Stimuli and percepts.
(a) A single frame of a typical motion stimulus. Unless noted otherwise, each stimulus consisted of two random dot surfaces: a center surface with an irregular outline (imaginary blue line) surrounded by an annulus. Also, the annulus, as well as the outline of the center, remained stationary in all stimuli. Subjects reported the depth-order of the center using a key press. (b,c) Space-time (ST) plots ,  of the two main motion stimuli used in this study (see Supporting Information S1 for the details of ST plot construction). Depending on the condition, the dots of the surround (top and bottom strips) remained static (b) or flickered to various degrees (stimulus in (d) denotes maximal flicker). In all stimuli, the dots of the center translated smoothly (upward in the case shown), and this motion was pixel-to-pixel identical across all stimuli regardless of the surround flicker, as denoted by the fact that the lines in the middle strip are identical between (b) and (c). Also see Demo Movies 2 and 1, downloadable from www.hegde.us/DFMdemo2.avi and www.hegde.us/DFMdemo1.avi, respectively. (d,e) Percepts elicited by the stimuli in (b) and (c), respectively. When the surround dots were static, the center was perceived as a moving far surface visible through a hole in the nearer surround (d). When the surround dots were flickering, the center appeared to be the near surface moving over the farther surround (e). The effect in (e) was previously reported by Anstis and Ramachandran , . The effect in (d) is original to this study to our knowledge. Note that the depth-order reversal of the center is entirely a contextual effect, in that it occurs solely as a result of the changes in the flicker of the surround in the total absence of changes in the center.