Proceedings of the Annual Meeting of the Cognitive Science Society

When a vertical test stimulus is presented simultaneously within a surrounding stimulus of orientation 10-30° clockwise from vertical, the test stimulus appears slightly counter-clockwise from vertical. In contrast, when the surrounding stimulus is 60-80° clockwise from vertical, the test stimulus appears slightly clockwise from vertical. Lateral inhibition between orientation-selective neurons can account for the former effect (repulsion), but not for the latter effect (attraction). H o w e v e r , if an orthogonal "virtual axis" is also present and exerts its o w n lateral inhibition, both effects can be accounted for. A mathematical m o d e l demonstrates quantitatively h o w this m a y occur in the visual system. O n e simulation with narrowly tuned orientation-selectivity functions produced tilt illusions of similar magnitude to that observed with h u m a n s at normal presentation durations. A simulation with m o r e broadly tuned functions produced tilt illusions of m u c h greater magnitude, as are found with h u m a n s at very short presentation durations. Based on the model's performance, h u m a n performance and neurophysiological data, it is suggested that: 1) lateral inhibition m a y be the immediate cause of both direct and indirect tilt illusions, and 2) the "virtual axis" m a y be a real neural mechanism and m a y be found in greater proportion in extrastriate cortex than in striate cortex.