Abstract
We study how the visual system groups local features extracted by early visual processes in order to determine global attributes of an object, specifically the global orientation of a bar in the presence of another intersecting bar that evokes the tilt illusion. Our psychophysical studies suggest that early visual processes, probably in V1, are involved in the neural grouping, and that the orientation information around the bar junction does not contribute to the apparent tilt. We then investigate computationally the cortical mechanism for the grouping by a V1 network model including long-range horizontal connections evident in superficial layers. The results show that the activities of V1 cells are grouped together excluding the junction region because of colinear synchronization. The model shows good quantitative agreement with human responses to tilt illusions consisting of various bar junctions
