The Site of Visual Adaptation

In response to background illumination, the adaptation properties of the b-wave are similar to those observed in the human eye with psychophysical methods. With increasing background luminance the b-wave sensitivity is diminished; except at the lowest background intensity the elevation of the log threshold is linearly related to the increase of background intensity, the relation having a slope of almost 1. The a-wave, however, behaves quite differently. At low background luminances it shows little adaptation. With higher background luminances the awave saturates, and no a-wave potential can be elicited with any stimulus intensity. The L-type S-potentials respond to background light in much the same way as the a-wave does. Thus, the b-wave is the first of the known responses in the visual system to show typical adaptation properties. This suggests that the site of visual adaptation may be in the bi-polarcell layer, the presumed locus of b-wave generation. Recent electron microscopic studies have demonstrated reciprocal synapses between the bipolar terminals and amacrine processes, and it is suggested that such a synaptic arrangement could account for visual adaptation by a mechanism of inhibitory feedback on the bipolar cells.