Spatial filter combination in human pattern vision: channel interactions revealed by adaptation.

Above threshold, two superimposed sinusoidal gratings of the same spatial frequency (eg 1 cycle deg-1), of equal moderate contrast (eg C1 = C2 = 6%), and with orientations of +/- 45 degrees, usually look like a compound structure containing vertical and horizontal edges (ie a blurred checkerboard). These feature orientations are very different from the dominant filter orientations in a wavelet-type (eg simple-cell) transform of the stimulus, and so present a serious challenge to conventional models of orientation coding based on labelled linear filters. Previous experiments on perceived structure in static plaids have led to the view that the outputs of tuned spatial filters are combined in a stimulus-dependent way, before features such as edges are extracted. Here an adaptation paradigm was used to investigate the cross-channel interactions that appear to underlie the spatial-filter-combination process. Reported are two aftereffects of selective adaptation: (i) adaptation to a 1 cycle deg-1 plaid whose component orientations are intermediate to those in a 1 cycle deg-1 test plaid 'breaks' perceptual combination of the components in the test plaid; (ii) adapting to a 3 cycles deg-1 plaid whose component orientations match those in a 1 cycle deg-1 test plaid facilitates perceptual combination of the components in the test plaid. The results are taken as evidence that spatial channels remote from those most responsive to a test plaid play a crucial role in determining whether the test plaid segments or coheres perceptually.