Abstract
Colored afterimages of steady fields are predominantly photoreceptor driven (Williams & MacLeod, 1979), but afterimages in other domains have implicated cortical loci. We demonstrate a new method to measure aftereffects of time-varying chromatic stimuli that can be used to probe properties of later color processes. If the colors of two halves of a disk start at the same point on a color-circle, and follow opposite paths for a half-cycle along the circumference so that they end at the same point, the two halves appear significantly different. This would be compatible with successive contrast from different adapting colors. If equal numbers of frames are subtracted progressively from the ends of the two animations, a point is reached where the two halves look identical to an observer, despite being physically distinct. Adaptation magnitude was estimated from the number of frames that had to be rewound for equalization. For excursions beginning and ending on the δ(L-M) and δ(S) cardinal axes, adaptation magnitude decreased from modulation frequencies of 0.5 to 2.0 Hz, both in phase and time. For half-cycle modulations along the color circles, the colors of the two halves go from the neutral point to opposite extreme points and back for one cardinal axis, and from the same extreme to the opposite extreme for the other axis. The adaptation effect of modulating solely along the cardinal axis with opposite directions was significantly less than the effect of the joint modulation along the color circle, especially at low frequencies, implicating neural interactions beyond the LGN. Adding the third harmonic at one-third power to the 0.5 Hz modulation gave a lower adaptation magnitude than subtracting it, by an amount larger than predicted from the sum of independent adaptations, indicating that excursion magnitude is more important than sharp transients.