Acoustic response properties of single units in the torus semicircularis of the goldfish, Carassius auratus.

Single units of the goldfish torus semicircularis (TS) were recorded in response to pure tones. Response areas (RA) were obtained by recording the number of spikes evoked by tones in a range of frequencies and levels within the units' dynamic range. RAs gave estimates of best sensitivity (BS), characteristic frequency (CF), most excitatory frequency at each level (BF), and Q10dB. Peri-stimulus-time histograms (PSTH), interspike interval histograms (ISIH), and period histograms were obtained at various frequencies and levels to describe the units' temporal response patterns. The distribution of CF is nonuniform with modes at 155, 455, and 855 Hz. The distribution of the coefficient of synchronization to standard tones is also nonuniform, revealing a dichotomy between units with little or no phase-locking and those that phase-lock strongly. PSTHs for units without significant phase-locking vary widely and include patterns resembling those of the mammalian auditory brainstem. Compared with saccular afferents, torus units tend to have lower spontaneous rates, greater sensitivity, and sharper tuning. Unlike saccular afferents, BF is independent of level for most torus units. Some torus units are similar to saccular afferents while others reveal significant transformations of information between the periphery and the midbrain.