Afterimages, grating induction and illusory phantoms

Under some conditions (dark or light inspection areas) illusory gratings of ten appear to be in-phase with the inducing gratings and under others (gray inspection area) illusory gratings often appear to be out-of-phase with th e inducing gratings. McCourt reported that point-by-point brightness matche s reveal only out-of-phase illusory gratings, no matter what the luminance of the inspection area (McCourt, M. E. (1994). Vision Research, 34, 1609-16 17). Since the technique used might have led to afterimages which mimic out -of-phase illusory gratings, the present series of experiments was undertak en to determine how such afterimages might bias illusory grating judgments. Afterimages were induced during fixation with brief flashes of inducing gr atings within the inspection area (Experiment 1), or by vertical shifts in the entire stimulus which exposed the retina to real gratings prior to judg ments within the inspection area (Experiment 2). Experiment 2 was replicate d with drifting inducing gratings (Experiment 3). The subjects were asked t o indicate whether illusory gratings appeared in- or out-of-phase. The resu lts of all three experiments reveal that out-of-phase illusory gratings pre dominate, and that afterimages can only bias judgments with stationary disp lays. It is suggested that grating induction is perceived when subjects att end to local contrast differences, while phantom visibility is facilitated when attention is captured by the more global aspects of the stimulus. (C) 1999 Elsevier Science Ltd. All rights reserved.