In the mesencephalic torus semicircularis of fish, visual processing is carried out in addition to acousticolateral processing, and takes place in unimodal and bimodal units. To study the encoding of visual stimuli in the torus of the rainbow trout, unit recordings made in the torus were compared with recordings made in the tectum.

The receptive fields (RFs) of the torus units in the rainbow trout (Salmo gairdneri) mostly measured more than 80°, generally had no well-defined borders and showed non-X type behavior. As in the tectum, the majority of the units showed On-responses and the RFs of On units were smaller than those of On/Off and Off units. From single unit and evoked potential data, obtained up to an eccentricity of 45°, it appears that the rostral visual field projects to the rostral and central part of the torus. In contrast to the features of tectal units, the RFs lacked spatial antagonism, wavelength coding was practically absent and directional selectivity rare. The responses, generally with latencies between 40 and 160ms, were often erratic. The most striking neurophysiological feature of the torus units as habituation to repetition of the stimulus, e.g. to an on-off flashing spot. The visuo-auditory interaction of most bimodal units showed algebraic summation, but for about 20% the visual modality enhanced the sensitivity to auditory stimulation.

The data from the trout confirm that the tectum plays an important role in visual functions such as acuity, pattern vision and vision of movement. The common feature of habituation in the torus suggests that the torus may be involved in signalling of sudden changes in the visual scenery.

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