Subjective contours, tilt aftereffects, and visual cortical organization.

The tilt aftereffect (TAE) was used to study interactions between real and subjective contours. Subjects adapted to either real or illusory lines and were then shown test stimuli containing real or illusory lines. In our first experiment, we found that there is a marked asymmetry in the interactions between real and subjective stimuli. Adaptation to real lines produces comparable TAEs with real and subjective test lines. With either type of test stimulus the maximum effect occurs with a 10-20 deg difference between the orientations of the adaptation and test stimuli. Also, there is a strong TAE when the adaptation and test stimuli contain only subjective lines. However, there is a significantly weaker TAE when the adaptation stimulus is subjective and the test stimulus is real. In a second experiment we find that interocular transfer of tilt aftereffects is greater when the test stimulus is subjective than when it is real. These results are consistent with physiological reports that a subset of orientation selective cells in visual cortex is responsive to subjective contours and that these cells are more binocular, on average, than those responsive only to real contours. Our findings also suggest that the perception of subjective contours is based on the activation of neurons with properties, such as orientation selectivity, which are characteristic of early visual cortical areas.