Visual motion priming reveals why motion perception deteriorates during mesopic vision.

We know empirically that the perception of moving objects deteriorates under mesopic vision, in which both rods and cones operate. The purpose of this study was to examine the cause of this degradation. We utilized a phenomenon called visual motion priming, in which the perceived direction of a directionally ambiguous test stimulus is influenced by the moving direction of a preceding stimulus. The spatial distances between the priming and the test stimuli were varied. At mesopic light levels, a stimulus that is presented at the central retina is presumably processed by the cone system, while a stimulus that is presented at the peripheral retina is processed by the rod system (Raphael & MacLeod, 2011). Subjects judged the perceived direction of 180° phase-shifted sine-wave grating that was followed by a smoothly drifting priming stimulus under different retinal illuminances. We found that, under mesopic conditions, the strength of motion priming was greatly reduced when the priming and test stimuli were presented separately at the center and the periphery, respectively. In contrast, motion priming was perceived in most of the trials under photopic and scotopic conditions or when both the priming and test stimuli were presented at the central retina under mesopic conditions. When the priming and test stimuli temporally overlapped, motion priming was conspicuous irrespective of the retinal illuminance. These results suggest that the incompleteness in the integration of signals that was induced by the temporal delay of rod pathways caused the degradation of motion perception under mesopic vision.