Patent Publication Number: US-8121698-B2

Title: Outer hair cell stimulation model for the use by an intra-cochlear implant

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/765,395, filed Jun. 19, 2007, which was in turn a divisional of U.S. patent application Ser. No. 11/003,155, filed Dec. 3, 2004 (now U.S. Pat. No. 7,242,985). Priority is claimed to both of these applications and both and incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to implantable neurostimulator devices and systems, for example, cochlear stimulation systems, and to sound processing strategies employed in conjunction with such systems. 
     BACKGROUND 
     Prior to the past several decades, scientists generally believed that it was impossible to restore hearing to the profoundly deaf. However, scientists have had increasing success in restoring normal hearing to the deaf through electrical stimulation of the auditory nerve. The initial attempts to restore hearing were not very successful, as patients were unable to understand speech. However, as scientists developed different techniques for delivering electrical stimuli to the auditory nerve, the auditory sensations elicited by electrical stimulation gradually came closer to sounding more like normal speech. The electrical stimulation is implemented through a prosthetic device, known as a cochlear implant, which is implanted in the inner ear to restore partial hearing to profoundly deaf patients. 
     Such cochlear implants generally employ an electrode array that is inserted into the cochlear duct. One or more electrodes of the array selectively stimulate different auditory nerves at different places in the cochlea based on the pitch of a received sound signal. Within the cochlea, there are two main cues that convey “pitch” (frequency) information to the patient. There are (1) the place or location of stimulation along the length of a cochlear duct and (2) the temporal structure of the stimulating waveform. In the cochlea, sound frequencies are mapped to a “place” in the cochlea, generally from low to high sound frequencies mapped from the apical to basilar direction. The electrode array is fitted to the patient to arrive at a mapping scheme such that electrodes near the base of the cochlea are stimulated with high frequency signals, while electrodes near the apex are stimulated with low frequency signals. 
     Accordingly, the present inventors recognized the need to account for the interaction between frequency bands and enhance the contrast between neighboring signals. 
       FIG. 1  presents a cochlear stimulation system  10  that includes a sound processor portion  12  and a cochlear stimulation portion  20 . The sound processor portion  12  includes a microphone  14  and a sound processor  18 . The microphone  14  can be connected directly to the sound processor  18 . Alternatively, the microphone  14  can be coupled to the sound processor  18  through an appropriate communication link  16 . The cochlear stimulation portion  20  includes an implantable cochlear stimulator  22  and an electrode array  24 . The electrode array  24  is adapted to be inserted within the cochlea of a patient. The electrode array  24  includes a plurality of electrodes (not shown) that are distributed along the length of the array and are selectively connected to the implantable cochlear stimulator  22 . 
     The electrode array  24  may be substantially as shown and described in U.S. Pat. Nos. 4,819,647 or 6,129,753, both patents incorporated herein by reference. Electronic circuitry within the implantable cochlear stimulator  22  allows a specified stimulation current to be applied to selected pairs or groups of the electrodes (not shown) included within the electrode array  24  in accordance with a specified stimulation pattern defined by the sound processor  18 . 
     The sound processor  18  and the implantable cochlear stimulator  22  are electronically coupled through a suitable communication link  26 . In an implementation, the microphone  14  and the sound processor  18  comprise an external portion of the cochlear stimulation system  10 , and the implantable cochlear stimulator  22  and the electrode array  24  comprise an internal, or implanted, portion of the cochlear stimulation system  10 . Thus, the communication link  26  is a transcutaneous (through the skin) link that allows power and control signals to be sent from the sound processor  18  to the implantable cochlear stimulator  22 . 
     In another implementation, the implantable cochlear stimulator  22  can send information, such as data and status signals, to the sound processor  18  over the communication link  26 . In order to facilitate bidirectional communication between the sound processor  18  and the implantable cochlear stimulator  22 , the communication link  26  can include more than one channel. Additionally, interference can be reduced by transmitting information on a first channel using an amplitude-modulated carrier and transmitting information on a second channel using a frequency-modulated carrier. 
     In an implementation in which the implantable cochlear stimulator  22  and the electrode array  24  are implanted within the patient, and the microphone  14  and the sound processor  18  are carried externally (not implanted) by the patient, the communication link  26  can be realized though use of an antenna coil in the implantable cochlear stimulator  22  and an external antenna coil coupled to the sound processor  18 . The external antenna coil can be positioned so that it is aligned with the implantable cochlear stimulator  22 , allowing the coils to be inductively coupled to each other and thereby permitting power and information, e.g., a stimulation signal, to be transmitted from the sound processor  18  to the implantable cochlear stimulator  22 . In another implementation, the sound processor  18  and the implantable cochlear stimulator  22  can both be implanted within the patient, and the communication link  26  can be a direct-wired connection or other suitable link as shown in U.S. Pat. No. 6,308,101, incorporated herein by reference. 
     In the cochlear stimulation system  10 , the microphone  14  senses acoustic signals and converts the sensed acoustic signals to corresponding electrical signals. The electrical signals are sent to the sound processor  18  over an appropriate communication link  16 , such as a circuit or bus. The sound processor  18  processes the electrical signals in accordance with a sound processing strategy and generates control signals used to control the implantable cochlear stimulator  22 . Such control signals can specify or define the polarity, magnitude, location (which electrode pair or group is intended to receive the stimulation current), and timing (when the stimulation current is to be applied to the intended electrode pair or group) of the stimulation signal, such as a stimulation current, that is generated by the implantable cochlear stimulator  22 . 
     It is common in the cochlear stimulator art to condition the magnitude and polarity of the stimulation current applied to the implanted electrodes of the electrode array  24  in accordance with a specified sound processing strategy. A sound processing strategy involves defining a pattern of stimulation waveforms that are applied as controlled electrical currents to the electrodes of an electrode array  24  implanted in a patient. Stimulation strategies can be implemented by modulating the amplitude of the stimulation signal or by modulating the frequency of the stimulation signal. 
     SUMMARY 
     The methods and apparatus described here implement techniques for clarifying sound as perceived through a cochlear implant. More specifically, the methods and apparatus described here implement techniques for using the outer hair cell model to enhance contrasts between stimulation signals as perceived through a cochlear implant. 
     In general, in one aspect, the techniques can be implemented to include dividing an audio signal into a plurality of input signals, wherein each input signal is associated with a frequency band; generating a plurality of envelope signals, including at least a first and a second envelope signal, by determining an envelope of each of at least two input signals, each input signal being associated with a corresponding frequency band; scaling at least one of the envelope signals in accordance with a scaling factor to generate at least one scaled envelope signal; and combining at least one envelope signal with at least one scaled envelope signal to generate an output signal. 
     The techniques also can be implemented to include multiplying the first envelope signal by a first weighting factor and multiplying the second envelope signal by a second weighting factor. The techniques can further be implemented to include determining a separation between a first frequency band and a second frequency band, and selecting a scaling factor based on the separation. Additionally, the techniques can be implemented to include scaling a plurality of envelope signals associated with frequency bands that neighbor a first frequency band to generate a plurality of scaled envelope signals, and combining the envelope signal associated with the first frequency band with the plurality of scaled envelope signals to generate an output signal associated with the first frequency band. 
     The techniques also can be implemented to include rectifying an input signal prior to determining the envelope of the input signal. Further, the techniques can be implemented to include full-wave rectifying the input signal. Additionally, the techniques can be implemented to include setting an average amplitude associated with an input signal to zero at the beginning of a frame and determining the average amplitude associated with the input signal for the frame. The techniques also can be implemented such that the generated output signal comprises an acoustic signal. Further, the techniques can be implemented to include mapping the generated output signal to an electrical signal and applying the electrical signal to one or more electrode pairs of a cochlear implant. Additionally, the techniques can be implemented such that the generated output signal is associated with a first frequency band. 
     The techniques also can be implemented such that combining further comprises generating an output signal in accordance with a frequency modulated stimulation strategy, such as that described in U.S. patent application Ser. No. 10/917,789, incorporated herein by reference. Further, the techniques can be implemented to include subtracting the at least one scaled envelope signal from the at least one envelope signal. Additionally, the techniques can be implemented such that scaling at least one of the envelope signals reduces the magnitude of the envelope signal. The techniques also can be implemented such that each of the plurality of envelope signals represents an average amplitude of a corresponding input signal. Further, the techniques can be implemented such that scaling in accordance with a scaling factor comprises using a scaling factor which ranges from 0 to 1. 
     In general, in another aspect, the techniques can be implemented to include a plurality of filters configured to divide an audio signal into a plurality of input signals, wherein each input signal is associated with a frequency band; a plurality of envelope detectors configured to generate a plurality of envelope signals, including at least a first and a second envelope signal, by determining an envelope of each of at least two input signals, each input signal being associated with a corresponding frequency band; and circuitry configured to scale at least one of the envelope signals in accordance with a scaling factor to generate at least one scaled envelope signal and to combine at least one envelope signal with at least one scaled envelope signal to generate an output signal. 
     The techniques also can be implemented to include circuitry configured to multiply the first envelope signal by a first weighting factor and multiply the second envelope signal by a second weighting factor. The techniques also can be implemented to include circuitry configured to determine a separation between a first frequency band and a second frequency band, and select a scaling factor based on the separation. Additionally, the techniques can be implemented to include circuitry configured to scale a plurality of envelope signals associated with frequency bands that neighbor a first frequency band to generate a plurality of scaled envelope signals and combine the envelope signal associated with the first frequency band with the plurality of scaled envelope signals to generate an output signal associated with the first frequency band. 
     The techniques also can be implemented to include a rectifier configured to rectify an input signal prior to the envelope detector determining the envelope of the input signal. Further, the techniques can be implemented such that the rectifier comprises a full-wave rectifier. Additionally, the techniques can be implemented such that the envelope detector is configured to set an average amplitude associated with an input signal to zero at the beginning of a frame and determine the average amplitude associated with the input signal for the frame. The techniques can also be implemented such that the generated output signal comprises an acoustic signal. 
     The techniques also can be implemented to include circuitry configured to map the generated output signal to an electrical signal and apply the electrical signal to one or more electrode pairs of a cochlear implant. Further, the techniques can be implemented such that the generated output signal is associated with a first frequency band. Additionally, the techniques can be implemented to include circuitry configured to generate the output signal in accordance with a frequency modulated stimulation strategy. 
     The techniques also can be implemented to include circuitry configured to subtract the at least one scaled envelope signal from the at least one envelope signal. Further, the techniques can be implemented such that scaling at least one of the envelope signals reduces the magnitude of the envelope signal. Additionally, the techniques can be implemented such that each of the plurality of envelope detectors is configured to generate an envelope signal representing an average amplitude of a corresponding input signal. Further, the techniques can be implemented such that the circuitry comprises one or more of a programmable logic device, a field programmable gate array, an application-specific integrated circuit, and a general purpose processor executing programmed instructions. The techniques can also be implemented such that the scaling factor ranges from 0 to 1. 
     In general, in another aspect, the techniques can be implemented to include dividing an audio signal into at least a first input signal and a second input signal, wherein each input signal is associated with a frequency band; determining an effect of the second input signal on the first input signal; and subtracting the effect of the second input signal from the first input signal to generate an output signal. 
     The techniques described in this specification can be implemented to realize one or more of the following advantages. For example, the techniques can be implemented to enhance the contrast between neighboring stimulation signals of a sound processing strategy and thus improve sound clarity and speech recognition, especially under difficult listening conditions. The techniques also can be implemented to decrease the power consumption of a cochlear implant system implementing a sound processing strategy. Further, the techniques can be implemented to reduce interaction between neighboring electrodes and the resulting influence on corresponding neurons by decreasing the stimulation level on one or more electrodes as a result of the stimulation level present on one or more neighboring electrodes. 
     These general and specific aspects can be implemented using an apparatus, a method, a system, or any combination of an apparatus, methods, and systems. The details of one or more implementations are set forth in the accompanying drawings and the description below. Further features, aspects, and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a cochlear stimulation system. 
         FIGS. 2-3  show a functional block diagram of a sound processing system. 
         FIG. 4  presents exemplary frequency maps that can be used in conjunction with a sound processing strategy. 
         FIG. 5  is a functional block diagram of a lateral suppression network of a sound processing system. 
         FIG. 6  presents results comparing a Continuous Interleaved Sampler (CIS) strategy used in conjunction with an outer hair cell model and a CIS strategy used without an outer hair cell model. 
         FIG. 7  is a flowchart of a method of stimulating a cochlea. 
     
    
    
     Like reference symbols indicate like elements throughout the specification and drawings. 
     DETAILED DESCRIPTION 
       FIG. 2  presents a functional block diagram of a conventional system arranged to implement a sound processing strategy. In the conventional sound processing system  100 , an audio signal  102  is provided as an input to a bank of bandpass filters  104 , which separates the audio signal  102  into individual frequency bands or channels. For example, if the audio signal  102  is provided to a bank of K bandpass filters, then the audio signal  102  is separated into K individual frequency bands. In another implementation, different types and combinations of filters can be employed to separate the audio signal  102  into individual frequency bands, such as notch filters, high-pass filters, and low-pass filters. 
     As the audio signal  102  is provided to the bank of bandpass filters  104 , individual bandpass filters output filtered signals. For example, the bank of bandpass filters  104  includes a bandpass filter  106  corresponding to a first frequency band, a bandpass filter  108  corresponding to a second frequency band, and a bandpass filter  110  corresponding to a k th  frequency band. As the audio signal  102  is provided to the bank of bandpass filters  104 , the bandpass filter  106  corresponding to the first frequency band outputs a filtered signal  112  associated with the first frequency band, the bandpass filter  108  corresponding to the second frequency band outputs a filtered signal  113  associated with the second frequency band, and the bandpass filter  110  corresponding to the k th  frequency band outputs a filtered signal  114  associated with the k th  frequency band. Thus, each filtered signal is associated with a frequency band that represents a subset of the audio signal  102 . 
     The bank of envelope detectors  116  includes an envelope detector  118  corresponding to the first frequency band, an envelope detector  120  corresponding to the second frequency band, and an envelope detector  122  corresponding to the k th  frequency band. The envelope detectors of the bank of envelope detectors  116  receive as input filtered signals output from the corresponding bandpass filters in the bank of bandpass filters  104 . For example, the envelope detector  118  corresponding to the first frequency band receives as input the filtered signal  112  associated with the first frequency band from the bandpass filter  106  corresponding to the first frequency band. 
     Each envelope detector of the bank of envelope detectors  116  is configured to determine an envelope associated with a received filtered signal and to output a representative envelope signal. For example, the filtered signal  112  associated with the first frequency band is input to the envelope detector  118  corresponding to the first frequency band, which determines the envelope of the filtered signal  112  and outputs an envelope signal E 1    124  associated with the first frequency band. Similarly, the filtered signal  113  associated with the second frequency band is input to the envelope detector  120  corresponding to the second frequency band, which determines the envelope of the filtered signal  113  and outputs an envelope signal E 2    134  associated with the second frequency band. Additionally, the filtered signal  114  associated with the k th  frequency band is input to the envelope detector  122  corresponding to the k th  frequency band, which determines the envelope of the filtered signal  114  and outputs an envelope signal E k    144  associated with the k th  frequency band. 
     The envelope signals output from the bank of envelope detectors  116  are converted to electrical signals using acoustic-to-electrical mappings. Each of the resulting electrical signals is then applied to electrodes of a cochlear implant to provide a stimulation signal. For example, the envelope signal E 1    124  output from the envelope detector  118  corresponding to the first frequency band is converted from an acoustic signal to an electrical signal using the acoustic-to-electrical mapping  132  associated with the first frequency band. Similarly, the envelope signals E 2    134  and E k    144  are converted to electrical signals using the acoustic-to-electrical mappings  142  and  152  associated with the second frequency band and the k th  frequency band respectively. 
     As described above, sound processing strategies do not account for the interaction of signals associated with neighboring frequency bands. A signal corresponding to a particular frequency band is either provided to an electrode array as a stimulation signal at full strength or the signal is completely suppressed. For example, the N of M algorithm simply determines the amplitude of the signals on each of M frequency bands and selects the N signals with the highest amplitudes to provide as stimulation signals. The remaining M-N signals are completely suppressed. 
     Further, sound processing strategies used to generate stimulation signals often incorporate the assumption that each frequency band, or channel, is represented independently in the cochlea. This assumption can result in poor sound quality and decreased comprehension of speech under difficult listening conditions, such as listening in a noisy environment. One factor believed to contribute to the poor performance is the interaction that occurs between frequency bands in cochlear implant subjects. Such frequency band interaction can smear or distort peaks in the stimulation signal that are essential to the identification of sounds. 
     Similar to  FIG. 2 ,  FIG. 3  presents a functional block diagram of a system arranged to implement a sound processing strategy. Such sound processing strategy can be implemented using any combination of circuitry and programmed instructions, including one or more of a programmable logic device, a field-programmable gate array, an application-specific integrated circuit, and a general purpose processor executing programmed instructions. 
     In the system  300 , an audio signal  102  is also provided to a bank of bandpass filters  104 , which separates the audio signal  102  into a plurality of frequency bands or channels. The bank of bandpass filters  104  can be configured to separate the audio signal  102  into frequency bands that correspond to frequencies defined in a frequency map associated with a specific sound processing strategy.  FIG. 4  presents an example of a frequency map associated with the Simultaneous Analog Stimulation (SAS) strategy  410  and a frequency map associated with the Continuous Interleaved Sampler (CIS) strategy  420 . Other sound processing strategies, such as the Frequency Modulated Stimulation (FMS) strategy, can be implemented using either of the frequency maps presented in  FIG. 4 , or by using alternative frequency maps. 
     In another implementation, the audio signal  102  can undergo other processing before being provided as input to the bank of bandpass filters  104 . For example, the audio signal  102  may originate as acoustic information sensed by a microphone, which is then converted into an electrical signal representing an audio signal. The electrical signal can further be converted to a digital signal in an analog-to-digital converter, and then subjected to automatic gain control (AGC) processing using an AGC algorithm. The AGC algorithm serves to compress the dynamic range of the audio signals to provide a more consistent level of stimulus to the electrodes and to equalize the level between sound sources that are removed from the listener by differing distances. 
     As discussed above, the bank of envelope detectors  116  includes an envelope detector  118  corresponding to the first frequency band, an envelope detector  120  corresponding to the second frequency band, and an envelope detector  122  corresponding to the k th  frequency band. The envelope detectors of the bank of envelope detectors  116  receive as input filtered signals output from the corresponding bandpass filters in the bank of bandpass filters. For example, the envelope detector  118  corresponding to the first frequency band receives as input the filtered signal  112  associated with the first frequency band from the bandpass filter  106  corresponding to the first frequency band. Each of the envelope detectors of the bank of envelope detectors  116  can include a rectifier, such as a half-wave rectifier or a full-wave rectifier, that rectifies the filtered signal output from the corresponding bandpass filter of the bank of bandpass filters  104  before the envelope of the filtered signal is determined. 
     In an implementation, the envelope detectors included in the bank of envelope detectors  116  can comprise integrators that determine an average amplitude of a signal for a given interval. For example, upon receiving the filtered signal  112  associated with the first frequency band, the envelope detector  118  corresponding to the first frequency band determines an envelope of the filtered signal  112  for an interval, such as a frame. At the end of the interval, the envelope detector  118  corresponding to the first frequency band outputs the envelope signal E 1    124 , which represents the average amplitude of the filtered signal  112  associated with the first frequency band for that interval. Each envelope detector of the bank of envelope detectors  116  also can be configured to set the average amplitude value of a received filtered signal to an initial state prior to or at the start of a new interval. 
     The bank of envelope detectors outputs envelope signals representing acoustic signal values. However, unlike the prior art describe with reference to  FIG. 2 , the envelope signals are not converted to electrical signals using an acoustic-to-electrical mapping. Instead, the envelope signals are transferred to a lateral suppression network  154 , which accounts for the interaction between envelope signals of neighboring frequency bands through the use of a lateral suppression model, such as the outer hair cell model, and outputs suppressed signals. A lateral suppression model, such as the outer hair cell model, can be used in conjunction with either a frequency modulated sound processing strategy, such as FMS, or an amplitude modulated stimulation strategy, such as CIS. The lateral suppression network  154  is discussed in greater detail with reference to  FIG. 5 . 
     Lateral suppression is the term used to describe the psychophysical effect by which the loudness perceived from one tone is diminished to some extent by the presence of a neighboring tone. The suppressive effect is particularly evident when a loud tone closely neighbors a quieter tone. Thus, lateral suppression operates to enhance the contrast between tones. However, the lateral suppression algorithm must be implemented such that it does not generate abnormal results. If a flat spectrum is input to the lateral suppression network  154 , a flat spectrum should also be output from the lateral suppression network  154 . Further, the lateral suppression network must account for the frequency bands representing the highest and lowest frequencies of the audio signal  102 , the edge frequency bands. In the system  300 , for example, the first frequency band and the k th  frequency band are the edge frequency bands. Because only one frequency band is immediately adjacent to each edge frequency band, each edge frequency band would be subjected to less suppression without additional compensation. Therefore, the lateral suppression network  154  must compensate by adjusting one or more factors, such as the weighting factor u associated with the edge frequency band or one or more of the scaling factors w employed by the lateral suppression processor associated with the edge frequency band. 
     The suppressed signals output from the lateral suppression network  154  are converted to electrical signals using the acoustic-to-electrical mapping associated with the corresponding frequency bands and provided as stimulation signals to one or more electrode pairs of a cochlear implant. For example, the envelope signals E 1    124 , E 2    134 , and E k    144  output from the bank of envelope detectors  116  are input into the lateral suppression network  154 . The lateral suppression network  154  then suppresses one or more of the envelope signals E 1    124 , E 2    134 , and E k    144  in accordance with envelope signals associated with neighboring frequency bands, including the envelope signals E 1    124 , E 2    134 , and E k    144 . The lateral suppression network  154  then outputs the corresponding suppressed signals S 1    130 , S 2    140 , and S k    150  respectively. 
     The suppressed signal S 1    130  associated with the first frequency band is then converted into an electrical signal using the acoustic-to-electrical mapping  132  corresponding to the first frequency band. Similarly, the suppressed signal S 2    140  associated with the second frequency band is then converted into an electrical signal using the acoustic-to-electrical mapping  142  corresponding to the second frequency band. Additionally, the suppressed signal S k    150  associated with the k th  frequency band is then converted into an electrical signal using the acoustic-to-electrical mapping  152  corresponding to the k th  frequency band. 
       FIG. 5  presents a functional block diagram detailing an implementation of a lateral suppression network  154  as it relates to the system  300  of  FIG. 3 . As described above, the envelope signals E 1    124 , E 2    134 , and E k    144  are provided as inputs to the lateral suppression network  154 . In the lateral suppression network  154 , each envelope signal can be combined with one or more scaled envelope signals to account for the influence that envelope signals associated with neighboring frequency bands have on a particular envelope signal. 
     One or more of the envelope signals output from the bank of envelope detectors  116  can be weighted by a factor u i  upon being provided to the lateral suppression network  154 , where i represents the frequency band with which the envelope signal is associated. Thus, an envelope signal that is determined to be of greater importance than the envelope signals associated with neighboring frequency bands can be emphasized, such as an envelope signal representing an amplitude that exceeds a particular threshold value. Further, an envelope signal determined to be of lesser importance can be deemphasized, such as an envelope signal representing an amplitude that falls below a particular threshold value. In an implementation, each of the envelope signals provided to the lateral suppression network  154  can be weighted, and the weight associated with envelope signals that should not be emphasized or deemphasized can be set to 1. 
     For example, the envelope signals E 1    124 , E 2    134 , and E k    144  output from the bank of envelope detectors  116  are provided as inputs to the lateral suppression network  154 . The lateral suppression processor  128  corresponding to the first frequency band multiplies the envelope signal E 1    124  by a weighting factor u i    126  associated with the first frequency band. Similarly, the lateral suppression processor  138  corresponding to the second frequency band multiplies the envelope signal E 2    134  by a weighting factor u 2    136  associated with the second frequency band. The lateral suppression processor  148  corresponding to the k th  frequency band multiplies the envelope signal E k    144  by a weighting factor u k    146  associated with the k th  frequency band. As a result, the suppressive effect of signals associated with neighboring frequency bands will be diminished on envelope signals deemed to be of greater importance and increased on envelope signals deemed to be of lesser importance. 
     Because the influence that an envelope signal has on a neighboring envelope signal decreases as the number of frequency bands separating the envelope signals increases, the scaling factor applied to an envelope signal to generate a scaled envelope signal is selected as a function of the separation of between the neighboring frequency bands. Therefore, a scaling factor w ij  is chosen, where i represents the frequency band associated with the envelope signal being suppressed and j represents the frequency band associated with the envelope signal that is producing the suppressive effect. With each increase in the frequency band separation, the scaling factor w ij  will further decrease the magnitude of the envelope signal being scaled. Additionally, as scaled envelope signals suppress an envelope signal, the scaling factor represents a negative value. 
     A laterally suppressed signal S i  is generated by combining an envelope signal associated with a particular frequency band E i  with one or more scaled envelope signals w ij E j  associated with neighboring frequency bands. As discussed above, the envelope signal being suppressed also can be weighted using a weighting factor u i . The combining operation can be expressed mathematically as shown in Equation 1. 
     
       
         
           
             
               
                 
                   
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     Because nonlinearities are known to exist in the response of the basilar membrane, Equation 1 can be generalized as expressed in Equation 2, where F i (x)=X and w ii =0. However, this simplification is not required and S i  can be generated using a non-linear function in another implementation. 
     
       
         
           
             
               
                 
                   
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     In an implementation, the envelope signal E 1    124  associated with the first frequency band is provided to a corresponding lateral suppression processor  128 . The lateral suppression processor  128  then multiplies the envelope signal E 1    124  by the weighting factor u 1    126 . The lateral suppression processor  128  also receives as input the scaled envelope signal w 12 E 2    156 , which represents the interaction of the envelope signal E 2    134  associated with the second frequency band with the envelope signal E 1    124  associated with the first frequency band. Additionally, the lateral suppression processor  128  receives as input the scaled envelope signal w 1k E k    158 , which represents the interaction of the envelope signal E k    144  associated with the k th  frequency band with the envelope signal E 1    124  associated with the first frequency band. Further, the lateral suppression processor  128  can also receive as inputs the scaled envelope signals associated with any or all of the remaining third through K−1 th  frequency bands. 
     The lateral suppression processor  128  combines the envelope signal E 1    124 , weighted by u 1    126 , with at least the scaled envelope signals w 12 E 2    156  and w 1k E k    158 , and outputs a laterally suppressed signal S 1    130  associated with the first frequency band. The laterally suppressed signal S 1    130  can then be converted to an electrical stimulation signal using the acoustic-to-electrical mapping  132  corresponding to the first frequency band. 
     A similar lateral suppression operation can be carried out for any or all of the envelope signals associated with the remaining frequency bands. For example, the lateral suppression processor  138  receives the envelope signal E 2    134  associated with the second frequency band. The lateral suppression processor  138  then multiplies the envelope signal E 2    134  by the weighting factor u 2    136 . The lateral suppression processor  138  also receives as inputs the scaled envelope signals w 21 E 1    160  and w 2k E k    162 , which are associated with the first and k th  frequency bands respectively. Additionally, the lateral suppression processor  138  can receive as inputs the scaled envelope signals associated with any or all of the remaining frequency bands. The lateral suppression processor  138  combines the envelope signal E 2    134 , weighted by u 2    136 , with the scaled envelope signals w 21 E 1    160  and w 2k E k    162 , and outputs a laterally suppressed signal S 2    140  associated with the second frequency band. The laterally suppressed signal S 2    140  is then converted to an electrical stimulation signal using the acoustic-to-electrical mapping  142  associated with the second frequency band. 
     In an implementation, each lateral suppression processor of the lateral suppression network  154  can be configured to receive as inputs the scaled envelope signals associated with each of the neighboring frequency bands. Therefore, each of the envelope signals can be suppressed by scaled envelope signals associated with each of the neighboring frequency bands. If an envelope signal E b  should not be used to suppress an envelope signal E a , the scaling factor w ab  can be set to 0. 
       FIG. 6  presents a graphical comparison of the stimulation signals generated in response to varying input signals using the CIS strategy employed in conjunction with the outer hair cell model and the CIS strategy employed without the outer hair cell model. The spectrum of the stimulation signals generated in response to the first input signal  610  using CIS processing in conjunction with the outer hair cell model are indicated in the first bar graph  612  by light colored bars, such as the light bar  614  associated with channel  7 . The spectrum of the stimulation signals generated in response to the first input signal  610  using CIS processing without the outer hair cell model are indicated in the first bar graph  612  using dark colored bars, such as the dark bar  616  associated with channel  7 . 
     As the amplitude of the dark colored bars always equals or exceeds the amplitude of the light colored bars, the dark colored bars are depicted behind the light colored bars. Each of the dark colored bars and each of the light colored bars represents an amplitude of a stimulation signal corresponding to a particular channel. Where a dark colored bar is visible, such as the dark colored bar  616  associated with channel  7 , the amplitude of the stimulation signal generated using CIS processing without the outer hair cell model exceeds the amplitude of the stimulation signal generated using CIS processing in conjunction with the outer hair cell model. 
     As depicted in the first bar graph  612 , even though the spectrum of the first input signal  610  is relatively flat, CIS processing used in conjunction with the outer hair cell model produces stimulation signals characterized by greater channel-to-channel amplitude differences than the stimulation signals produced using CIS processing without the outer hair cell model. This is especially true for channels adjacent to signal peaks, such as the stimulation signal represented by the light bar  618  of channel  3 . Therefore, CIS processing performed in conjunction with the outer hair cell model provides enhanced stimulation signal contrast over CIS processing performed without the outer hair cell model. 
     The second bar graph  622  corresponding to the second input signal  620  indicates that as the peaks in the spectrum of the input signal become more pronounced, CIS processing in conjunction with the outer hair cell model produces stimulation signals characterized by even greater channel-to-channel amplitude differences than the stimulation signals produced using CIS processing without the outer hair cell model. As discussed above, this is especially true for channels adjacent to peaks in the spectrum, such as the stimulation signal represented by the light bar  624  of channel  3 . 
     The third bar graph  632  corresponding to the third input signal  630  provides further indication that, as the peaks of the spectrum of the input signal become very pronounced, CIS processing in conjunction with the outer hair cell model produces stimulation signals characterized by even greater channel-to-channel amplitude differences than the stimulation signals produced using CIS processing without the outer hair cell model. Again, this is especially true for channels adjacent to large peaks in the spectrum, such as the stimulation signal  634  of channel  3 . 
       FIG. 7  describes a method of stimulating a cochlea using a lateral suppression strategy, such as the outer hair cell model. In a first step  710 , as described above, an audio signal  102  is divided into a plurality of input signals using a bank of bandpass filters  104 . Each of the input signals generated from the audio signal  102  is associated with a frequency band. In a second step  720 , a plurality of envelope signals, including at least a first and second envelope signal, are generated by a bank of envelope detectors  106 . The individual envelope detectors included in the bank of envelope detectors  106  each determine an envelope of an input signal associated with a corresponding frequency band. Once the plurality of envelope signals has been generated, the third step  730  is to scale at least one envelope signal in accordance with a scaling factor to generate a scaled envelope signal. This step generates signals for use in the lateral suppression network  154 . In a fourth step  740 , at least one envelope signal is combined with at least one scaled envelope signal to generate an output signal. 
     A number of implementations have been disclosed herein. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the claims. Accordingly, other implementations are within the scope of the following claims.