Incremental Responses of VIIIth Nerve Units

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Abstract
Incremental responses of single units were studied in the auditory nerve of anesthetized, immobilized guinea pigs. The results confirm and extend earlier studies on the cochlear nucleus [R. L. Smith, J. Acoust. Soc. Amer. 52, 117(A) (1972)]. Positive and negative intensity increments were added to tone pedestals. Increments and pedestals always had the same sound frequency and phase and usually were at the unit's characteristic frequency (CF). The time delay from pedestal onset to increment onset was maintained at 150 msec. The change in firing rate at the onset of the incremental response served as a measure of incremental sensitivity. The pedestal intensity was varied, while the ratio between the incremental intensity and the pedestal intensity was held constant (constant decibel difference). The greatest incremental sensitivity was obtained for pedestals within 20 dB of the unit's threshold. If frequencies below CF were used the threshold became higher, and the region of maximum incremental sensitivity shifted to higher pedestal intensities. These results suggest that, for sufficiently high pedestal intensities, detection of intensity increments depends mainly upon units stimulated outside their CFs.

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