The visual system tends to group clusters of similar parts into a single shape. The perceived shape of such an object – consisting of many similar small parts – can be manipulated by creating a perceptual conflict between the orientation and the position of the parts. This has the interesting consequence that the position of parts and intermediate illusory contours can appear displaced compared to their physical position. Here, we used this shape-dependent change in perceived position as a pointer to identify visual regions involved in shape perception. We hypothesized that if the percept of a shape is primarily evoked bottom-up through (illusory) contours generated at an early level in visual cortex, it should be possible to read-out the displaced contour activity in V1 and V2, as these areas have previously been associated with contour integration. On the other hand, if it is the perceived shape that drives the perceived positional change of the parts and contours, activity in early visual cortex should reflect the physical position of the parts and not differ depending on the perceived shape.
In the experiment, participants viewed relatively large shapes composed of gabor patches, while their brains were scanned using fMRI. Data was analyzed by determining shifts in the position of contour-evoked cortical activity.
Collaborator: Frans W. Cornelissen