Speech discrimination in deaf subjects with cochlear implants

Electrical stimulation of the auditory nerve is being investigated as a way to provide information useful for speech communication in the profoundly deaf. Single-channel systems that tend to stimulate all fibers alike have had little success in achieving this goal. Multichannel systems that allow excitation of more complex temporal-spatial patterns of activity are now being introduced. Psychoacoustical experiments providing evidence that electrodes of a multichannel implant are able to separately excite distinct groups of neural elements are reviewed. New results using multiple electrodes and speech-like stimuli are presented. The synthetic stimuli were vowels (/a/, /i/, /u/) and consonant-vowel (CV) syllables (/ba/, /da/, /ga/, /ta/). Vowels and CV syllables were presented in an AXB discrimination task with different signal processing schemes and electrode configurations. A four-channel, frequency-selective system produced faultless discrimination scores for all stimuli and spontaneous recognition of the vowels while the scores for the single-channel system were generally much lower. Although understanding free running speech by the profoundly deaf does not seem imminent, the results presented indicate that the multichannel system tested shows more promise of approaching this goal than the single-channel scheme.