ABSTRACT
In the free-swimming electrode-implanted goldfish, the neural response in the medulla to a constant auditory stimulus may exhibit reversible fluctuations in amplitude which are abolished by anaesthesia.
The results are consistent with the action of an auditory control system which can reduce or enhance the input following a click.
Noise-masking effects and reflex muscular control were excluded by demonstrating the relative constancy of the rectified microphonic during simultaneous changes in the click-evoked action potential at the medulla.
There are three kinds of response modification: habituation, rapid inhibitory feedback, and facilitation.
Both feedback and habituation act predominantly on high-threshold auditory fibres. Low-threshold fibres do not become habituated, and dishabituation does not occur.
As in the mammal, anaesthetic reduces the tendency of the system to become habituated by an amount which depends on the dosage. Auditory fibres with highest threshold have the greatest tendency to become habituated and are the least affected in this respect by anaesthetic.
Simple conditioning experiments indicate that control influences exerted over the input can be biased by positive or negative reinforcement which follows the auditory stimulus.
The control system may work in attention, in frequency analysis, or in suppressing input to self-made sounds.
A new hypothesis is made on the biological significance of hearing in fish. A fish may be able to tell if other swimming fish are approaching, receding, or moving tangentially by analysing the proportions in time of the compressions and rarefactions present in the swimming sounds, which are proposed to be asymmetrical.
