The interaction of first- and second-order cues to orientation

The visual system is sensitive to orientation information defined both by f irst-order (luminance) and by second-order (texture) cues. We consider how these orientation cues are computed and how they affect one another. We mea sured the perceived orientation of the first and second-order components of Gabor patches (the carrier and envelope. respectively) and report a depend ence of the perceived orientation of each on the orientation of the other, and on the spatial frequency of the carrier. Fixing the carrier orientation near that of the envelope interferes with envelope orientation judgements. This interference is reduced by adding a small (subthreshold) rotation to the carrier indicating that the site of interference is early. When the gro ss relative orientation of carrier and envelope is varied, the carrier appe ars systematically tilted towards the envelope. However, provided envelope and carrier are separated by more than approximately 10 degrees, the percei ved envelope orientation appears tilted away from the carrier. The size of these effects increases with decreasing carrier spatial frequency, and with increasing exposure duration. When the envelope and carrier are both non p arallel and non-perpendicular Fourier energy is distributed asymmetrically across orientation. We demonstrate that, for a channel-based orientation co de, this asymmetry induces a shift in mean orientation that is sufficient t o explain illusory tilting of carriers. The illusory tilting of the envelop e, as a function of carrier orientation and spatial frequency, demonstrates that human ability to demodulate contrast information is far from ideal an d cannot be explained by existing two-stage filter-rectify-filter models. W e propose that illusory tilting of the envelope is due to selective connect ivity between first- and second-stage filters whose purpose is to dissociat e the type of image structure producing each class of cue. (C) 1999 Elsevie r Science Ltd. All rights reserved.