Peripheral crowding is invariant under different luminance levels

Peripheral crowding is the difficulty in recognizing objects in cluttered peripheral vision, affecting tasks like reading, visual search, and driving. The luminance of the environment also plays a significant role in these tasks. However, the potential interaction between crowding and luminance is still not clear. This study examined peripheral crowding (10 deg) by measuring its magnitude and critical spacing across a wide range of luminance levels, spanning from scotopic to photopic conditions. Ten participants binocularly identified the orientation of a target presented alone or flanked by four flankers. Responses were made by adjusting a central reference stimulus. Luminance was manipulated using neutral-density filter goggles (0.02-200 cd/m²). The distribution of differences between reported and actual target orientations was obtained, and the standard deviation of these distributions was taken as the perceptual error for each condition. Crowding magnitude was calculated by dividing perceptual error in the flanked condition by that in the isolated condition. Critical spacing was defined as the maximum separation between target and flanker for crowding to occur. It was calculated by fitting a hinged-line to the perceptual errors as a function of target-flanker-distance. Crowding magnitude and critical spacing were both measured for flankers with either no gap, a gap with a size matching each individual's luminance-specific acuity threshold, or a suprathreshold (200 % of worst-acuity) gap. Results showed that both crowding magnitude and critical spacing were similar under all luminance and gap conditions. Our findings suggest that the same peripheral crowding mechanisms operate under scotopic and photopic conditions.