Representation of tones in noise in the responses of auditory nerve fibers in cats. I. Comparison with detection thresholds.

Rate and temporal responses evoked by 1-kHz or 8-kHz tones in continuous broadband noise are described for large populations of auditory nerve fibers in anesthetized cats. The signal-to-noise ratios (S/Ns) of the tone and noise stimuli were above behavioral detection thresholds. Stimulus combinations were presented (1) over a range of moderate to high noise intensities at a constant S/N and (2) using high intensity noise and varying S/N. Responses of low (less than 1 spike/sec) and medium (1 to 19 spikes/sec) spontaneous rate (SR) fibers were compared with those of high SR (greater than 19 spikes/sec) fibers. Low and medium SR fibers with best frequencies in the region of the test tone frequency exhibited tone-evoked rate changes at all sound levels tested. High SR fibers, in contrast, exhibited much weaker tone-evoked rate changes at the lowest noise level tested. In the presence of high intensity noise, high SR fibers did not exhibit tone-evoked rate changes due to saturation by the noise. Fibers with best frequencies in the region of 1 kHz also exhibited strong phase-locking to the 1 kHz tone which increased as the tone level increased but which did not differ for the various SR groups. Results suggest that information in the rate responses of low and medium SR fibers can account for the encoding of information about tones in noise by the nervous system.