Across-Fiber Coding of Temporal Fine-Structure: Effects of Noise-Induced Hearing Loss on Auditory-Nerve Responses

Recent psychophysical evidence suggests that listeners with sensorineural hearing loss (SNHL) have a reduced ability to use temporal fine-structure cues. These results have renewed an interest in the effects of SNHL on the neural coding of fine structure. The lack of convincing evidence that SNHL affects within-fiber phase locking has led to the hypothesis that degraded across-fiber temporal coding may underlie this perceptual effect. Spike trains were recorded from auditory-nerve (AN) fibers in chinchillas with normal hearing and with noise-induced hearing loss. A spectro-temporal manipulation procedure was used to predict spatiotemporal patterns for characteristic frequencies (CFs) spanning up to an octave range from the responses of individual AN fibers to a stimulus presented with sampling rates spanning an octave range. Shuffled cross-correlogram analyses were used to quantify across-CF fine-structure coding in terms of both a neural cross-correlation coefficient and a characteristic delay. Neural cross-correlation for fine-structure decreased and the estimated traveling-wave delay increased with increases in CF separation for both normal and impaired fibers. However, the range of CF separations over which significant correlated activity existed was wider, and the estimated traveling-wave delay was less for impaired AN fibers. Both of these effects of SNHL on across-CF coding have important implications for spatiotemporal theories of speech coding.