Improved temporal coding of sinusoids in electric stimulation of the auditory nerve using desynchronizing pulse trains

Abstract

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.

Neural and Behavioral Sensitivity to Interaural Time Differences Using Amplitude Modulated Tones with Mismatched Carrier Frequencies

“…Certainly we would expect accurate place/pitch matching to yield better results, as has been shown in some studies (Long et al 2003;Wilson et al 2003). Other studies, however, have shown that ITD information can be obtained even over a range of as many as eight contacts, spanning many millimeters in the cochlea (van Hoesel and Clark 1997).…”

“…Implant listeners tend to have acute sensitivity to the presence of amplitude modulation (Shannon 1992), but there is some evidence that the ability to detect small changes in stimulus rate is relatively poor (Zeng 2002). Furthermore, in at least one case, better ITD discrimination was found for an electrode pair that was mismatched in place/pitch than for the matched pair (Long et al 2003). It is tempting to suggest that in this case, the matched pair was in the saturated range and the better result with the mismatched pair was caused by moving into a nonsaturated state, similar to off-CF listening.…”

“…In general, bilaterally implanted patients show IID performance similar to that of normal-hearing listeners. In con-trast, thresholds for ITD discrimination tend to be higher than in normal-hearing listeners (van Hoesel and Clark 1995;van Hoesel and Clark 1997;Lawson, Wilson et al 1998;Long, Eddington et al 2003;van Hoesel and Tyler 2003;Schoen et al 2005). This result is not altogether unexpected given the nature of ITD stimuli provided by most cochlear implant speech processors.…”

“…To adjust for these differences, pitch matching can be performed to identify contacts that are most likely to maximize the stimulation of overlapping populations. Some laboratories report an improvement in encoding ITDs when careful pitch matching and subsequent stimulation of the matched contacts is performed (Long et al 2003;Wilson et al 2003). Other studies report little or no differences in threshold to ITDs across wide ranges of electrode contacts, including those identified through careful pitch matching (van Hoesel and Clark 1997).…”

Electrically Evoked Auditory Steady State Responses in Cochlear Implant Users

“…Different approaches for artifact removal can be found in the literature, ranging from the use of linear and nonlinear filters (Whittington et al 2003(Whittington et al , 2005Sennels et al 1997;Parsa et al 1998;Gnadt et al 2003;Liang and Lin 2002) to the subtraction of artifact templates generated by a variety of methods (Hashimoto et al 2002;Lin et al 1996;Lin and McCallum 1998;Wagenaar and Potter 2002;Litvak et al 2003;Yuwaraj and Kunov 1995;Grieve et al 2000;Blogg and Reid 1990). For multielectrode recordings with a high number of recording electrodes, blind source separation, independent component analysis, and optimized differential reference techniques allow for the spatial localization of artifact sources and potentials (Gilley et al 2006; Castañeda-Villa and James , which makes it possible to select electrodes that are less affected by artifacts and to remove artifact components from these recordings.…”