Speech recognition under conditions of frequency-place compression and expansion

In normal acoustic hearing the mapping of acoustic frequency information onto the appropriate cochlear place is a natural biological function, but in cochlear implants it is controlled by the speech processor. The cochlear tonotopic range of the implant is determined by the length and insertion depth of the electrode array. Conventional cochlear implant electrode arrays are designed for an insertion of 25 mm inside the round window and the active electrodes occupy 16 mm, which would place the electrodes in a cochlear region corresponding to an acoustic frequency range of 500-6000 Hz. However, some implant speech processors map an acoustic frequency range from 150 to 10000 Hz onto these electrodes. While this mapping preserves the entire range of acoustic frequency information, it also results in a compression of the tonotopic pattern of speech information delivered to the brain. The present study measured the effects of such a compression of frequency-to-place mapping on speech recognition using acoustic simulations. Also measured were the effects of an expansion of the frequency-to-place mapping, which produces an expanded representation of speech in the cochlea. Such an expanded representation might improve speech recognition by improving the relative spatial (tonotopic) resolution, like an "acoustic fovea." Phoneme and sentence recognition was measured as a function of linear (in terms of cochlear distance) frequency-place compression and expansion. These conditions were presented to normal-hearing listeners using a noise-band vocoder, simulating cochlear implant electrodes with different insertion depths and different number of electrode channels. The cochlear tonotopic range was held constant by employing the same noise carrier bands for each condition, while the analysis frequency range was either compressed or expanded relative to the carrier frequency range. For each condition, the result was compared to that of the perfect frequency-place match, where the carrier and the analysis bands were perfectly matched. Speech recognition in the matched conditions was generally better than any condition of frequency-place expansion and compression, even when the matched condition eliminated a considerable amount of acoustic information. This result suggests that speech recognition, at least without training, is dependent on the mapping of acoustic frequency information onto the appropriate cochlear place.