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Visually evoked potentials and single unit activity in the superior colliculus of the cat.

In contemporary investigations of visual information processing the focus of interest has been shifted from the lateral geniculate nucleus (LGN), toward the tectum and pretectum. It has turned out that neurons of the cat's tectum and pretectum are more sensitive to moving stimuli than to changes in diffuse illumination Further observations demonstrated that there is a basic difference in the structure of receptive fields of cells in the visual centers mentioned above: the fields of tectal cells appeared to be homogeneous, i.e. stimulation of each part of receptive field evoked approximately the same responses (Harutiunian-Kozak et. al. 1968a); there is no classical concentric arrangement of the field consisting of a central area and an antagonistic periphery so typical for the cells in the retina and the LGN (Kufller 1953, Hube'l 1960, Wiesel 1960, Kozak et al. 1965). More recent investigations have shown that most neurons in the superior colliculus were direction-sensitive, i.e. they responded with increasing rate of their action potentials to stimuli moving in one preferred direction and their activity was suppressed when the stimuli moved in the opposite direction (the null direction-Barlow and Levick 1965). This phenomenon has been described by many authors in many species: in the frog's retina (Maturana et al. 1960), in the rabbit's retina and LGN