MECHANISMS OF PURELY SUBJECTIVE CONTOUR TILT AFTEREFFECTS

Neurones tuned for second-order stimuli-those which have edges defined by properties other than luminance and colour-have been frequently ob served in prestriate cortex and in area V2 there are neurones which ex plicitly and unambiguously signal the orientation of purely subjective contours, i.e. contours with no Fourier components at the orientation of the perceived edge [von der Heydt, R. & Peterhans, E. (1989) Journ al of Neuroscience, 9, 1732-1748]. No neurones in area V1 showed simil ar tuning characteristics. In addition, it has been demonstrated that like real contours, purely subjective test contours are subject to til t aftereffects following prolonged viewing of an adapting stimulus, wh ether that stimulus is real or subjective [Paradiso, M. A., Shimojo, S . & Nakayama, K. (1989) Vision Research, 29, 1205-1213]. This result s upports the assertion that the cortical processes responsible for real contour perception are also those giving rise to subjective contour p erception. The data reported here further examined this hypothesis. Fo ur experiments show that purely subjective contours exhibit both direc t and indirect tilt aftereffects and tilt illusions like those observe d with real contours. Further, they provide evidence that direct and i ndirect subjective contour effects, like direct and indirect real cont our effects, arise via the operation of two mechanisms: a low level pr ocess, possibly lateral inhibition between orientation channels, and a second ''higher-order'' process. The data suggest that processing of orientation information beyond the striate cortex is similar to that w hich occurs in area V1 and the data are consistent with models of cont our processing which assume that all perceived contours, both real and subjective, arise from a common mechanism.