Are there separate first-order and second-order mechanisms for orientationdiscrimination?

In a series of experiments we compared orientation discrimination performan ce for Gabor stimuli in which the stimulus profile was either matched to th e receptive field profile of single V1 simple cells ('simple'), or in which the carrier and envelope orientations were different ('tigertails'). In th e first Experiment, using small, high spatial frequency, peripheral stimuli to minimise the number of detectors involved, we found that simple stimuli were more detectable than tigertails of the same contrast energy, and that orientation discrimination thresholds for simple stimuli were lower than f or tigertails of equal detectability. In later experiments with larger stim uli we measured thresholds for detecting tilts of the envelope with the car rier fixed in orientation. Envelope thresholds were similar for different c arrier orientations, but carrier orientation had a strong biasing effect up on perceived envelope orientation. When the orientation difference between envelope and carrier was small, the carrier orientation was attracted to th at of the envelope; when the difference was large (> 10 degrees) repulsion was found. The biases were reduced by half-wave rectifying the stimuli, put atively making the envelope visible to a first-order filter (Experiment 2). Discrimination thresholds for envelope orientation were higher than those for carrier orientation, and this difference was greater for briefly-presen ted parafoveal stimuli than for long duration foveal stimuli (Experiments 3 and 4). We conclude from these results that there are separate mechanisms for envelope and carrier orientation discriminations for large stimuli, but that first- and second-order mechanisms are not independent in the discrim ination of orientation. (C) 2000 Elsevier Science Ltd. All rights reserved.