Discriminating local continuity in curved figures

We assessed whether the visual system's ability to discriminate subtle perturbations from smoothness in curved shapes was based on 1st-order properties or 2nd-order properties. We investigated which of the two would determine performance in a task where the observer had to detect spatial jitter on aligned, unaligned or unoriented Gabor patches forming either an open or enclosed path. Surprisingly, performance was no better in the conditions employing aligned micropatterns, implicating the use of 2nd-order properties. Varying the peak spatial frequency or the size, (standard deviation of the Gaussian envelope), produced little change in the jitter threshold. By contrast, increasing the spacing between the Gabor patches had a large detrimental effect. Randomizing the orientation of the Gabors also hampered performance. These results indicate that orientation linking may only aid psychophysical performance in detection tasks. If variance was imposed on the size of the blobs (a 2nd-order property), performance was degraded. Variance on the carrier spatial frequency (a 1st-order property) resulted in a smaller worsening of performance. Overall, our results imply that shape discrimination is performed by mechanisms sensitive to 2nd-order micropattern properties, although some dependence on 1st-order properties exists. (C) 1999 Elsevier Science Ltd. All rights reserved.