Electrophysiological evidence for early access to object memories during figure assignment in humans

During figure assignment configural cues (e.g., area, closure, convexity, symmetry) along the same side of a contour cooperate in assigning that side figural status; cues across a contour compete. Here we report human electrophysiological evidence that object memories play a role in figure assignment. We created silhouettes in which the cue balance along the bounding contour biased the inner region to be seen as figure; the inner regions portrayed novel shapes. Control silhouettes suggested novel shapes on the outside of their edges as well as on the inside. Experimental silhouettes suggested familiar objects on the outside, which nonetheless appeared as shapeless grounds. Electroencephalographic (EEG) activity was recorded from 29 scalp sites while the observers viewed experimental and control silhouettes for 50 ms in the context of a cover task in which they categorized line drawings of familiar and novel objects shown 500 ms after the silhouettes. Event-related potentials (ERPs) were created by averaging the EEG traces across trials with respect to the onset of the silhouettes. The P100 component of the ERP was smaller for experimental than for control silhouettes. This difference was maximal over midline occipital, and right hemisphere occipital and parietal-occipital sites (ps < 0.05). The differential P100 responses may reflect either 1) access to object memories, which occurs on the outside of the experimental, but not the control, silhouettes during cross-contour competition; or 2) the inhibition of the object memories by cues on the side of the contour ultimately seen as figure. Thus, object memory influences are evident in the first 100 ms of processing the silhouettes, even though the familiar objects in the grounds were not perceived consciously. Physiological data obtained in monkey V1 by other investigators follows a similar time course. Results are discussed within the context of models of figure-ground assignment and shape perception.