The Journal of the Acoustical Society of America, 2003
Current cochlear implant processors poorly represent sound waveforms in the temporal discharge pa... more Current cochlear implant processors poorly represent sound waveforms in the temporal discharge patterns of auditory-nerve fibers (ANFs). A previous study [Litvak et al., J. Acoust. Soc. Am. 114, 2079-2098 (2003)] showed that the temporal representation of sinusoidal stimuli can be improved in a majority of ANFs by encoding the stimuli as small modulations of a sustained, high-rate (5 kpps), desynchronizing pulse train (DPT). Here, these findings are extended to more complex stimuli by recording ANF responses to pulse trains modulated by bandpass filtered vowels. Responses to vowel modulators depended strongly on the discharge pattern evoked by the unmodulated DPT. ANFs that gave sustained responses to the DPT had period histograms that resembled the modulator waveform for low (< 5%) modulation depths. Spectra of period histograms contained peaks near the formant frequencies. In contrast, ANFs that gave a transient (< 1 min) response to the DPT poorly represented the formant frequencies. A model incorporating a linear modulation filter, a noisy threshold, and neural refractoriness predicts the shapes of period histograms for both types of fibers. These results suggest that a DPT-enhanced strategy may achieve good representation of the stimulus fine structure in the temporal discharge patterns of ANFs for frequencies up to 1000 Hz. It remains to be seen whether these temporal discharge patterns can be utilized by cochlear implant subjects.
The Journal of the Acoustical Society of America, 2003
Rubinstein et al. [Hearing Res. 127, 108-118 (1999)] suggested that the representation of electri... more Rubinstein et al. [Hearing Res. 127, 108-118 (1999)] suggested that the representation of electric stimulus waveforms in the temporal discharge patterns of auditory-nerve fiber (ANF) might be improved by introducing an ongoing, high-rate, desynchronizing pulse train (DPT). To test this hypothesis, activity of ANFs was studied in acutely deafened, anesthetized cats in response to 10-min-long, 5-kpps electric pulse trains that were sinusoidally modulated for 400 ms every second. Two classes of responses to sinusoidal modulations of the DPT were observed. Fibers that only responded transiently to the unmodulated DPT showed hyper synchronization and narrow dynamic ranges to sinusoidal modulators, much as responses to electric sinusoids presented without a DPT. In contrast, fibers that exhibited sustained responses to the DPT were sensitive to modulation depths as low as 0.25% for a modulation frequency of 417 Hz. Over a 20-dB range of modulation depths, responses of these fibers resembled responses to tones in a healthy ear in both discharge rate and synchronization index. This range is much wider than the dynamic range typically found with electrical stimulation without a DPT, and comparable to the dynamic range for acoustic stimulation. These results suggest that a stimulation strategy that uses small signals superimposed upon a large DPT to encode sounds may evoke temporal discharge patterns in some ANFs that resemble responses to sound in a healthy ear.
The Journal of the Acoustical Society of America, 2010
Optimizing pitch allocation in a cochlear implant. [The Journal of the Acoustical Society of Amer... more Optimizing pitch allocation in a cochlear implant. [The Journal of the Acoustical Society of America 128, 2257 (2010)]. Leonid M. Litvak, Lakshmi N. Mishra. Keywords. Auditory prostheses, hearing aids.
Both simultaneous (SI) and sequential stimulation of intracochlear electrodes can be used to gene... more Both simultaneous (SI) and sequential stimulation of intracochlear electrodes can be used to generate pitches that are intermediate to the physical electrodes (PEs). The goal of this study was to compare the spread of neural excitation for SI and sequential dual-electrode stimulation with the spread of neural excitation for the intermediate electrode using electrically evoked compound action potentials. Seven Advanced Bionics cochlear implant users with either CII or HiRes 90k implant and HiFocus 1 or HiFocus 1j electrode array participated in this study. A masker-probe subtraction method was used to derive neural excitation patterns for SI nonadjacent dual-electrode stimulation, apical and basal-first sequential nonadjacent dual-electrode stimulation, and the intermediate PE. For apical-first sequential (SEa) stimulation, the masker pulse on the apical electrode immediately preceded the masker pulse on the basal electrode, and vice versa for basal-first sequential stimulation (SEb). The electrodes used for dual-electrode stimulation were separated by an intermediate PE, which represents a spatial distance of approximately 2 mm. Current levels necessary to achieve comfortable loudness were determined for each masker and probe stimulus. During the evoked compound action potential measurements, the masker was fixed in location, whereas the probe was varied across a subset of electrodes in the array. Neural responses were calculated by subtracting the response to the probe from the masked response. Neural excitation patterns were normalized to their peak and analyzed in terms of their area and center of gravity. The area and center of gravity for SI nonadjacent dual-electrode stimulation were similar to those of the intermediate PE. In contrast, the area for the two modes of sequential nonadjacent dual-electrode (SEa and SEb) stimulation differed significantly from the intermediate PE. The center of gravity for SEa stimulation also differed significantly from the intermediate PE, whereas there was no significant difference in the center of gravity between SEb stimulation and the intermediate PE. Peripheral neural activation patterns suggest a similar spread of excitation for SI dual-electrode stimulation and the intermediate PE. The spread of excitation associated with sequential dual-electrode stimulation is generally different from the intermediate PE, and it varies depending on the order of the sequential pulses.
The Journal of the Acoustical Society of America, 2003
Current cochlear implant processors poorly represent sound waveforms in the temporal discharge pa... more Current cochlear implant processors poorly represent sound waveforms in the temporal discharge patterns of auditory-nerve fibers (ANFs). A previous study [Litvak et al., J. Acoust. Soc. Am. 114, 2079-2098 (2003)] showed that the temporal representation of sinusoidal stimuli can be improved in a majority of ANFs by encoding the stimuli as small modulations of a sustained, high-rate (5 kpps), desynchronizing pulse train (DPT). Here, these findings are extended to more complex stimuli by recording ANF responses to pulse trains modulated by bandpass filtered vowels. Responses to vowel modulators depended strongly on the discharge pattern evoked by the unmodulated DPT. ANFs that gave sustained responses to the DPT had period histograms that resembled the modulator waveform for low (< 5%) modulation depths. Spectra of period histograms contained peaks near the formant frequencies. In contrast, ANFs that gave a transient (< 1 min) response to the DPT poorly represented the formant frequencies. A model incorporating a linear modulation filter, a noisy threshold, and neural refractoriness predicts the shapes of period histograms for both types of fibers. These results suggest that a DPT-enhanced strategy may achieve good representation of the stimulus fine structure in the temporal discharge patterns of ANFs for frequencies up to 1000 Hz. It remains to be seen whether these temporal discharge patterns can be utilized by cochlear implant subjects.
The Journal of the Acoustical Society of America, 2003
Rubinstein et al. [Hearing Res. 127, 108-118 (1999)] suggested that the representation of electri... more Rubinstein et al. [Hearing Res. 127, 108-118 (1999)] suggested that the representation of electric stimulus waveforms in the temporal discharge patterns of auditory-nerve fiber (ANF) might be improved by introducing an ongoing, high-rate, desynchronizing pulse train (DPT). To test this hypothesis, activity of ANFs was studied in acutely deafened, anesthetized cats in response to 10-min-long, 5-kpps electric pulse trains that were sinusoidally modulated for 400 ms every second. Two classes of responses to sinusoidal modulations of the DPT were observed. Fibers that only responded transiently to the unmodulated DPT showed hyper synchronization and narrow dynamic ranges to sinusoidal modulators, much as responses to electric sinusoids presented without a DPT. In contrast, fibers that exhibited sustained responses to the DPT were sensitive to modulation depths as low as 0.25% for a modulation frequency of 417 Hz. Over a 20-dB range of modulation depths, responses of these fibers resembled responses to tones in a healthy ear in both discharge rate and synchronization index. This range is much wider than the dynamic range typically found with electrical stimulation without a DPT, and comparable to the dynamic range for acoustic stimulation. These results suggest that a stimulation strategy that uses small signals superimposed upon a large DPT to encode sounds may evoke temporal discharge patterns in some ANFs that resemble responses to sound in a healthy ear.
The Journal of the Acoustical Society of America, 2010
Optimizing pitch allocation in a cochlear implant. [The Journal of the Acoustical Society of Amer... more Optimizing pitch allocation in a cochlear implant. [The Journal of the Acoustical Society of America 128, 2257 (2010)]. Leonid M. Litvak, Lakshmi N. Mishra. Keywords. Auditory prostheses, hearing aids.
Both simultaneous (SI) and sequential stimulation of intracochlear electrodes can be used to gene... more Both simultaneous (SI) and sequential stimulation of intracochlear electrodes can be used to generate pitches that are intermediate to the physical electrodes (PEs). The goal of this study was to compare the spread of neural excitation for SI and sequential dual-electrode stimulation with the spread of neural excitation for the intermediate electrode using electrically evoked compound action potentials. Seven Advanced Bionics cochlear implant users with either CII or HiRes 90k implant and HiFocus 1 or HiFocus 1j electrode array participated in this study. A masker-probe subtraction method was used to derive neural excitation patterns for SI nonadjacent dual-electrode stimulation, apical and basal-first sequential nonadjacent dual-electrode stimulation, and the intermediate PE. For apical-first sequential (SEa) stimulation, the masker pulse on the apical electrode immediately preceded the masker pulse on the basal electrode, and vice versa for basal-first sequential stimulation (SEb). The electrodes used for dual-electrode stimulation were separated by an intermediate PE, which represents a spatial distance of approximately 2 mm. Current levels necessary to achieve comfortable loudness were determined for each masker and probe stimulus. During the evoked compound action potential measurements, the masker was fixed in location, whereas the probe was varied across a subset of electrodes in the array. Neural responses were calculated by subtracting the response to the probe from the masked response. Neural excitation patterns were normalized to their peak and analyzed in terms of their area and center of gravity. The area and center of gravity for SI nonadjacent dual-electrode stimulation were similar to those of the intermediate PE. In contrast, the area for the two modes of sequential nonadjacent dual-electrode (SEa and SEb) stimulation differed significantly from the intermediate PE. The center of gravity for SEa stimulation also differed significantly from the intermediate PE, whereas there was no significant difference in the center of gravity between SEb stimulation and the intermediate PE. Peripheral neural activation patterns suggest a similar spread of excitation for SI dual-electrode stimulation and the intermediate PE. The spread of excitation associated with sequential dual-electrode stimulation is generally different from the intermediate PE, and it varies depending on the order of the sequential pulses.
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