Frequency Specific Stimulation Sequences

[0001] This application claims priority from U.S. Provisional Patent Application 62/212,642, filed September 1, 2015, and from U.S. Provisional Patent Application 62/212,643, filed September 1, 2015, both of which are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

[0002] The present invention relates to hearing implant systems, and more specifically, to techniques for producing electrical stimulation signals in such systems.

BACKGROUND ART

[0003] A normal ear transmits sounds as shown in Figure 1 through the outer ear 101 to the tympanic membrane 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for

approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.

[0004] Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, hearing prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.

[0005] Figure 1 also shows some components of a typical cochlear implant system, including an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110.

[0006] Typically, the electrode array 110 includes multiple electrode contacts 112 on its surface that provide selective stimulation of the cochlea 104. Depending on context, the electrode contacts 112 are also referred to as electrode channels. In cochlear implants today, a relatively small number of electrode channels are each associated with relatively broad frequency bands, with each electrode contact 112 addressing a group of neurons with an electric stimulation pulse having a charge that is derived from the instantaneous amplitude of the signal envelope within that frequency band.

[0007] In some coding strategies, stimulation pulses are applied at a constant rate across all electrode channels, whereas in other coding strategies, stimulation pulses are applied at a channel-specific rate. Various specific signal processing schemes can be implemented to produce the electrical stimulation signals. Signal processing approaches that are well- known in the field of cochlear implants include continuous interleaved sampling (CIS), channel specific sampling sequences (CSSS) (as described in U.S. Patent No. 6,348,070, incorporated herein by reference), spectral peak (SPEAK), and compressed analog (CA) processing.

[0008] Figure 2 shows the major functional blocks in a typical cochlear implant signal processing system wherein band pass signals are processed and coding to generate electrode stimulation signals to stimulation electrodes in an implanted cochlear implant electrode array. For example, commercially available Digital Signal Processors (DSP) can be used to perform speech processing according to a 12-channel CIS approach. The initial acoustic audio signal input is produced by one or more sensing microphones, which may be omnidirectional and/or directional. Preprocessor Filter Bank 201 pre-processes the initial acoustic audio signal with a bank of multiple band pass filters, each of which is associated with a specific band of audio frequencies— for example, a digital filter bank having 12 digital Butterworth band pass filters of 6th order, Infinite Impulse Response (IIR) type— so that the acoustic audio signal is filtered into some M band pass signals, B _\ to 5M where each signal corresponds to the band of frequencies for one of the band pass filters. Each output of the CIS band pass filters can roughly be regarded as a sinusoid at the center frequency of the band pass filter which is modulated by the envelope signal. This is due to the quality factor (Q « 3) of the filters. In case of a voiced speech segment, this envelope is approximately periodic, and the repetition rate is equal to the pitch frequency. Alternatively and without limitation, the Preprocessor Filter Bank 201 may be implemented based on use of a fast Fourier transform (FFT) or a short-time Fourier transform (STFT). Based on the tonotopic organization of the cochlea, each electrode contact in the scala tympani often is associated with a specific band pass filter of the external filter bank.

[0009] Figure 3 shows an example of a short time period of an audio speech signal from a microphone, and Figure 4 shows an acoustic microphone signal decomposed by bandpass filtering by a bank of filters into a set of signals. An example of pseudocode for an infinite impulse response (IIR) filter bank based on a direct form II transposed structure is given by Fontaine et al. , Brian Hears: Online Auditory Processing Using Vectorization Over Channels, Frontiers in Neuroinformatics, 2011 ; incorporated herein by reference in its entirety:

for j ~ 0 to number of channels - i do

les - 1. do · A _{: :} * Y (s)

. ,. _: ^■ _t .. x/s) .· . _5j . ·* v (s)

end lor

end for [0010] The band pass signals to 5M (which can also be thought of as frequency channels) are input to a Signal Processor 202 which extracts signal specific stimulation information— e.g., envelope information, phase information, timing of requested stimulation events, etc.— into a set of N stimulation channel signals Si to <¾ that represent electrode specific requested stimulation events. For example, channel specific sampling sequences (CSSS) may be used as described in U. S. Patent 6,594,525, which is incorporated herein by reference in its entirety. For example, the envelope extraction may be performed using 12 rectifiers and 12 digital Butterworth low pass filters of 2nd order, IIR-type.

[0011] A Pulse Generator 205 includes a Pulse Mapping Module 203 that applies a nonlinear mapping function (typically logarithmic) to the amplitude of each band-pass envelope. This mapping function— for example, using instantaneous nonlinear compression of the envelope signal (map law)— typically is adapted to the needs of the individual cochlear implant user during fitting of the implant in order to achieve natural loudness growth. This may be in the specific form of functions that are applied to each requested stimulation event signal Si to <¾ that reflect patient-specific perceptual characteristics to produce a set of electrode stimulation signals A\ to AM that provide an optimal electric representation of the acoustic signal. A logarithmic function with a form- factor C typically may be applied as a loudness mapping function, which typically is identical across all the band pass analysis channels. In different systems, different specific loudness mapping functions other than a logarithmic function may be used, with just one identical function is applied to all channels or one individual function for each channel to produce the electrode stimulation signals A\ to AM outputs from the Pulse Mapping Module 203.

[0012] The Pulse Generator 205 also includes a Pulse Shaper 204 that develops the set of electrode stimulation signals A\ to A _M into a set of output electrode pulses E\ to E _M for the electrode contacts in the implanted electrode array which stimulate the adjacent nerve tissue. The electrode stimulation signals A\ XO AM may be symmetrical biphasic current pulses with amplitudes that are directly obtained from the compressed envelope signals.

[0013] In the specific case of a CIS system, the stimulation pulses are applied in a strictly non-overlapping sequence. Thus, as a typical CIS-feature, only one electrode channel is active at a time and the overall stimulation rate is comparatively high. For example, assuming an overall stimulation rate of 18 kpps and a 12 channel filter bank, the stimulation rate per channel is 1.5 kpps. Such a stimulation rate per channel usually is sufficient for adequate temporal representation of the envelope signal. The maximum overall stimulation rate is limited by the minimum phase duration per pulse. The phase duration cannot be arbitrarily short because, the shorter the pulses, the higher the current amplitudes have to be to elicit action potentials in neurons, and current amplitudes are limited for various practical reasons. For an overall stimulation rate of 18 kpps, the phase duration is 27 μβ, which is near the lower limit.

[0014] In the CIS strategy, the signal processor only uses the band pass signal envelopes for further processing, i.e., they contain the entire stimulation information. For each electrode channel, the signal envelope is represented as a sequence of biphasic pulses at a constant repetition rate. A characteristic feature of CIS is that the stimulation rate is equal for all electrode channels and there is no relation to the center frequencies of the individual channels. It is intended that the pulse repetition rate is not a temporal cue for the patient (i.e., it should be sufficiently high so that the patient does not perceive tones with a frequency equal to the pulse repetition rate). The pulse repetition rate is usually chosen at greater than twice the bandwidth of the envelope signals (based on the Nyquist theorem).

[0015] Another cochlear implant stimulation strategy that does transmit fine time structure information is the Fine Structure Processing (FSP) strategy by Med-El. Zero crossings of the band pass filtered time signals are tracked, and at each negative to positive zero crossing, a Channel Specific Sampling Sequence (CSSS) is started. Typically CSSS sequences are only applied on the first one or two most apical electrode channels, covering the frequency range up to 200 or 330 Hz. The FSP arrangement is described further in Hochmair I, Nopp P, Jolly C, Schmidt M, SchoBer H, Garnham C, Anderson I, MED-EL Cochlear Implants: State of the Art and a Glimpse into the Future, Trends in

Amplification, vol. 10, 201 -219, 2006, which is incorporated herein by reference.

[0016] Many cochlear implant coding strategies use what is referred to as an N-of-M approach where only some number n electrode channels with the greatest amplitude are stimulated in a given sampling time frame. If, for a given time frame, the amplitude of a specific electrode channel remains higher than the amplitudes of other channels, then that channel will be selected for the whole time frame. Subsequently, the number of electrode channels that are available for coding information is reduced by one, which results in a clustering of stimulation pulses. Thus, fewer electrode channels are available for coding important temporal and spectral properties of the sound signal such as speech onset.

[0017] One method to reduce the spectral clustering of stimulation per time frame is the MP3000™ coding strategy by Cochlear Ltd, which uses a spectral masking model on the electrode channels. Another method that inherently enhances coding of speech onsets is the ClearVoice™ coding strategy used by Advanced Bionics Corp, which selects electrode channels having a high signal to noise ratio. U.S. Patent Publication 2005/0203589 (which is incorporated herein by reference in its entirety) describes how to organize electrode channels into two or more groups per time frame. The decision which electrode channels to select is based on the amplitude of the signal envelopes.

[0018] In addition to the specific processing and coding approaches discussed above, different specific pulse stimulation modes are possible to deliver the stimulation pulses with specific electrodes— i.e. mono-polar, bi-polar, tri-polar, multi-polar, and phased - array stimulation. And there also are different stimulation pulse shapes— i.e. biphasic, symmetric triphasic, asymmetric triphasic pulses, or asymmetric pulse shapes. These various pulse stimulation modes and pulse shapes each provide different benefits; for example, higher tonotopic selectivity, smaller electrical thresholds, higher electric dynamic range, less unwanted side-effects such as facial nerve stimulation, etc. But some stimulation arrangements are quite power consuming, especially when neighboring electrodes are used as current sinks. Up to 10 dB more charge might be required than with simple mono-polar stimulation concepts (if the power-consuming pulse shapes or stimulation modes are used continuously).

[0019] It is well-known in the field that electric stimulation at different locations within the cochlea produce different frequency percepts. The underlying mechanism in normal acoustic hearing is referred to as the tonotopic principle. In cochlear implant users, the tonotopic organization of the cochlea has been extensively investigated; for example, see Vermeire et al. , Neural tonotopy in cochlear implants: An evaluation in unilateral cochlear implant patients with unilateral deajhess and tinnitus, Hear Res, 245(1-2), 2008 Sep 12 p. 98-106; and Schatzer et al., Electric-acoustic pitch comparisons in single-sided- deaf cochlear implant users: Frequency-place functions and rate pitch, Hear Res, 309, 2014 Mar, p. 26-35 (both of which are incorporated herein by reference in their entireties).

[0020] In a normal hearing ear, one frequency component consecutively stimulates multiple neural populations. This phenomenon was described as the "travelling wave" as shown in Figure 5 from Von Bekesy, Georg. Experiments in hearing. Ed. Ernest Glen Wever. Vol. 8. New York: McGraw-Hill, 1960 (incorporated herein by reference in its entirety). That is, in response to a pure tone, the basilar membrane resonates in a travelling wave (the ascending numbers within Fig. 5) which gradually grows in amplitude (the dashed lines in Fig. 5) as it moves along the cochlear duct from the stapes (base) toward the helicotrema (apex).

[0021] One quality of the travelling wave that is partly reflected in modern cochlear implant systems is that each frequency component reaches a peak amplitude at a specific spot within the cochlea (the tonotopic principle discussed above). These spectro-temporal properties can also be observed in the activity of cat's cochlear nerve fibres shown in Figure 6 from Seeker Walker et al, Time domain analysis of auditory nerve fiber firing rates, J Acoust Soc Am , 88(3), 1990, p. 1427-1436 (incorporated herein by reference in its entirety). Figure 6 shows neural activity in the cochlear nerve over time at nerve fibres with different characteristic frequencies in response to synthetic vowels. One dominant frequency component in the synthetic vowel stimuli is the fundamental frequency (F0), which in Fig. 6 can be clearly identified as a regular pattern starting at high frequencies and ending several milliseconds later at low frequencies. The black curve in the shaded box in Fig. 6 indicates the frequency-specific time delays or the neural responses. Higher frequency components also can be observed between the F0 structures; for example, harmonics that are visible between 1800 and 1000 Hz. Similar to the F0 structures, they start at high frequency fibers and end some milliseconds later at low frequency fibers. This spectro-temporal excitation behaviour is not currently explicitly implemented in cochlear implant systems. [0022] Loeb G. , Are cochlear implant patients suffering from perceptual dissonance? Ear Hear, 26, 2005, p. 435-450 (incorporated herein by reference in its entirety) describes that phase-locking occurs over a substantial length of the cochlea. Furthermore, the action potentials exhibit a coherent spatial gradient with the steepest and most rapidly changing gradient of the phase occurring next to the place of the resonant frequency. At this point, the travelling wave starts to significantly slow down and dissipates. The phase gradient is believed to substantially contribute to pitch perception, especially in loud situations where harmonics are not resolved.

[0023] Existing coding approaches take into account some of the temporal properties of the acoustic signal. CIS determines frequency-specific envelopes which inherently contain a certain amount of information about individual low frequency components such as the fundamental frequency. More advanced approaches for calculating band specific envelopes also have been described; for example, U.S. Patent Publication 2006/0235486 (which is incorporated herein by reference in its entirety). The latter and CIS both sample the band pass envelopes with fixed rate stimulation pulses to resemble rudimentary properties of the basilar membrane movement. Other advanced systems as described in U. S. Patent Publication 2011/0230934 (which is incorporated herein by reference in its entirety) explicitly extract temporal characteristics of a band pass signal by identifying phase characteristics such as zero crossings. The described system triggers channel- specific sequences of stimulation pulses at each detected zero crossing. Each of the foregoing arrangements attributes certain frequency components to certain stimulation places. U. S. Patent Publication 2011/0230934 also explicitly takes into account the timing of certain frequency components.

[0024] Vocoder-based cochlear implant stimulation arrangements such as CIS and N-of- M do not take into account the travelling wave properties of normal acoustic hearing. The acoustic signal is analysed by filter banks or FFT and assigned either to single intracochlear electrodes, or to simultaneous stimulation of multiple adjacent electrodes. While filter banks can mimic the latencies of single frequency components at the place of stimulation, they are not able to mimic other aspects of the travelling wave behaviour such as the spectro-temporal distribution of this component to neighbouring stimulation sites, starting at a more basal site with low amplitude and ending at a more apical stimulation site with a maximum of stimulation at a site in between. An FFT, also used for mimicking the tonotopic principle in a cochlear implant is no better able to replicate the general latency differences between the frequency components (at the place of stimulation) nor does it provide the spectro-temporal behaviour described above.

SUMMARY OF THE INVENTION

[0025] Embodiments of the present invention are directed to a signal processing arrangement and corresponding method that generates electrode stimulation signals to electrode contacts in an implanted cochlear implant array. An input sound signal is analyzed to determine characteristic frequency components. For each frequency component, one or more stimulation events are requested based on the timing and amplitude of the frequency component. For each requested stimulation event, a frequency- specific stimulation sequence (FSSS) is generated for stimulation of adjacent electrode contacts. The FSSS starts with a stimulation pulse to the highest-frequency, most-basal electrode contact of the adjacent electrode contacts, ends with a stimulation pulse to the lowest-frequency, most-apical electrode contact of the adjacent electrode contacts, and reaches a maximum stimulation amplitude at a frequency-specific location within the cochlea corresponding to a natural traveling wave maximum. The electrode stimulation signals are then generated from the FSSS for delivery by the electrode contacts to adjacent auditory neural tissue.

[0026] In further specific embodiments, each stimulation pulse within the FSSS activates either a single electrode contact, or a plurality of adjacent electrode contacts

simultaneously and in-phase. Simultaneous stimulation pulses may be amplitude corrected based on Channel Interaction Compensation (CIC). The FSSS may be shorter in time for higher frequency components and longer in time for lower frequency components. The FSSS may be at least partially simultaneous on two or more electrode contacts. For each electrode contact, the FSSS may be a Channel Specific Sampling Sequence (CSSS). The timing of each frequency component may reflect a phase characteristic and/or frequency- specific latency characteristic of the frequency component.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 shows a section view of a human ear with a typical cochlear implant system designed to deliver electrical stimulation to the inner ear.

[0028] Figure 2 shows various functional blocks in a continuous interleaved sampling (CIS) processing system.

[0029] Figure 3 shows an example of a short time period of an audio speech signal from a microphone.

[0030] Figure 4 shows an acoustic microphone signal decomposed by band-pass filtering by a bank of filters into a set of band pass signals.

[0031] Figure 5 shows the concept of the travelling wave within the cochlea.

[0032] Figure 6 shows an example neurogram of auditory nerve fibers of a cat over time.

[0033] Figure 7 shows various logical steps in developing electrode stimulation signals according to an embodiment of the present invention.

[0034] Figure 8 shows various waveforms related to producing frequency-specific stimulation sequences for a low frequency component according to an embodiment of the present invention.

[0035] Figure 9 shows various waveforms related to producing frequency-specific stimulation sequences for a high frequency component according to an embodiment of the present invention.

[0036] Figure 10 shows different shapes of frequency-specific stimulation sequences.

[0037] Figure 11 shows different amplitude and phase delay shapes of frequency- specific stimulation sequences.

[0038] Figure 12 shows an example of steered multi-polar travelling wave stimulation. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0039] Embodiments of the present invention add to a cochlear implant system an emulation of a normal auditory physiological process which is important for frequency perception in normal hearing individuals, the travelling wave response of the cochlea. The added spectro-temporal features reflect the rise and sharp fall of excitation along the cochlea from the travelling wave, as well as its slowing down. Embodiments of the invention use arrangements that detect a number of relevant (i.e. spectrally spread or psychophysical^ unmasked) frequency components and translates them into stimulation sequences which can be super-positioned. This approach can be configured for the specific number of individual information channels of a given patient by skipping frequency components while transmitting distinguishable components in a highly natural way.

[0040] Figure 7 is a flow chart showing various logical steps in producing electrode stimulation signals to electrode contacts in an implanted cochlear implant array according to an embodiment of the present invention. A pseudo code example of such a method can be set forth as:

Input Frequency and Component Level Estimation :

FilterAnalyze (input sound, frequency components)

Frequency Specific Stimulation Sequences :

Code (frequency components, stim events)

FSSS ( stim_events , fsss_seqs)

Stimulation Pulse Generation:

Generate (fsss_seqs, output_pulses )

The details of such an arrangement are set forth in the following discussion.

[0041] As in the arrangement discussed above with respect to Figure 2, a preprocessor signal filter bank 201 can be configured to decompose an input sound signal into band pass frequency component signals to 5M, step 701, representing an estimate of instantaneous input frequency/timing and component level/amplitude such that each band pass frequency component signal to 5M changes over time in characteristic timing and amplitude. The timing of the band pass frequency component signals to 5M typically may reflect frequency-specific response latencies and/or phase characteristics. The signal processing module 202 then processes the band pass frequency component signals B _\ to 5M to code each frequency component, step 702, as a sequence of requested stimulation events based on the frequency component timing and amplitude.

[0042] For each requested stimulation event, a frequency-specific stimulation sequence (FSSS) output Si to <¾ is generated, step 703, for at least partially simultaneous stimulation of adjacent electrode contacts. Figure 8 shows various waveforms related to the signal processing module 202 producing an FSSS according to an embodiment of the present invention for a low frequency component. The top panel A in Fig. 8 shows a low frequency component where the circles (zero crossings) indicate specific stimulation events. Each stimulation event triggers an FSSS. The middle Panel B in Fig. 8 shows the timing of the desired neural response to the input frequency in terms of location along the cochlea over time. The thin horizontal lines in Panel B indicate examples of electrode- specific locations/frequencies. The lower Panel C in Fig. 8 shows FSSS stimulation sequences on two adjacent electrode contacts that apply weighted partially simultaneous stimulation in the specific form of Channel Specific Sampling Sequences (CSSS as described in U.S. Patent 6,594,525; incorporated herein by reference in its entirety). Specifically, Panel C shows that the FSSS starts with the left-most stimulation pulse to the highest-frequency, most-basal electrode contact (n+1), ends with the right-most stimulation pulse to the lowest-frequency, most-apical electrode contact (n). The FSSS is defined so as to reach a maximum stimulation amplitude at a frequency-specific location within the cochlea that corresponds to a natural traveling wave maximum. Each given stimulation pulse within the FSSS activates either a single electrode contact, or multiple adjacent electrode contacts simultaneously and in-phase.

[0043] For the low frequency component shown in Fig. 8, there is a relatively large phase delay and longer duration FSSS. The FSSS is typically shorter in time for higher frequency components and longer in time for lower frequency components. Thus, Figure 9 shows various waveforms related to producing frequency-specific stimulation sequences for a high frequency component where the phase delay is smaller and the FSSS is shorter in duration. And Figure 10 shows different potential shapes of frequency-specific stimulation sequences.

[0044] The pulse generator 205 is configured to convert the requested stimulation events Si to ¾to produce a corresponding sequence of unweighted stimulation signals A _\ to A _M that provide an optimal electric representation of the acoustic signal, and then apply a linear mapping function (typically logarithmic) and pulse shaping to produce weighted output pulse sequences electrode stimulation signals E\ to EM for delivery by the electrode contacts to adjacent auditory neural tissue, step 704. Simultaneous stimulation pulses may be amplitude corrected based on Channel Interaction Compensation (CIC). The weighted output pulse sequences electrode stimulation signals Ei to EM also are adapted to the needs of the individual implant user based on a post-surgical fitting process that determines patient-specific perceptual characteristics.

[0045] The length of the FSSS can vary based on the number of electrode channels and the number of the CSSS per channel. The lengths of the electrode channel CSSS per FSSS may be constant, however, varying CSSS lengths per FSSS also may be possible, such as longer CSSS at more apical channels or longer/shorter CSSS at the maximum level of the FSSS, etc. Some embodiments also may apply a Channel Interaction Compensation (CIC) algorithm (e.g., U.S. Patent 7,917,224; incorporated herein by reference in its entirety) to the amplitudes of simultaneous FSSS to provide a desired loudness level to the user. The onset of the CSSS within a FSSS is controlled by the phase of the travelling wave.

Subthreshold stimulation on individual electrode channels may be applied within a single FSSS in order to support and maintain spontaneous action potentials at the stimulation locations.

[0046] Frequency specific characteristics of the FSSS such as amplitude shape, spread over electrode positions, and duration (of entire FSSS and channel specific CSSS per FSSS) can be stored as templates in system memory that is accessible to the signal processing module 202. Figure 11 shows some specific examples of different amplitude and phase delay shapes of frequency- specific stimulation sequences, with a low frequency component shown on the left, and a high frequency component shown on the right. The vertical lines in Fig. 11 represent time instances at which simultaneous stimulation is elicited. And an FSSS can be calculated for any desired frequency component by using interpolation.

[0047] Temporal overlap of an FSSS can be handled by applying simultaneous stimulation of all necessary electrodes, i.e. superposition. Spectral overlap of two simultaneously requested interleaving FSSS can also be omitted.

[0048] The FSSS can be optimized in duration, number of stimulations, and amplitude shape to produce a the most tone-like percept in response to an acoustic presentation of a pure tone. The amplitude shape and timing of the FSSS can reproduce the envelope of the traveling wave by representing portions of the traveling wave at consecutive positions along the cochlea (Figure 11) starting at the base of the cochlea with low amplitude and rising with a shallow slope up to the maximum frequency and then falling with a steep slope towards the apex of the cochlea. Alternatively, in simplest form, the amplitude shape of the FSSS can consist of a single stimulation event, occurring simultaneously on two or more adjacent electrode channels. The various FSSS may overlap in time so that they can be applied in a superimposed manner. The timing of the stimulation event reflects the phase delay of the encoded frequency component as shown in Figures 8 and 9. The amplitude weightings of the simultaneous stimulation events on the adjacent electrodes reflect the frequency specific place of the component along the cochlea. And, in some specific embodiments, the stimulation events may be selected as described in U.S. Patent Publication 2009/0125082, which is incorporated herein by reference in its entirety.

[0049] Stimulation positions which are intermediate to physical electrode positions can be produced by weighted simultaneous stimulation of one or more adjacent electrodes. Alternatively, a FSSS can also be compiled from a series of focused stimulation modes, e.g. tri-or multipolar stimulation such as phased array stimulation as shown in Bonham et al. , Current focusing and steering: modeling, physiology, and psychophysics, Hear Res , 242(1-2), Aug 2008, pp. 141-153; incorporated herein by reference in its entirety. The focus, amplitude and timing of the stimulation will follow the tempo-spectral shape of the traveling wave envelope. Figure 12 shows an example of steered multi-polar traveling wave stimulation where the focus of stimulation (circles) follows the temporal course of the traveling wave. [0050] Embodiments of the invention may be implemented in part in any conventional computer programming language. For example, preferred embodiments may be implemented in a procedural programming language (e.g., "C") or an object oriented programming language (e.g. , "C++", Python). Alternative embodiments of the invention may be implemented as pre-programmed hardware elements, other related components, or as a combination of hardware and software components.

[0051] Embodiments can be implemented in part as a computer program product for use with a computer system. Such implementation may include a series of computer instructions fixed either on a tangible medium, such as a computer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk) or transmittable to a computer system, via a modem or other interface device, such as a communications adapter connected to a network over a medium. The medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented with wireless techniques (e.g., microwave, infrared or other transmission techniques). The series of computer instructions embodies all or part of the functionality previously described herein with respect to the system. Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies. It is expected that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the network (e.g., the Internet or World Wide Web). Of course, some embodiments of the invention may be implemented as a combination of both software (e.g. , a computer program product) and hardware. Still other embodiments of the invention are implemented as entirely hardware, or entirely software (e.g. , a computer program product).

[0052] Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention.