Abstract
From a bright day to a dark night, extreme light levels may deteriorate our vision. Daylight vision is mediated by the cone's photoreceptor, accumulates in the center of the retina (fovea), and is responsible for maximal resolution and form vision. At low light levels, it is considered that vision is mediated by the rod's photoreceptors (spread in the retina's periphery) which are less sensitive to resolution. Studies suggest that foveal neuronal mechanisms are vital for visual processing at low light levels. We explored the central visual processing underlying range of light levels. Adult observers (N=10) with normal or corrected-to-normal vision were tested. The luminance levels were controlled using natural density filters (Rosco Laboratories) and measured using a photometer. The tasks were 1) Identifying the direction of an E target in a clutter (crowding) under daylight (50 cd/m2) and low luminance (0.003 cd/m2) for different target-flankers-spacing. 2) crowding at low contrast that was adjusted individually for each subject's threshold. 3) measuring lateral masking under low luminance using Gabor patches. The foveal crowding effect under daylight was significantly increased at a target-flanker spacing of half-letter. Interestingly, the crowding effect was abolished under low luminance and even performed with some facilitation. Crowding at low contrast (letter and distractors) task showed no crowding effect. Results of lateral masking under low luminance showed significant facilitation for all tested target-flanker separations for the collinear but not for the orthogonal configurations. The absence of crowding and the existence of collinear facilitation at low luminance is consistent with the results of an absence of crowding for low contrast, suggesting less spatial suppression under night vision. Thus, under challenging visual conditions, such as low luminance, foveal vision may be able to process form by increasing the excitatory level to increase spatial summation at the fovea rather than the periphery.