Electrophysiology and anatomy of direction-specific pretectal units in Salamandra salamandra

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Abstract
Pretectal cells of the European fire salamander were recorded extracellularly during binocular and monocular horizontal optokinetic stimulations. The locations of the individual units within the pretectal nucleus were verified with Alcian blue injections. The particular anatomical properties of single cells were demonstrated after single-unit recordings by means of horseradish peroxidase preparations. Direction selective pretectal cells were found to be predominantly (2/3) sensitive to temporo-nasal movements in the visual field of the contralateral eye. They usually possessed large receptive fields centered on the visual equator with restricted diameters in the dorso-ventral axis. Their resting discharge was low, and in some cells no spontaneous discharges were observed. The cells preferred low stimulus velocities, most of them being optimally stimulated with velocities of 1 to 10 deg/s. A group of units was exclusively sensitive to accelerated movements. A subclass of them was transiently responsive when the stimulus stopped. In the anterior and most dorsal part of the pretectal nucleus, binocularly influenced units were found. These cells responded best with binocular optokinetic stimulations and less vigorously or with less pronounced direction selectivity if only the contralateral eye was stimulated. With ipsilateral stimulations alone no response could be elicited. This response type could be explained by inhibitory inputs from the ipsilateral eye via direct ipsilateral projections or crossing pretectal fibers. The responses of these cells are well correlated to behavioral results showing that OKN performance in salamanders, as in some other vertebrates, is different with binocular as compared to monocular stimulations. The direction-sensitive pretectal cells usually possess extensive dendritic arborizations within the ipsilateral pretectal neuropil. Most of the cell bodies were scattered in the white substance or in the superficial layers of the periventricular gray. In the cases where the efferent fiber of a particular cell could be clearly recognized, the axon projected to the basal optic neuropil of the accessory optic system, the contralateral pretectum or, in two cases, to the medulla oblongata into a region which might be homologous to the inferior olive of higher vertebrates.