulus applied to C 2  of intensity level  3  causes a corresponding stimulus of intensity level  2  to appear on non-stimulated channel C 1 , and another stimulus of intensity level  2  to appear on non-stimulated channel C 3 . Thus, a C 1  stimulus equal to ⅔ of the applied stimulus C 2  appears on non-stimulated channel  1  when a stimulus is applied to C 2 , and a C 3  stimulus equal to ⅔ of the applied stimulus C 2  appears on non-stimulated channel  3  when a stimulus is applied to C 2 . 
     Likewise, a stimulus applied to C 3  of intensity level  3  causes a corresponding stimulus of intensity level  1  to appear on non-stimulated channel C 1 , and another stimulus of intensity level  2  to appear on non-stimulated channel C 2 . That is to say, a C 1  stimulus equal to ⅓ of the applied stimulus C 3  appears on non-stimulated channel  1  when a stimulus is applied to C 3 , and a C 2  stimulus equal to ⅔ of the applied stimulus C 3  appears on non-stimulated channel  2  when a stimulus is applied to C 3 . 
     The above channel interactions can be summarized in a channel interaction matrix, or CIM, as depicted on the right side of  FIG. 4 . The first row of the CIM contains the channel interactions resulting from application of a stimulus to C 1 , the second row contains the channel interactions resulting from application of a stimulus to C 2 , and the third row contains the channel interactions resulting from application of a stimulus to C 3 , where the first column of the matrix corresponds to C 1 , the second column to C 2 , and the third column to C 3 . 
     It is the primary purpose of the present invention to minimize the channel interactions occurring on the non-stimulated channels. This is done by first stimulating each channel and collecting data from all the non-stimulated electrodes simultaneous with the applied stimulation. This data is used to build a matrix of the impedances between the stimulated electrode and the non-stimulated electrodes. This matrix of impedances comprises the channel interaction matrix, described above. 
     For example, consider an eight channel system. The channel interaction matrix, or CIM, is created by measuring the impedance between E 1  and each of the other seven electrodes. Such impedance is preferably measured by applying a stimulus of known current I 1  to electrode E 1  and by measuring the resulting voltage that appears at E 2 , E 3 , E 4  . . . E 8 . The impedance Z is simply Z=V/I, Thus, the impedance at E 2  resulting from application of current I 1  to E 1 , Z 1 , 2 , is simply
 
 Z   1 , 2 = V   2 / I   1 
 
and, in general, the impedance at any electrode Ej (j=1−8) resulting from application of a current of known magnitude to electrode Ei (l=1−8) may be expressed as
 
 Zi,j=Vj/Ii.  
 
     The measured impedances may be used to create an impedance matrix, or channel interaction matrix, CIM, as follows: 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 Z1,1 
                 Z1,2 
                 Z1,3 
                 Z1,4 
                 Z1,5 
                 Z1,6 
                 Z1,7 
                 Z1,8 
               
               
                 Z2,1 
                 Z2,2 
                 Z2,3 
                 Z2,4 
                 Z2,5 
                 Z2,6 
                 Z2,7 
                 Z2,8 
               
               
                 Z3,1 
                 Z3,2 
                 Z3,3 
                 Z3,4 
                 Z3,5 
                 Z3,6 
                 Z3,7 
                 Z3,8 
               
               
                 Z4,1 
                 Z4,2 
                 Z4,3 
                 Z4,4 
                 Z4,5 
                 Z4,6 
                 Z4,7 
                 Z4,8 
               
               
                 Z5,1 
                 Z5,2 
                 Z5,3 
                 Z5,4 
                 Z5,5 
                 Z5,6 
                 Z5,7 
                 Z5,8 
               
               
                 Z6,1 
                 Z6,2 
                 Z6,3 
                 Z6,4 
                 Z6,5 
                 Z6,6 
                 Z6,7 
                 Z6,8 
               
               
                 Z7,1 
                 Z7,2 
                 Z7,3 
                 Z7,4 
                 Z7,5 
                 Z7,6 
                 Z7,7 
                 Z7,8 
               
               
                 Z8,1 
                 Z8,2 
                 Z8,3 
                 Z8,4 
                 Z8,5 
                 Z8,6 
                 Z8,7 
                 Z8,8 
               
               
                   
               
             
          
         
       
     
     Once the impedance matrix, or channel interaction matrix (CIM) has been created, the channel interaction of the system may be modeled as shown in  FIG. 5A . (Here, it should be emphasized, that while only eight channels are shown as being present in the CIM, such number is only exemplary. The principles taught herein apply to any number of channels in a multichannel neural stimulation system.) 
     In  FIG. 5A , the desired stimulation current, l 1 , l 2 , l 3 , . . . l 8 , represents the current that the processor, or equivalent circuit, of the neural stimulation system has determined should be applied through the respective channel to the corresponding electrode E 1 , E 2 , . . . E 8 . However, due to channel interaction, the desired current may not be applied through the corresponding electrode because channel interaction may make the current more of less than the desired amount. 
     In order to assure that the correct current is applied through the corresponding electrode as determined by the processor, or equivalent circuitry, of the implantable neural stimulator, the present invention determines the voltage [V] that should appear on each electrode if the desired stimulation current [I D ] were applied to the electrode. This is done by taking the diagonal terms of the CIM matrix, [CIM D ], and finding the voltage [V] as follows:
 
[ V ]=[ CIM   D ]×[ I   D ]
 
This process is also illustrated in  FIG. 5B  for an eight channel system.
 
     Once the voltage [V] that should result when the correct stimulation currents are applied has been determined, as depicted above and in  FIG. 5B , then the required stimulation current [I R ] that will produce that voltage in the presence of the measured channel interaction is determined as follows:
 
[ I   R ]=[ CIM ] −1 ×[ V ].
 
This process is illustrated in  FIG. 5C  for an eight channel system.
 
     Advantageously, the channel interaction matrix [CIM] and its inverse [CIM] −1  need only be determined once, e.g., during a fitting session. Once determined, such matrices can be saved and recalled as needed as different required stimulus currents [I R ] are produced in order to respond to the demands of the implantable neural stimulator (INS) to produce a set of desired stimulation currents [I D ] in order to carry out the desired function of the INS. 
     Further, as should be evident from the above steps, the steps may be combined in a way that eliminates the need to determine the voltage matrix [V] that should appear when the desired currents are applied through the various channels. That is, the steps represented above and in  FIGS. 5B and 5C  may be combined as:
 
[ I   R ]=[ CIM ] −1 ×[ V ] where
 
[ V ]=[ CIM   D ]×[ I   D ]
 
Substituting the value of [V] into the first equation from the second equation yields:
 
[ I   R ]=[ CIM ] −1 ×{[ CIM   D ]×[ I   D ]}.
 
     It is thus seen that the present invention provides a method for minimizing channel interaction in a multichannel neural stimulator system, where the multichannel neural stimulation system has a multiplicity of electrodes adapted to interface with body tissue or nerves, and where means are provided for simultaneously applying a stimulus waveform of a desired intensity to selected ones of the multiplicity of electrodes. Such method comprises the steps of: (a) applying a stimulus waveform of a known intensity to one of the multiplicity of electrodes; and (b) measuring an interaction waveform appearing at any of the other of the multiplicity of electrodes as a result of the stimulus waveform applied in step (a); (c) repeating steps (a) and (b) for each of the multiplicity of electrodes; (d) creating a channel interaction matrix, [CIM], that defines the interaction waveforms measured in step (b) as a result of the stimulus waveforms, applied in step (a); (e) determining the inverse of the channel interaction matrix, [CIM] −1 , created in step (d); and thereafter (f) using the inverse of the channel interaction matrix, [CIM] −1 , and a desired stimulation waveform matrix, [I D ], to determine the actual stimulation waveforms [I R ] that must be applied in order to produce, after channel interaction, the desired stimulation waveforms. 
     Turning next to  FIG. 6A , a flow chart is shown that summarizes the process used by the invention to minimize channel interaction. As a first portion of the process, the channel interaction matrix, or CIM (also referred to as the impedance matrix) must be created. This is done in steps (or blocks)  70 - 76  of  FIG. 6A . In a first step, a stimulation pulse, STIM, is applied to one of the electrodes Ei (block  70 ). At the same time that the STIM pulse is applied to electrode Ei, the effective interaction pulse that appears on the electrodes other than Ei is measured (block  72 ). This process is continued so as to build the CIM (block  74 ), using the superposition principle as required. Once the CIM has been created, it is stored (block  76 ). Then, the inverse of the CIM is computed and stored (block  78 ). Advantageously, the steps required to build the CIM, and to compute its inverse, need only be performed once, e.g., during a fitting session. 
     Once the CIM has been created and stored, and its inverse has been determined and stored, such matrices may be used to minimize channel interaction as the implantable neural stimulation (INS) carries out its intended function. That is, the INS determines the desired stimulation pulses, STIM, that should be applied to each of the electrodes Ej in accordance with its intended or programmed function (block  80 ). The processing circuits within the INS then use the inverse of the CIM to determine the required STIM that needs to be applied to each electrode Ej to produce the desired STIM (block  82 ). Then the required STIM pulses are applied to the electrodes Ej through the appropriate channels of the INS (block  83 ). 
     A preferred manner for carrying out the steps indicated at blocks  80 ,  82  and  83  in  FIG. 6A  is depicted in more detail in  FIG. 6B . This involves determining the desired stimulation currents to be applied to the electrode Ej (block  80   a ), and then using the diagonal terms of the CIM to determine the voltage resulting from the desired stimulation currents (block  82   a ). That is,
 
[ V ]=[ CIM   D ]×[ I   D ]
 
where [V] represents the voltage appearing on the electrodes, [CIM D ] represents the diagonal terms of the channel interaction matrix, and [I D ] represents the desired stimulation currents.
 
     Once the voltage values [V] have been determined, then the required stimulation currents [I R ] needed to produce those voltages are determined using the inverse of the channel interaction matrix [CIM] −1  and the voltage values [V] (block  82   b ). Or, as previously indicated,
 
[ I   R ]=[ CIM ] −1 ×[ V ].
 
     Moreover, as previously indicated, the steps indicated at blocks  82   a  and  82   b  may be combined. That is, the required stimulation currents [I R ] may be determined as follows:
 
[ I   R ]=[ CIM ] −1 ×{[ CIM   D ]×[ I   D ]}.
 
     Once the required stimulation currents have been determined (at block  82   b ), then those required stimulation currents are applied to the tissue (block  83   a ). Once applied, a determination is made as to whether the INS is to continue its operation, i.e,. whether more stimulation currents need to be applied (block  83   b ). If so (YES branch of block  83   b ), then the process repeats by completing the steps set forth at blocks  82   a ,  82   b  and  83   a . If not, e.g., if the INS is to be shut down or placed in a sleep mode, or otherwise turned OFF, then the process ends (NO branch of block  83   b ). 
     Using the invention to minimize channel interaction has proven very effective. For example, as illustrated in  FIG. 7 , significant interelectrode interaction is minimized through use of the invention.  FIG. 7  shows the voltages appearing on electrodes E 2  through E 15  of a sixteen-electrode neural stimulation system when a current stimulus is applied to electrode E 1 . The stimulation is carried out in a monopolar fashion relative to a reference electrode. As can be seen in  FIG. 7 , without using the invention (“un-adjusted” data points in  FIG. 7 ), there is significant interaction between the 16 electrodes, with the voltage that appears on electrodes E 2 –E 15  being only about an order of magnitude less (1.00E-01) than the stimulation value applied to electrode E 1 . However, when the invention is used (“nulled” data points in  FIG. 7 ), the interaction between the 16 electrodes is effectively eliminated. 
     It should be appreciated that while a preferred INS configuration for practicing the invention involves applying current stimulation pulses to the electrodes, the invention may also be practiced when voltage stimulation pulses are applied to the electrodes. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.