T-junctions in inhomogeneous surrounds.

In an inhomogeneous checkerboard surround, the lighter check darkens an incremental test patch more than the darker check lightens it. However, decremental test patches are influenced equally [Schirillo & Shevell, 1996. Vision Research, 36, 1783-1796]. In the current study, we manipulate the spatial arrangement of a checkerboard surround to produce T-junctions that perceptually group the checks with the test patch. These stimuli alter the inducing effects of the checks. For one modified surround, increments appeared approximately 8% darker and decrements appeared approximately 10% lighter over the original checkerboard surround prior to modification. In a second modified surround, that resembled White's illusion [White, 1979. Perception, 8, 413-416], increments again appeared approximately 8% darker, while decrements appeared a dramatic approximately 23% lighter over the original checkerboard surround prior to modification. These enhanced induction effects are postulated to result from the addition of specific T-junctions. However, these grouping effects remain subservient to the asymmetrical induction effects found by Schirillo and Shevell (1996).