The effects of exposure duration and surrounding frames on direct and indirect tilt aftereffects and illusions.

Direct and indirect tilt illusions (TIs) have been shown to have different mechanisms, because spatial parameters that affect the one do not affect the other, and vice versa. The indirect TI, for example, is reduced markedly by a surrounding vertical square frame, a manipulandum that has no effect on the direct TI (Wenderoth & Johnstone, 1988a). In six experiments, we show that both direct and indirect TIs are enhanced in magnitude with short (10-60 msec) exposures; that tilt aftereffects (TAEs) induced with short test exposures are entirely comparable in magnitude; that a surrounding square frame reduces indirect TAEs but not direct TAEs, just as occurs with the TI; and that when the surrounding frame is present during adaptation only, test only, and both or neither, the greatest indirect TAE reduction occurs when the frame is present during the test. These results are consistent with the view (Wenderoth & Johnstone, 1987, 1988a, 1988b) that indirect TIs and TAEs may not reflect temporary neural modification based on V1 lateral inhibitory processes but rather the operation of more global, possibly extrastriate, orientation-constancy mechanisms.