Central control of auditory input in the goldfish. I. Effect of shocks to the midbrain.

Abstract

ABSTRACT Shocks applied to three midbrain regions reversibly suppressed the click-evoked action potential at the medulla. Clicks in air did not activate lateral-line fibres. Only one midbrain region, the torus semicircularis, showed a distinct afferent auditory response; the other two regions (anterior optic tectum and midbrain below) were used to avoid unwanted antidromic activation of medullary afferents by shocks to the torus. Muscular action on the mechanical auditory pathway was excluded by showing that shocks had no effect on the saccular microphonie or receptor potential. The results are consistent with the action of a descending inhibitory pathway that suppresses auditory input either at the medulla or at the sacculus. One brief shock is effective, but the system is mainly dependent on temporal and spatial summation. The dependency on click intensity may be complex, both high-threshold and low-threshold fibres can be inhibited and occasionally a component of the compound spike shows facilitation instead. Shocks affect the two-click recovery cycle in a complex maimer. Recovery is enhanced if the clicks are presented less than 40 msec after the shocks but depressed at longer intervals, even beyond 500 msec.