Neural grouping and geometric effect in the determination of apparent orientation

We propose that neural grouping of retinotopically distributed responses in the primary visual cortex (V1) is essential for the determination of apparent tilt, including the tilt illusion. Our psychophysical study shows that apparent tilt is independent of stereo disparity, hue, or contrast of bars, which determine the ownership of their intersection. This leads us to suspect that the neuronal responses within the intersection are excluded from the computation of apparent tilt. To investigate the underlying cortical mechanisms, we developed and examined a V1 network model including the collinear connections observed in the superficial layers. The model shows good agreement with the results of psychophysical experiments, including segmentation independence, contrast dependence, and apparent tilt for various stimuli. The results suggest that collinear connections underlie the neural grouping that excludes the intersection region and establishes the independence of segmentation.