ALBERT

Notes: Summary 1.600 neurons of the cat's superior colliculus and pretectum were recorded and marked with stainless-steel microelectrodes. The position and structure of receptive fields were tested with stationary flickering and moving stimuli. The position of the stimuli in the visual field was determined by the direction of the lamp projecting the light-points because animal and lamp were arranged in a fixed relationship to the screen. The positions of the stimuli were described in a coordinate system based on the horizontal-and vertical zeromeridean of the retina. 2.About 55% of tectal neurons are directionally selective and signal mainly movements directed to the periphery of the visual field. Neurons of the pretectum have the same response characteristics as neurons of the superior colliculus but in addition some are selectively responsive to movements towards the animal or away from it (S-neurons). 3.Neurons in one functional column (diameter 0.5 mm, length 3.6 mm) perpendicular to the surface of the superior colliculus react to the same position and preferred direction of a moving stimulus. The size, complexity and directional selectivity of the receptive fields increase with the depth of the recorded neurons. The projection of the vertical zero-meridean passes across the rostral part of the colliculus but does not form the rostral border of the superior colliculus. The nasal part of the ipsilateral visual field projects to the most rostral part of the superior colliculus. The projection of the horizontal zeromeridean passes rostro-caudally in a nearly sagittal plane down the middle of the colliculus. Along this projection-line the resolving power is 13°/column in the caudal part and 6°/column in the rostral part of the superior colliculus. The size of the receptive fields increase with their excentricity in the visual field. (Average of field diameters: 26±13°). 4.The diameter of receptive fields in the pretectum was 21±11°, except for a few very large fields (70° and larger). Along the medio-lateral axis of the pretectum there was a retinotopic organization identical to that in the colliculus. Along the caudo-rostral axis, the retinotopic organization was the mirror image of that in the colliculus. No retinotopic organization was observed in the so-called “deep pretectal nucleus” or in the nucleus of the posterior commissure. Neurons of these nuclei may represent more complex levels in the visual pathway. 5.The more superficial neurons of the colliculus (0.1–1.8 mm deep) react mainly with uniform on-, off- or on-off responses to stationary flickered stimuli, i.e. their receptive fields (7–20° in diameter) are uniformly on-, off- or on-off. The deeper neurons (2 mm and deeper) have receptive fields (20–40° in diameter) with compound but not antagonistic structure. No receptive fields showed on- or off-inhibition. Binocularly driven neurons have the same position and structure of their receptive fields for both eyes. 6.A survey of the literature reveals that all vertebrates so far investigated show small differences in the destination and retinotopic organization of their retinofugal fibre projections and in the types of tectal receptive fields. These differences seem to indicate an adaption to the development of binocular representation of the center of the visual field, of a specialized area of the retina and of a retino-cortical system.