Abstract
Current spread is known to limit the number of independent spectral channels in cochlear implants. The outcome of an experiment employing cochlear implant simulations indicated that current spread is not the only limitation on the benefit of increasing the number of electrodes: for both sentences and digit triplets, improvements in speech reception threshold slowed markedly once more than seven electrodes/processing channels were simulated. Factor analysis of speech envelopes from the output of an auditory filterbank confirmed that speech contains 6-8 independent sources of information, causing finer spectral filtering to produce redundant information in adjacent channels. It is possible that factor analysis can be used to refine the frequency maps used in cochlear implants in order to minimise the effects of current spread.
Highlights
Current spread limits the benefit of increased numbers of electrodes in cochlear implants, because ganglion cells at a given location on the spiral ganglion are stimulated by multiple electrodes (e.g. Abbas et al [1])
In the present study, we use SPIRAL to show that the increasing benefit of additional electrodes shows a marked inflection at around seven electrodes even in the absence of current spread, indicating the existence of a more fundamental limiting factor
Materials and vocoding Speech and noise were mixed and vocoded using SPIRAL [3], a vocoder designed to simulate listening through a cochlear implant (CI) with normally-hearing listeners
Summary
Current spread limits the benefit of increased numbers of electrodes in cochlear implants, because ganglion cells at a given location on the spiral ganglion are stimulated by multiple electrodes (e.g. Abbas et al [1]). Friesen et al [2] showed that the percent-correct sentence recognition in noise of cochlear implant users plateaued once about seven electrodes were activated. They attributed this effect to the influence of current spread. Using SPIRAL, Grange et al confirmed a limiting effect of current spread on the benefit of additional electrodes. In the present study, we use SPIRAL to show that the increasing benefit of additional electrodes shows a marked inflection at around seven electrodes even in the absence of current spread, indicating the existence of a more fundamental limiting factor
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