Acoustic response properties of single units and unit clusters in the midbrain of the goldfish.

Abstract

Single units and small unit clusters were recorded from the torus semicircularis (TS) of the goldfish using indium electrodes. Response areas (RA) were formed as iso-level, spike count functions (75 to 1250 Hz) at 5-dB intervals throughout the unit’s dynamic range. Spike count versus level functions and tuning curves were obtained from the RAs, giving characteristic frequency (CF), best sensitivity (BS), and Q10dB. Peri-stimulus time (PSTH), interval, and period histograms were determined at CF and other frequencies. Units fall into three clusters with CFs at about 150, 400, and 1000 Hz, as is the case for saccular afferents. Tuning tends to be sharper in the TS than in the periphery. Lowest BS as a function of CF corresponds closely to the behavioral audiogram. Inhibitory effects are suggested by nonmonotonic level functions and by suppression of spontaneous activity. Slopes of spike count versus level functions are steep near CF and much shallower above and below CF. The first units encountered in a dorso-ventral electrode track tend to be tonic, short latency, and phase lock as accurately as saccular afferents. These are often two-unit clusters with different CFs but with phase- locked spikes 180° out-of-phase. Encountered next are more phasic, longer-latency responses with little phase locking, and occasionally with complexly shaped PSTHs. The frequency at which iso-level functions peak is generally independent of level, unlike saccular afferents. Thus frequency is far better represented by spike rate in the TS than in primary afferents. [Work supported by a Center Grant from NIDCD.]