Holes in hearing.

Previous experiments have demonstrated that the correct tonotopic representation of spectral information is important for speech recognition. However, in prosthetic devices, such as hearing aids and cochlear implants, there may be a frequency/place mismatch due in part to the signal processing of the device and in part to the pathology that caused the hearing loss. Local regions of damaged neurons may create a "hole" in the tonotopic representation of spectral information, further distorting the frequency-to-place mapping. The present experiment was performed to quantitatively assess the impact of spectral holes on speech recognition. Speech was processed by a 20-band processor: SPEAK for cochlear implant (CI) listeners, and a 20-band noise processor for normal-hearing (NH) listeners. Holes in the tonotopic representation (from 1.5 to 6 mm in extent) were created by eliminating electrodes or noise carrier bands in the basal, middle, or apical regions of the cochlea. Vowel, consonant, and sentence recognition were measured as a function of the location and size of the hole. In addition, the spectral information that would normally be represented in the hole region was either: (1) dropped, (2) assigned to the apical side of the hole, (3) assigned to the basal side of the hole, or (4) split evenly to both sides of the hole. In general, speech features that are highly dependent on spectral cues (consonant place, vowel identity) were more affected by the presence of tonotopic holes than temporal features (consonant voicing and manner). Holes in the apical region were more damaging than holes in the basal or middle regions. A similar pattern of performance was observed for NH and CI listeners, suggesting that the loss of spectral information was the primary cause of the effects. The Speech Intelligibility Index was able to account for both NH and CI listeners' results. No significant differences were observed among the four conditions that redistributed the spectral information around the hole, suggesting that rerouting spectral information around a hole was no better than simply dropping it.