William S. Battersby

Neural Limitations of Visual Excitability. I. The Time Course of Monocular Light Adaptation

Monocular visual excitability was studied psychophysically by determining the threshold intensity for a small (40-min) brief (5-msec) “test” flash centered within a larger (3°) “conditioning” flash, both presented at 7° of arc in the temporal half-field. Interval between flashes was varied from −200 (test flash preceding onset of conditioning) to +1500 msec (test flash following), nine combinations of conditioning flash intensity and duration being used as parameters. For all combinations, threshold rose between −200 and −100 msec (before the start of the conditioning flash), reaching a maximum increment when the onsets of the two flashes were synchronized. When test was superimposed upon conditioning flash in time (positive intervals), threshold declined to approach an asymptote near the end of the latter flash. Following termination of the conditioning flash, threshold returned to resting level. The maximum rise in threshold, as well as the time taken to achieve an asymptote, varied with both intensity and duration of the conditioning flash. When a rough estimate of the photochemical contribution to these threshold changes was subtracted from the raw data, residual threshold increments were obtained and attributed to neural processes. On the basis of time course, it is suggested that these neural processes involve central (geniculocalcarine) as well as peripheral (neuro-retinal) factors.

Neural limitations of visual excitability V. Cerebral after-activity evoked by photic stimulation

Abstract In locally anesthetized and in lightly anesthetized cats, a 5-msec flash of light to the eye evokes from the visual cortex a “primary” potential, followed by a train of after-discharges which may last 300 msec or longer. The duration, frequency and amplitude of after-discharges are variable, but it is clear that the total duration becomes longer with increasing intensity or duration of the photic stimulus. These after-discharges can be disrupted by the response to a subsequent longer flash when the onset of the second flash occurs before the total response to the first flash is over (i.e. within 300 msec). If one plots, on the ordinate, the reduction in the duration of the after-discharges elicited by a preceding test flash and, on the abscissa, the conditioning flash-test flash interval, the curve is similar to those obtained in previous studies of the psychophysical threshold in the human subject. These findings suggest that the perception of a single flash of light is related to the whole complex of waves elicited by a light flash and not just to the “primary” response.

Neural limitations of visual excitability VIII: Binocular convergence in cat geniculate and cortex

Abstract Excitability cycles for the lateral geniculate body and the visual cortex were obtained in 49 locally anesthetized-immobilized cats, by plotting the response changes to a test Mash ( F t ) against the time interval from a supraliminal conditioning flash ( F c ), presented either to the same (monocular stimulation) or to the contralateral (interocular stimulation) eye. For both forms of stimulation, the conditioning flash response disrupted the test flash response, either in amplitude or duration, depending upon the temporal interval. With monocular stimulation, increasing either F c duration or luminance prolonged the recovery cycle at both cortex and LG. With interocular stimulation, however, cortical recovery was more rapid, and only slightly affected by F c parameters. Interocular interaction could not be demonstrated at LG with closely spaced bipolar derivations. These findings suggest that both peripheral and central processes limit visual excitability, cortical rather than geniculate mechanisms being more significant in the latter instance.

Light Adaptation Kinetics: The Influence of Spatial Factors

Reducing the target diameter of an adapting (conditioning) flash of light results in a progressive rise in the conventional light adaptation curve, as measured with a small superimposed test flash presented at the end of adapting flashes of variable duration. When both targets are the same size, an abrupt and marked rise in threshold is obtained, resulting from a unique effect that occurs near the termination of the adapting flash. This effect can be demonstrated by means of a variable delay procedure, and it indicates that neural as well as photo-chemical processes limit the time course of light adaptation.