The effect of borders and contours on threshold during early dark adaptation

The present series of experiments were designed to investigate whether the drop in threshold during early dark adaptation was due to activity within a receptor (or within the direct neural channel from receptor to higher visual centers) or whether the drop in threshold reflected the decay of activity extending over a lateral spatial region. The results show that conditions maximizing lateral effects increased the magnitude of the drop in threshold during early dark adaptation; and that spatial integration of light rapidly changed during the course of early adaptation. It is concluded that early dark adaptation reflects a mechanism sensitive to the spatial distribution of light and consequently rules out an interpretation of early dark adaptation as a process arising solely within a receptor. It is also argued that the mechanism responsible for these changes may not be retinal.