Electrocochleographic documentation of temporal findings of speech perception in normal and hearing-impaired individuals.

Compound action potentials (CAPs) evoked by the short Japanese syllables /a/ and /ka/ were recorded by extratympanic electrocochleography in 17 subjects with normal hearing (17 ears) and 34 patients with sensorineural hearing losses (35 ears) to investigate the temporal aspects of speech coding for perception. In normal ears, three characteristics were found common to the temporal patterns of all CAPs: (1) a prominent CAP at the beginning of both stimuli; (2) periodic CAPs with the same interval as the pitch period through the vowel part of both stimuli; (3) absence of a prominent CAP at the onset of voice. These characteristics may help to produce consonant recognition. Among the subjects with sensorineural hearing loss, some ears showed the following two characteristics different from those with normal hearing: (1) a significantly lower CAP at the onset of both stimuli than in those with normal hearing; (2) decay of CAPs during the voiced part. These findings may result in abnormal loudness perception in sensorineural hearing loss as produced by loudness recruitment and pathological adaptation. Such different temporal patterns of CAPs may have an adverse influence on the speech discrimination of patients with sensorineural hearing impairments.