Neural limitations of visual excitability. VII. Nonhomonymous retrochiasmal interaction

Abstract

Visual excitability changes were obtained from two trained observers by measuring threshold with a brief test flash of light at varying temporal intervals from the onset of a longer, supraliminal conditioning flash. The test target was either a 1°20' or a 5°20' solid disc, and was exposed to the right eye at 6°40' along the horizontal meridian in the temporal half field. The conditioning target was a 6°40' disc containing central masks of various angular subtenses, and was so placed in the left eye that the resulting annuli surrounded the test target in the binocularly fused field of view. For all conditions, threshold rose when test preceded conditioning flash in time, reaching a maximum at about o-msec interval between onsets. As the test flash was progressively delayed in time, thresholds fell to an asymptote, returning to resting level after termination of the conditioning flash. The smaller the mask diameter in the center of the conditioning target, i.e., the tighter the fit around the binocularly fused test target, the greater the maximum rise in threshold in the other eye. With small test targets, greater threshold changes were produced by conditioning targets with small masks than by those without a mask (solid targets). Observations on a third observer indicate that similar findings obtain when conditioning stimuli are equated for total luminous flux. These data indicate that retrochiasmal interaction is greater at target borders, and are compatible with recent unit studies on the cortical receptor field.