Electrophysiological evidence for a post-perceptual influence of global visual context on perceived orientation.

Visual orientation cues largely determine our perceptions of "vertical," as illustrated in the Rod and Frame Illusion (RFI): an upright rod appears slanted in the opposite direction of a surrounding tilted frame. We used event-related potentials (ERPs) to examine the cortical time course of the frame's illusory influence on the perceived orientation of the rod. ERPs to an orientation discrimination task revealed: 1) the amplitudes of the P1 and N1 (indices of early visual processing) were not affected by frame tilt, but the amplitude of the P3 (an index of later processing) was, and 2) increasing the perceptual difficulty of the rod tilt judgment did not affect the P1 or N1, but increased response time and attenuated P3 amplitude. Results suggest that global orientation context effects are manifest during later, post-perceptual stages of information processing.