2nd-order processes in the human perception of motion and texture
2nd-order lateral inhibition. When a central texture patch C, is surrounded by a texture field S, the perceived contrast of C depends substantially on the contrast of the surround S (with mean luminance held constant). When C is surrounded by a high-contrast texture with similar spatial frequency content, it appears to have less contrast than when it is surrounded by a uniform background. The induced reduction of apparent contrast is greatly diminished when (i) C and S are filtered into non-overlapping spatial frequency bands, or when (ii) C and S are presented to different eyes. When C and S are special textures designed to selectively stimulate either the on-center or off-center visual system, the reduction of C's apparent contrast does not vary with the combination of on-center and off-center textures. When C and S gratings are perpendicular, S causes less of a reduction in C's apparent contrast than it does when they are parallel. This orientation specificity increases with increasing spatial frequency and decreasing stimulus contrast. These results indicate a 2nd-order lateral inhibition with a neural locus central to that of on-center/off-center interaction and mediated by an early cortical or precortical cell population.
Mechanisms of 2nd-order motion perception. Two kinds of stimuli that are invisible to 1st-order (Fourier) motion mechanisms are tested: (1) stimuli for which motion information is lost following full-wave rectification ("half-wave stimuli") and (2) stimuli for which motion information is lost following half-wave rectification ("full-wave stimuli"); given sufficient contrast, both convey motion. All observers perceive full-wave motion; only one-third perceive half-wave motion. Remarkably, full-wave stimuli are perceived slightly more efficiently than Fourier stimuli, and much more efficiently than half-wave stimuli. When either full-wave and Fourier or half-wave and Fourier gratings are presented simultaneously, there is a wide range of relative contrasts within which the directions of both gratings are easily determinable. Conversely, when half-wave and full-wave gratings are combined, the direction of only one of these gratings can be determined with high accuracy. Conclusion. Fourier ("1st-order") and non-Fourier ("2nd-order") motion systems are fully separate; full-wave and half-wave mechanisms share resources.