Peripheral Crowding Magnitude is Similar Under Photopic and Scotopic Luminance Conditions

Visual crowding is the inability to distinguish a stimulus (target) in the presence of surrounding objects (flankers) and impacts various visual functions. Some of these functions are influenced by the luminance level of the environment, such as visual acuity, and contrast sensitivity. However, the potential interactions between crowding and luminance remain unclear. In this study, we explored the magnitude of crowding under five different luminance conditions, ranging from photopic to scotopic levels. Eight participants were asked to report the orientation of the gap in a white Landolt-C (2°diameter) on a gray background positioned at 10° eccentricity. The target Landolt-C was either presented alone or flanked by four Landolt-C’s (2°diameter) placed at one of five center-to-center distances (2°,2.52°,3.22°,4.18°,5.5°). Participants responded by adjusting the orientation of a reference Landolt-C gap (5°diameter) placed at 0° eccentricity. Goggles with neutral density filters were used to manipulate stimulus luminance (0.02 to 200 cd/m²). Landolt-C gap sizes were individually adjusted to equalize acuity performance at isolated levels across luminance conditions. Participants' perceptual error (PE) was calculated based on the difference between their response and the actual orientation of the target gap. Crowding magnitude was defined as PE flanked - PE isolated for each luminance and distance conditions. Results showed that crowding magnitude was similar under all luminance conditions (BF10=0.23), while a monotonous decrease in crowding magnitude was revealed as the target-flanker distance increased in all luminance conditions. Moreover, participants’ PEs were similar in all isolated conditions, confirming similar acuity performances (BF10=0.18). Our results support the idea that the same neural mechanisms operate under scotopic and photopic conditions for peripheral crowding. Additionally, our results align with prior research on contour interaction in scotopic conditions, suggesting a shared underlying process between contour interaction and crowding.