Polarity selectivity of spatial interactions in perceived contrast.

The apparent contrast of a texture is reduced when surrounded by another texture with high contrast. This contrast-contrast phenomenon has been thought to be a result of spatial interactions between visual channels that encode contrast energy. In the present study, we show that contrast-contrast is selective to luminance polarity by using texture patterns composed of sparse elongated blobs. The apparent contrast of a texture of bright (dark) elements was substantially reduced only when it was surrounded by a texture of elements with the same polarity. This polarity specificity was not evident for textures with high element densities, which were similar to those used in previous studies, probably because such stimuli should inevitably activate both on- and off-type sensors. We also found that polarity-selective suppression decreased as the difference in orientation between the center and surround elements increased but remained for orthogonally oriented elements. These results suggest that the contrast-contrast illusion largely depends on spatial interactions between visual channels that are selective to on-off polarity and only weakly selective to orientation.