Measurements of chromatic and achromatic afterimages.

Several types of measurement were made of the negative afterimages formed by viewing chromatic and achromatic sine-wave conditioning gratings that were stabilized on the retina. We varied the spatial frequency, contrast, and duration of the conditioning stimulus and the interval between its offset and the afterimage measurement. Different methods of measuring afterimage contrast were also compared. We conclude that (1) an isoluminant chromatic stimulus creates an isoluminant chromatic afterimage; (2) afterimage contrast is linearly related to conditioning contrast; (3) chromatic and achromatic afterimages have similar low-pass spatial-frequency characteristics; (4) both types of afterimage build up and decay exponentially, with a (1/e) time constant of 4-8 s; (5) most important, both chromatic and achromatic afterimages raise the threshold for a chromatic flashed grating, but neither affects the threshold for an achromatic flashed grating; (6) we can explain these results by postulating that negative afterimages are subserved only by the sustained, or parvocellular, pathways.