Abstract
We know empirically that perception of moving objects deteriorates in the dusk. The purpose of this study is to reveal the reason of such sensitivity degradation under mesopic vision, when both cones and rods operate. For the purpose, 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 priming stimulus. Participants judged the perceived direction of 180 deg phase-shifted, thus directionally ambiguous, sine-wave grating (test stimulus) followed by a smoothly drifting priming stimulus under three different retinal illuminance levels: photopic, mesopic, and scotopic levels, respectively. The spatial distance between priming and test stimuli was varied from 0 deg to 10 deg in visual angle. When the stimuli were high-contrast, the test stimulus was perceived to move in the same direction as the primer (positive priming) under photopic level, while the test stimulus was perceived to move in the opposite direction of the primer (negative priming) under scotopic level. Neither positive nor negative priming was observed under mesopic level. When the stimuli were low-contrast and spatially separated, however, only negative priming was observed regardless of the retinal illuminance level. These results suggest that a higher-order motion system such as a feature-tracking mechanism is functioning to induce visual motion perception under photopic level, while a first-order center-surround motion system is functioning under scotopic level. We speculate that the concurrent activation of different motion mechanisms induces a degradation of motion sensitivity under mesopic vision.