Abstract
Motion silencing (Suchow & Alvarez, 2005) is a striking illusion. Participants easily perceive color changes in an array of 100 dots. But if the array of dots rotates around a circle, the perception of change becomes difficult, ‘silenced’. The predominant suggestion has been that it reveals a failure of change detection, perhaps owing to a limitation of small receptive fields. Some illusions indeed reflect properties of visual ‘wetware’. But often illusions reveal an interpretive mistake in the visual system, a wrong decision about what is there given the input. We suggest that the interpretation of structure from motion produces silencing. Note that motion around a circle —the standard stimulus— can be interpreted as a case of structure: a rotating surface. To test our theory, we sought a stimulus in which we could untangle motion and the implication of structure. We, therefore, turned to the classic structure-from-motion rotating cylinder. By overlaying two groups of oppositely translating dots, participants perceive a 3D rotating cylinder. We produced such a stimulus in which the dots changed color. When the dots were static, changes were easily noticed. But when the dots moved —producing the cylinder percept— silencing was observed (as measured by asking participants to report which stimulus had color changes at a faster rate). Critically, when the two sets of dots translated in the same direction, removing the cylinder percept, no silencing was reported. (i.e. Participants chose this stimulus and the static stimulus as changing faster, equally often). If silencing reflects a limitation of change detection, the limitation should be equally present when the dots translate in the same or opposite directions. But it was only present when the total motion implied underlying structure, revealing that silencing is an interpretation not a limitation of perception.