Neural limitations of visual excitability: After-effects of subliminal stimulation

Abstract

Changes in the threshold luminance for a 5 msec, 30′, “test” flash ( F t ), were studied as a function of temporal delay from a 5 msec subliminal (55% of threshold) “conditioning” flash ( F c ), set at either 30′, 45′, or 60′ in diameter. In one stimulating mode (“monocular”), both F c and F t were exposed to the same (right) eye; in another (“interocular”), Fc was presented to the left and F t to the right eye. Temporal summation was obtained under both modes, and was greater in magnitude for monocular stimulation, was independent of F c diameter, but was specific for the same (monocular), or homotopic (interocular) retinal locations. In all cases, perfect summation (energy additivity) was limited to about 20 msec interflash interval, and was followed by an additional 30 msec of partial summation. Collectively, the foregoing findings imply that central neural mechanisms are involved in determining threshold sensitivity.