Early visual adaptation in goldfish retinal ganglion cells

Abstract

THE human visual system responds with transient losses of sensitivity when background light levels are suddenly increased or decreased. Crawford¹ measured the early threshold changes in humans by means of a small circular test flash presented before, during and after presentation of a larger diameter adapting light. He found that threshold increased just before the onset of the adapting light, reached a peak shortly after onset, and then decreased during the subsequent 0.5 s. Immediately before the offset of the adapting light, threshold increased slightly, peaked at offset and then returned to its original level. Other investigators have reported essentially the same results under various conditions^2–4. The increase in visual sensitivity during the first few seconds of light adaptation and the peaked loss of sensitivity at the offset of the adapting light are in the wrong direction as predicted by bleaching of the photopigments. Consequently, Baker⁵ suggested that neural factors account for the early change in visual adaptation. Unfortunately, the question of mechanisms involved to account for these rapid changes in visual sensitivity cannot be answered from psychophysical experiments since the response range cannot be assessed during these periods.