Abstract
Several studies have shown that reduced contrast causes an illusory reduction in the perceived speed of objects or surfaces (Thompson, 1982). Does this illusion also occur for visual perception of self-motion? To answer this question, we tested the effects of reduced contrast and limited field of view (FOV) on visual perception of (I) object-motion and (II) self-motion. In Experiment I, participants matched the apparent speed of two rotating disks. Like the “Thompson Effect,” reduced contrast produced a significant decrease in perceived speed of rotation. Perceived speed was not affected, however, when FOV was reduced to 5°. In Experiment II, posture and vection were recorded while participants viewed the interior of a large rotating drum, which created roll vection. Unlike the “Thompson Effect,” reduced contrast had no effect on perception of self-motion or posture. Both measures decreased significantly, however, when FOV was reduced to 5°. These results represent a double dissociation of two modes of motion perception: Contrast affects perception of object-motion but not perception of self-motion; whereas, reduced FOV affects perception of self-motion but not perception of object-motion. Consistent with previous studies of the effects of blur and reduced luminance on vection and vehicle control (Leibowitz et al., 1979; Owens et al. 1999, 2007, 2010), the present findings add evidence for two functionally distinct “modes of vision”: a focal mode that mediates object perception and an ambient mode that mediates perception of posture and self-motion (Schneider, 1967; Held, 1968; Trevarthen, 1968; Mishkin et al., 1983; Goodale & Milner, 1992).