Effects of background light on the human dark-adapted electroretinogram and psychophysical threshold

We compared the effects of background light on the sensitivities of two components of the human electroretinogram, the cornea-negative scotopic threshold response (STR) and the cornea-positive PII (beta wave), as well as on the psychophysical sensitivity in a ganzfeld. The background illuminance necessary to reduce the STR (an inner retinal signal) measurably was approximately five times greater than that needed to raise the psychophysical threshold. A background illuminance at least 1 log unit greater still was needed to reduce PII (a signal-reflecting activity of bipolar cells). These findings suggest (1) that the weakest backgrounds that reduce retinal sensitivity have their effect at a site that is proximal to the bipolar cells, a site that involves amacrine or ganglion cells, and (2) that very weak backgrounds have their effect on visual sensitivity at a site more proximal than the scotopic threshold response generator and perhaps more central than the retina.