Neural limitations of visual excitability. IV: spatial determinants of retrochiasmal interaction

¹ Department of Neurology, Mt. Sinai Hospital, New York City

Visual excitability changes were obtained from two trained observers by measuring threshold with a test flash of light at varying temporal intervals from a supraliminal conditioning flash. In monocular observation the two flashes were presented to the same eye; in binocular observation the conditioning flash was exposed to one eye and the test to the homonymous location in the other eye. The conditioning target size was varied while the concentrically placed test flash was held constant. In all instances, threshold rose when test preceded conditioning flash in time, reaching a maximum at about a o-msec interval. As test flash was progressively delayed with respect to conditioning flash onset, thresholds fell to an asymptote, returning to resting level only after termination of the conditioning flash. Both monocularly and binocularly, an increase in the magnitude of threshold rise was produced by making the conditioning target smaller, the greatest proportionate effect being obtained binocularly. These findings indicate that central (retrochiasmal) processes are critical with respect to spatial interaction in the visual system, a conclusion compatible with recent studies on the cortical receptor field.

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