Figure 2.

(Upper) Experiments on the perception of second-order motion, using the stimuli described in Fig. 1, illustrated here for Experiment I. Observers indicated the perceived motion direction by pressing one of two buttons. For group A (Lower, left), the heterogeneous motion direction dominates perception as the contrast modulation of the texture type v, which consists of the low spatial frequency sectors, increases. Group B observers show the opposite behavior (Lower, center): They perceive only the homogeneous direction of motion, independent of the contrast modulation of the texture type v. The dominance of the heterogeneous direction (for high contrast stimuli) is displayed for all observers in a bar chart (Lower, right). Observers: group A: AW, WB, YH, CS, WG, and CE; group B: DB, SC, ABW, and MS.