Perceived direction of glass patterns can flip by 90°: A neural model

We show a 90° shift in the perceived direction of a well-known motion illusion, based on the dot-pair separation in dynamic Glass patterns. The change can be explained with a cortical model incorporating extra-classical receptive fields. We employed dynamic Glass patterns: a sequence of patterns of randomly positioned, but consistently oriented dot-pairs, presented in rapid succession. Although dynamic Glass patterns contain no coherent motion, observers perceive strong motion in either direction parallel to the dot-pair orientation for small dot-pair separations (< 30 arc min). This has been attributed to the visual system interpreting the dot-pairs as motion streaks of fast-moving objects. The motion illusion has been explained by a model of the orientation selectivity of simple cells in primary visual cortex for small dot-pair separations. However, those models did not include the influence of end-stopping, an extra-classical receptive field mechanism that enhances length-selectivity to oriented bars. We incorporated end-stopping into the model and showed that increasing dot separation in Glass patterns shifts the directional preference of simple cells from parallel with the dot-pairs to the orthogonal direction. Our psychophysical experiments confirmed that Glass patterns perceived as rotating, with small dot-pair separations, were perceived as expanding/contracting with large separations (> 53 arc min). Furthermore, this shift to radial motion was eliminated when dot-pairs with opposite contrast polarities were used, consistent with our end-stopping model. The results provide new insight into the interaction between motion and form cues in the visual system, highlighting the role of extra-classical receptive fields in motion perception.