ABSTRACT
Three techniques to measure the contrast of afterimages were tested; the most accurate and reproducible one was a cancellation technique. The conditioning grating and the afterimages were produced in the same way as before, but instead of a flashed grating we presented a steady, canceling grating on top of the afterimage. This canceling grating was always of the same type and spatial frequency as the conditioning grating and was always presented in spatial counterphase with the afterimage. We measured the contrast of the canceling grating that was required to cancel out the perceived afterimage. We varied the contrast (10-900Jo), duration (1-12 s), and spatial frequency (0.25-4.0 c/deg) of the conditioning grating, and the time
Figure 34 shows the data for one of the low spatial frequencies. Higher conditioning contrast yields proportionately higher afterimage contrast for both chromatic and achromatic afterimages. The slope of the best-fitting straight line is the ratio of afterimage contrast to the contrast of the conditioning grating. That slope is defined as the relative afterimage contrast in upcoming figures.
