The factor analysis of speech: limitations and opportunities for cochlear implants

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