Psychophysics and neurophysiology of repetition noise processing in a vertebrate auditory system

The psychophysics and neurophysiology of repetition noise (RN) processing was studied in the goldfish. RN is the sum of a noise waveform with its delayed (by T s) repetition, which may be attenuated (by A dB), and inverted relative to the undelayed signal. Such a signal has a periodic spectrum with peaks separated by 1/T Hz, and a prominence in its autocorrelation function at T s. In usual environments, RN contains information about sound-reflecting surfaces. Delays in the range of 0.5-20 ms create pitch sensations in man. Psychophysical experiments using classical respiratory conditioning investigated the masking effectiveness of RN on tones, the detection of changes in delay (T) at various values of T, A and overall noise level, and the values of A required to bring a 20% delay discrimination to threshold. While the masking data define detection filters quite broadly tuned compared with man, various measures of delay discrimination are comparable to those for man. Unit responses from the auditory nerve are consistent with broadly tuned psychophysical filters, but in all cells studied show prominent inter-spike-interval (ISI) peaks which predict the delay values used to generate the RN. We conclude that the qualitative features of RN are coded in ISIs, and are processed by the CNS in the time domain. Similar mechanisms may be used by other vertebrate species in processing repetition noise.