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Journal of Experimental Biology 144,357-375 (1989)
Published by Company of Biologists 1989

Response Features of Visual Units in the Lower Midbrain of the Rainbow Trout

NICO A. M. SCHELLART ¹ and WALTHER E. I. RIKKERT ¹

¹ Laboratory of Medical Physics, University of Amsterdam, Academic Medical Centre, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands

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.

Key words: visual system, torus semicircularis, receptive fields, tectum, trout

Accepted on March 20, 1989

This article has been cited by other articles:

				R. J. Wubbels and N.A.M. Schellart Neuronal Encoding of Sound Direction in the Auditory Midbrain of the Rainbow Trout J Neurophysiol, June 1, 1997; 77(6): 3060 - 3074. [Abstract] [Full Text] [PDF]