Spatial Suppression of the Perceived Contrast is Polarity Selective

The apparent contrast of a texture is reduced when it is surrounded by another texture with high contrast. This contrast-contrast illusion has been thought to be a result of spatial interactions between visual channels that encode contrast energy. In the present study, we show that the contrast-contrast is selective to the 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 found for textures of dense elements, which was similar to those used in previous studies, probably because such stimuli should inevitably activate both on- and off-type sensors. The polarity selective suppression decreased as the difference in orientation between the centre and surround elements increased, but still remained as for orthogonally-oriented elements. These results suggest that the contrast-contrast illusion largely depends on spatial interactions between visual channels that are selective to the luminance polarity and partially to the orientation.