Local and global factors in spatially-contingent coloured aftereffects.

Dodwell and O'Shea's [(1987) Vision Research, 27, 569-580] conclusions that contingent coloured aftereffects (CAEs) depend on gobal pattern organization were investigated in four experiments. In Expt 1, we replicated findings that CAEs can be induced with complex patterns (concentric circles; radial spokes) under conditions of systematic eye movements. Contrary to Dodwell and O'Shea's argument that eye movements should uniformly cancel local orientation-colour contingencies, leaving only global effects, we reduced CAE magnitude by halving the diameter of the test stimuli. This suggests that cancellation did not occur uniformly over whole patterns, and that CAEs observed on these patterns are the residuals of uncancelled local orientation-colour contingencies. In Expt 2 we used central-fixation induction procedures to demonstrate that it is possible to induce CAEs with randomly-organized and locally-orthogonal orientation components. These findings are inconsistent with Dodwell and O'Shea's failure to observe CAEs under these conditions, and with their conclusion that global organization is necessary for CAE induction. However, CAEs induced with randomly-organized components were significantly weaker than those induced with globally-organized components. We examined the contribution of global organization in two additional experiments. In Expt 3 we induced CAEs with randomly-organized components under conditions in which the need for central fixation was removed, and found that CAE strength was directly related to the organization as well as the density of local-orientation components. In Expt 4, we found that the global organization of local-orientation components enhanced CAE strength only in regions away from the edges of these components: pattern organization did not affect the strength of CAEs at edges. We interpret these findings as evidence that CAEs may involve separate edge- and spread-colour components, and conclude that such components may account for observations previously attributed to global pattern geometry.