Photopic Mechanisms of Early Dark Adaptation*

Abstract

Adaptive equivalence has been demonstrated by Crawford for long-term dark adaptation; he showed that after the observer remained a certain time in the dark, increment thresholds equivalent to the absolute threshold were the same for all target diameters used and indicated that a single variable controls the spatial integration of light. Equivalence was investigated in the present study for early dark adaptation using two target diameters (0.25 and 0.96 deg) and four combinations of chromatic test and adapting stimuli (red on red, blue on blue, red on blue, and blue on red). In general, the results showed that adaptive states as produced by steady and transient conditions of adaptation do not appear to be equivalent during this early period of dark adaptation, and indicated that more than one process controls the spatial integration of light under the conditions investigated. It is hypothesized that these processes are excitation and inhibition in the visual receptive field. One subject, however, showed approximate equivalence for the homochromatic conditions (red on red and blue on blue), but none for the heterochromatic conditions (red on blue and blue on red) indicating perhaps interaction between color mechanisms. Spatial effects were also noted as a result of varying target size; these effects suggested lateral interaction of groups of receptors rather than the activity of single receptors during early dark adaptation.