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.