Abnormal processing of temporal fine structure in speech for frequencies where absolute thresholds are normal

The identification of nonsense syllables that were lowpass filtered at 1.5 kHz was compared for subjects with normal hearing and subjects with mild-to-severe hearing loss at high frequencies but with normal or near-normal hearing at low frequencies. Absolute thresholds were mostly within the normal range (< 20 dB hearing level) for both groups for frequencies below 1.5 kHz. Performance was assessed with intact speech, speech that had been processed to preserve only temporal envelope cues in a few frequency bands (E speech), and speech that had been processed to remove envelope cues as far as possible while preserving temporal fine structure cues, again in a few frequency bands (TFS speech). For the intact speech and E speech, the hearing-impaired subjects performed slightly more poorly than the normal-hearing subjects, but this effect was significant only for the intact speech. For the TFS speech, the hearing-impaired subjects performed significantly more poorly than the normal-hearing subjects, with 12 out of 16 of the former performing at chance. The results indicate that, for people with hearing loss at medium to high frequencies, the processing of the TFS of speech can be degraded for frequencies where absolute thresholds are within the normal range.