Attention-dependent brief adaptation to contour orientation: a high-level aftereffect for convexity?

In contrast to the abundant literature investigating how orientation coding depends on edges defined by various image features, relatively little is known about how coding of orientation might also depend on the two distinct functional roles that oriented edges commonly play. Oriented lines can delineate outline contours of a figure or they can form texture. The results of five experiments using orientation aftereffects measured with brief tests (27 ms, backward masked; adapt-to-test interval=201 ms) provided evidence that brief stimuli (<135 ms) selectively adapt coding of contour-line orientation rather than coding of line-texture orientation. Furthermore, parametric results revealed that the rapidly adapting aftereffects for contour orientation are characterized by (1) broad orientation tuning (peaking at +/-30 degrees to +/-50 degrees from test orientation), (2) indifference as to how the contours are defined (e.g. bright lines, high-pass-filtered lines, faint lines generated by the spatial inhomogeneity of visual sensitivity), (3) rapid saturation at low contrast energy, (4) strong modulation by selective attention, and (5) relative size tolerance. These characteristics appear to parallel those of cells in the high end of the visual form processing pathway (such as inferotemporal cortex). It is thus suggested that the rapidly adapting contour orientation aftereffects reported here may be mediated by high-level neural units that encode global configurations of orientation (e.g. convexity and concavity).