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 ari se in VI and are sensitive to differences in spatial and temporal parameter s between test and inducing stimuli, whereas indirect effects arise in extr astriate cortex and are insensitive to such parameters. When Wolfe (Vision Research 1984; 24: 1959-64) reported that large direct tilt after-effects o ccurred with short lest flashes, he postulated that either there are distin ct 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 other s. Results: In three experiments we demonstrate that large direct tilt illusio ns can be induced when parameters other than duration are manipulated, incl uding contrast and spatial frequency, and that such large effects can occur when stimulus parameters ape chosen to favour preferentially either the tr ansient (magnocellular-like) system or the sustained (parvocellular-like) s ystem. Conclusions: These results are thus consistent with Wolfe's second hypothes is. None of these stimulus manipulations had any effect on indirect tilt il lusions, consistent with previous findings and hypotheses about the differe nt mechanisms of the direct and indirect effects.