Abstract

Psychophysical measurements using visual thresholds have revealed a course of dark adaptation in which the threshold rises before cessation of the adapting light, then falls during dark adaptation. In the present research, measurements were made of the ERG to brief stimulus flashes before and during very early dark adaptation, using a contact-lens electrode and a precisely timed light stimulator to investigate this peculiar effect. No significant decrease of the b-wave potential could be detected prior to the onset of darkness, but the b wave showed a rapid increase in response after the cessation of the adapting light. Although the latency of the b wave to a weak test light is prolonged, it is not long enough to be correlated with the early threshold rise, which occurs as much as 0.2 sec before darkness. The discrepancy also seems to preclude any suggestion that the threshold rise can be due to inhibition by the negative potential of the off effect.

Relationships between x and b waves during dark adaptation were also studied, particularly with a red test light. The data emphasize that the human ERG is a dual manifestation of both scotopic and photopic functions of the retina.

© 1963 Optical Society of America

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