Neural substrates of the tilt illusion

Purpose: It has been suggested that direct and indirect tilt illusions and after-effects have different mechanisms, namely that the direct effects arise in V1 and are sensitive to differences in spatial and temporal parameters between test and inducing stimuli, whereas indirect effects arise in extra-striate cortex and are insensitive to such parameters. When Wolfe (Vision Research 1984; 24: 1959–64) reported that large direct tilt after-effects occurred with short test flashes, he postulated that either there are distinct mechanisms which process brief and longer duration stimuli or that there are distinct mechanisms that are not primarily concerned with duration but are differentially responsive to temporal parameters amongst several others. Results: In three experiments we demonstrate that large direct tilt illusions can be induced when parameters other than duration are manipulated, including contrast and spatial frequency, and that such large effects can occur when stimulus parameters are chosen to favour preferentially either the transient (magnocellular-like) system or the sustained (parvocellular-like) system. Conclusions: These results are thus consistent with Wolfe’s second hypothesis. None of these stimulus manipulations had any effect on indirect tilt illusions, consistent with previous findings and hypotheses about the different mechanisms of the direct and indirect effects.