The effect of field size and pattern on the change of visual sensitivity with time

Abstract

Results are recorded showing the effect of size of a circular conditioning field on brightness threshold measurements made at the centre of the field : first, with the field steadily exposed and viewed ; when the conditioning field has been cut off and the eye is returning to a state of dark adaptation. Conditioning and test fields are both illuminated by white light. No field size effect is detectable over the range 60° diameter down to 3° diameter. Below 3°, for a steady state of adaptation, various rather complex effects of interference between the conditioning fields are shown ; these are explained in terms of the effect of distant parts of the conditioning field in raising the threshold and a perceptual interference between the conditioning fields are shown ; these are explained in terms of the effect of distant parts of the conditioning field in raising the threshold and a perceptual interference between the patterns of conditioning and test fields. For changing states of adaptation the effect of field size cancels out─except in an isolated case─if the initial conditioning field brightness are adjusted to give the same steady value of threshold : then, after cutting off these various conditioning fields, the decrease of brightness threshold follows the same course. However the test patch of retina be brought to a given steady state of adaptation, as measured by the brightness threshold, it will recover its dark adaptation in the same way. These considerations apply both to foveal vision and to vision in the parafovea at 8° from the fovea.