Quan Lei¹, Taylor Shupsky¹; ¹Wichita State University

Humans can simultaneously track a limited number of moving objects, a capability known as multiple object tracking (MOT). Whereas many visual functions are compromised under low lighting conditions, it is yet unknown how MOT performance is affected by reduced illumination. This study was aimed to measure MOT performance under mesopic illumination and examine the role of object contrast and spacing. In Experiment 1, participants tracked 1-4 targets out of 10 objects (dots of 1° diameter with a Weber contrast of 0.1) moving along random trajectories (speed: 6°/s) within a 30°×30° field for 6s, after which they were probed on one of the objects (target or distractor). They performed the task under either photopic (~100 cd/m2) or mesopic illumination (~1 cd/m2) implemented by wearing goggles fitted with neutral density filters. Both target number and illumination level had an effect on tracking performance (percent correct of target identification). There was also a significant interaction between target number and illumination level, such that tracking performance was similar between photopic and mesopic illuminations when tracking only one target but performance was worse under mesopic illumination than photopic illumination when tracking more than one target. In Experiment 2, participants performed the same MOT task but with increased contrast of the objects (Weber contrast of 1); in Experiment 3, object contrast being low at 0.1, a minimal spacing between objects (which equals eccentricity*0.4 following Bouma’s law) was dynamically maintained throughout the motion phase. In both experiments, there was a main effect of target number on tracking performance, but more importantly, there was neither an effect of illumination nor an interaction between target number and illumination. The results suggest that illumination level is a limiting factor in MOT and the performance decrement under mesopic illumination might reflect a data-limited process due to reduced contrast sensitivity and acuity.