Acousticolateral processing in the torus semicircularis of the trout Salmo gairdneri

Characteristics of single units in the trout torus semicircularis to acousticolateral stimulation were investigated with tone bursts. Auditory (A) units (33%, N = 67), exclusively sensitive to frequencies higher than 125 Hz, and low frequency (L, 12%) units responding to tones up to 125 Hz were found next to the third type, the broadband (B,55%) unit, sensitive to both low and high frequencies. For many B units sensitivity to intermediate frequencies is absent. The sensitivity range of 85% of the units does not exceed 450 Hz. Most units (70%) show an increase in spike firing during the tone burst irrespective of frequency (ON units), but for a minority (9%) firing is maximal in the silent intervals (OFF units). OFF units are mostly found among A units. All L units are ON units. The remaining ON/OFF (ON behaviour for some and OFF behaviour for other frequencies) or ON + OFF (ON and OFF behaviour for all frequencies) units are all B units. Tonic ON units are most common. Some units show habituation and only sensitive ones may exhibit phase-locking. The different behaviour of some B units for low versus high frequencies, as expressed in their ON and OFF behaviour and latencies, presumably reflects convergence of inputs from acousticolateral subsystems with distinct properties.