Afterimages of sinusoidal, square-wave and compound gratings

Negative afterimages were observed after steady fixation of sinusoidal gratings at low spatial frequencies, and quantified using contrast-matching and cancellation methods. Afterimage contrast increased as a power function of "exposure", defined as the product of inspection contrast and inspection duration. A single function, linear on a log-log plot, describes the afterimages of gratings at different spatial frequencies, contrasts and durations. The matching method yielded a lower slope (about 0.42) than the cancellation method (0.62), probably because contrast adaptation attenuates perceived afterimage contrast in the first method, but not the second. Square-wave gratings, and those containing two or three harmonic components gave much weaker afterimages (assessed by contrast-matching) than sine-waves did. Contrast adaptation may again be responsible. Hemi-field asymmetries in contrast perception were noted. The contrast of real gratings and afterimages was nearly additive, especially at lower exposures, but the results cannot distinguish between linear filter and gain control models of "local adaptation".