A re-evaluation of curvature-specific chromatic aftereffects

Abstract Prolonged adaptation to red convex and green concave curves results in an aftereffect of the opposite color on test patterns composed of convex and concave arcs. This result has recently been ascribed to the adaptation of curvature detectors in the human visual system; the adaptation of lineorientation detectors, thought to be responsible for the McCollough effect, has been rejected as an explanation. We performed several experiments whose results lead to a different conclusion. The aftereffect was measured by two techniques: a color-naming method, and a cancellation procedure. We find that: (1) When fixation is at the centers of the arcs during both adaptation and testing, the aftereffect is stronger at the edaes of the arcs than at the centers. (2) The color of the aftereffect reverses when the test patterns are fixated beyond the edges of the arcs. (3) No aftereffect is observed, if during adaptation, the obsener moves his eyes perpendicular to the axis through the centers of the arcs. We conclude that these curvaturespecific aftereffects are the result of the adaptation of line-orientation detectors, and not of curvature detectors.