Abstract
Objects changing in hue, luminance, size or shape appear to stop changing when they move–this is silencing (“silent updating”, Suchow & Alvarez, VSS 2010). But there are many kinds of motion, and we sought to determine which of them cause silencing. Consider that when an object moves but the observer's gaze does not, two types of motion occur simultaneously: the object moves in space, and its image moves on the retina. We created two complementary displays that dissociate these two kinds of motion. In the first, the changing objects remain stationary while the observer tracks a fixation mark that moves across the display; this produces motion on the retina, but not in space. In the second, the changing objects move across the display and the eyes follow; this produces motion in space, but not on the retina. Comparing silencing across the two conditions revealed that motion on the retina is responsible for the full effect of silencing, whereas motion in space is irrelevant. Next, we asked whether silencing is produced by the sensation of motion or by movement. To clarify this distinction, consider the motion aftereffect and the “motion without movement” illusion (Freeman, Adelson, & Heeger, 1991), both of which produce the appearance of motion even though the “moving” objects remain in fixed locations. Here, after prolonged exposure to a rotating pinwheel, observers viewed a display with stationary objects that changed in luminance, and they reported the apparent rate of change. We found that the motion aftereffect produces silencing, which shows that it is the sensation of motion, not movement, that causes silencing. Together, these results suggest that local motion signals interfere with processing of changes in hue, luminance, size, and shape; this interference likely comes in the form of suppression or misattribution of non-motion changes to motion.
This work was partially funded by National Science Foundation CAREER Award BCS-0953730 to G.A.