Seeing history: The perception of causal history induces illusory motion perception in static shapes

Yi-Chia Chen & Brian Scholl
Yale University

It has been argued that seeing shapes entails seeing time. A bitten cookie, for example, is seen as a whole cookie that was subsequently bitten. Similarly, a twisted towel is represented as an untwisted towel that was subsequently twisted. Do such inferences arise during visual processing itself? To explore this, we tested whether the perception of history in static shapes could trigger the perception of motion. Observers were told that they would see short movies of a shape (e.g. a square) changing from its complete form into a truncated form, with a "piece" of it missing, and that this change could occur in two ways: (1) all at once, in a single flash; or (2) gradually, with the missing piece quickly "growing" into the shape (as when you poke your finger into a lump of clay). We manipulated the contours of the missing piece to look either as if another shape had "intruded" on the original shape, or as if there had been no such past gradual transformation. When the shape suggested an intrusion, sudden changes were much more likely to be (mis-)perceived as being gradual, in a type of transformational apparent motion. This effect was exceptionally robust, and occurred reliably in every observer. Additional experiments demonstrated that this effect: (1) was due to perception and not response biases (since the effect did not persist with real gradual changes); (2) was not due to differences in lower-level visual features (since it replicated while controlling for geometric features such as area and opening width); and (3) was due to perceived causal history per se (since the effect disappeared when the very same shapes were presented so as to suggest occlusion rather than intrusion). In sum, the perception of causal history in static shapes is powerful enough to induce illusory motion percepts.