Coding of amplitude-modulated tones in the central auditory system of catfish.

In the catfish central acoustic system information is coded by two subsets of units. Type I units show little or no adaptation, type II units adapt rapidly, and some units are transitional, showing moderate adaptation. The two groups of units also respond differently when exposed to sinusoidal amplitude modulation of the signal's carrier frequency. The tonic, less readily adapting type I units code over an intensity range of about 30 dB, are fairly insensitive to intensity changes, and follow stimulus envelopes of 60 Hz and less. They apparently discharge in response to the actual intensity of the signal rather than in response to something in its temporal pattern. The onset-sensitive, fast-adapting type II units on the other hand are restricted to an intensity range of only 10 dB, show greater sensitivity to intensity changes, and are capable of following the temporal pattern of amplitude-modulated stimuli exceeding 100 Hz. These units appear to code the temporal changes in the stimulus intensity irrespective of the absolute intensity of the signal.