Auditory responses in the torus semicircularis of the cane toad, Bufo marinus. II. Single unit studies

The responses of single units to brief tone bursts presented in the free field have been recorded in the torus semicircularis of the toad, Bufo marinus. The characteristic frequencies of the units fell into three main groups. The peri stimulus time histograms (p.s.t.h.s) for units in the torus showed a variety of forms but most histograms had a peak near the beginning of the response. The latencies of these peaks varied between 10 and 90 ms after the onset of the stimulus. As the intensity of a stimulus at the characteristic frequency was increased, the number of spikes elicited increased and the latency of the first peak in the p.s.t.h. decreased. All units that exhibited a high sensitivity to changes in stimulus intensity had characteristic frequencies in the low region of the auditory range and relatively long latencies. The number of spikes elicited and the latencies of the responses also varied as the location of the stimulus was changed. For those units whose spike output was highly sensitive to stimulus direction, the latency of the response was longer and showed greater variation with stimulus direction than the less sensitive units. The characteristic frequencies of the more highly sensitive units were also restricted to the low region of the auditory range. The properties of units with a high sensitivity to stimulus intensity were similar to the properties of units that were highly sensitive to stimulus direction. The difference between these properties and those of units with lower sensitivity to stimulus intensity and direction suggests that the auditory system in the brainstem of anura may be composed of at least two functionally distinct pathways. One of these pathways probably involves a more complex neuronal integration of the afferent input following low frequency stimulation.