Noise susceptibility and immunity of phase locking in amphibian auditory-nerve fibers

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Abstract
Recordings from auditory‐nerve fibers in the anesthetized frog revealed that addition of broadband noise results in a reduction in the ability of a fiber to phase lock to a continuous pure tone. In particular, our results suggest that: (i) there is a threshold below which masking noise has little or no effect on vector strength (VS); then with increasing masking noise level, VS appears to decrease monotonically for all test frequencies (TFs); (ii) there exist subpopulations of auditory‐nerve fibers in the frog for which the deterioration of phase locking to tones in wideband noise depends critically on the relationship of the TF to the fiber’s CF. Specifically, in one subpopulation (43% of the fibers studied), the rate of VS decrease with increasing levels of masking noise is greater for CF tones than it is for TFs>CF. The net result is a ‘‘crossing’’ of the VS versus masking noise functions (e.g., Fig. 6); (iii) there exists a small subpopulation of amphibian papillar (a.p.) fibers for which the rate of VS decrease with increasing levels of masking noise is less for TFsCF, whereas, for stimulus tones at or below CF, the preferred firing phase is nearly noise‐level independent; (v) the remainder of the sample consists of fibers in which the VS‐falloff rates appear to be test‐frequency independent; (vi) addition of wideband masking noise to a CF tone, and increasing the CF‐tone level in the absence of noise, produced (qualitatively) similar effects on the preferred firing phase of auditory‐nerve fibers (e.g., Figs. 1 and 7). Thus amphibian auditory‐nerve fibers appear to be energy detectors, i.e., exhibit phase shifts corresponding to the total energy within the filter passband defined by the frequency‐threshold curve.