Illusory Motion Due to Causal Time Filtering

Static patterns by Kitaoka (2006), the most well known of which is the “Rotating Snake”, elicit forceful illusory motion. The patterns are composed of repeating patches of asymmetric intensity profile, in most cases organized circularly. Motion perception depends on the size of the patches and is found to occur in the periphery for larger patches and closer to the center of the eye for small patches. We propose as main cause for these illusions erroneous estimation of image motion due to eye movements. The reason is that image motion is estimated from the spatial and temporal energy of the image signal with filters which are symmetric in space, but asymmetric (causal) in time. In other words, only the past, but not the future, is used to estimate the temporal energy. It is shown that such filters mis-estimate the motion of locally asymmetric intensity signals for a range of spatial frequencies. This mis-estimation predicts the perceived motion in the different patterns of Kitaoka as well as the peripheral drift illusion, and accounts for the effect at varying patch size. This study builds upon our prior work on the distortion of image features and movement (Fermüller and Malm 2004). Kiatoka (2006): http://www.ritsumei.ac.jp/~akitaoka/index-e.html. C. Fermüller and H. Malm (2004).“ Uncertainty in visual processes predicts geometrical optical illusions ”, Vision Research, 4, 727–749.