Onset Of Flash Research Articles

Neural limitations of visual excitability. IV. Spatial determinants of retrochiasmal interaction.

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.

Neural limitations of visual excitability: II. Retrochiasmal interaction

Binocular visual excitability changes were obtained from two trained observers by presenting a "conditioning" flash of light to one eye and a brief "test" flash to a homologous retinal position in the other. Interval between flashes ranged from –200 msec. (test flash preceding conditioning) to +600 msec. (test following onset of conditioning flash). Results showed that threshold rose when test preceded conditioning flash, reaching a maximum increment of 0.5 log ml at –25 msec. separation. When test was superimposed upon conditioning flash in time, threshold dropped sharply to reach a relatively constant but elevated value for the duration of the conditioning flash. No excitability changes were obtained, however, if the test and conditioning flashes were displaced so as to stimulate adjacent rather than "corresponding" retinal points. These findings indicate that central (retrochiasmal) factors can influence visual sensitivity, the magnitude of this influence varying with the spatial and the temporal relations between successive light flashes. This conclusion is compatible with recent electrophysiological data on excitability changes in the cerebral visual system.

Effect of flash and field luminance upon human reaction time.

Reaction time is measured from the onset of a signal flash to the manual release of a microswitch. Responses are made by two trained subjects over wide ranges of stimulus intensity and under varying conditions of signal flash and background field luminance. Two types of experimental conditions prevail: (1) variable flash luminance at each of three field levels, and (2) variable field luminance at three constant flash intensities. Thirty-six reactions are collected for every flash-field combination at each of two fixation positions (foveal and peripheral).The results are described by the following equation: T=1B logI/I0+K,where B, I0, and K depend upon the field luminance.

The effect of duration of onset and cessation of light flash on the intensity time relation in the peripheral retina.

Two experiments are reported. The first experiment studies the intensity-duration function for peripheral threshold stimulation of the human eye, over a range of durations from 0.02 second to 0.24 second. The form of the flashes presented involves a rapid onset and cessation (i.e., a nearly rectangular wave). The second experiment determines the total energy required for threshold excitation for flashes of various wave forms (i.e., with different onset and cessation times), the maximum duration always being below the critical duration obtained in the first experiment.The results of the first experiment agree with previous studies in showing that below a certain critical duration the product of light intensity and duration is constant. Above this critical duration the intensity remains constant. The transition from the relationship It=C to I=Const. is abrupt and occurs at 0.10 second.The results of the second experiment indicate that changes in the wave form, within the limits of variation introduced, do not affect the total amount of energy required for threshold below the limit of a critical duration. These findings give additional generality to the Bunsen-Roscoe law and to the view that the essential element in producing a constant effect in response to flashes of light is the total energy of the stimulus.