PHYSIOLOGICAL EXCITATION OF VISUAL CORTEX IN CAT'S UNANAESTHETIZED ISOLATED FOREBRAIN

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Abstract
The responses of single neurons in the visual cerebral cortex to excitation of the retina with patterned light have been studied. Eye movements were prevented with gallamine. All visual units examined displayed continual activity in the unstimulated preparation. An appropriately located change of retinal illumination, produced by movement or flashing of a pattern, would drive these units away from their mean frequency of discharge for a short time. Stationary, continuously illuminated patterns produced no detectable influence upon the behaviour of cortical neurones except at ON and OFF. The behaviour of cortical units was not the same following each of a series of identical retinal stimuli. The relation between stimulus and response was therefore measured in terms of the probability that a visual unit would fire at various times after retinal excitation. The mean frequency of discharge per minute of visual units is commonly unaffected by repetitive retinal excitation. Cells that gave ON responses to patterns in one part of the visual field often gave OFF responses to the same pattern in another part of the field. Small cyclical oscillations in the position of retinal patterns provided an extremely efficient way of exciting cortical units. Oscillations of a black-white border (in one line at right angles to the boundary) were most exciting when the amplitude was 50 min. arc and the frequency was 2-4c./sec. The smallest movement producing adetectable cortical response was 2-5 min. arc at the retina, or about the diameter of one cone. During stimulation with an oscillating black-white border, the greatest excitation is produced by the boundary between black and white regions.