Adaptation to the spatial distortions introduced by prisms is a well-known phenomenon (Gibson,
1966; Harris,
1965; Held & Freedman,
1963; Rock,
1966; Welch,
1974). In particular, a subject wearing wedge prisms experiences compressions, extensions, and distortions of the visual scene that depend on his eye and head movements. With time, subjects partially adapt to these unusual correlations between movements and sensory changes. As a consequence, when the prisms are removed, aftereffects are obtained that can be measured by the strength of prisms, oriented in the opposite direction, necessary to cancel the perceptual distortion experienced when eye or head movements are performed.
Literature on prism adaptation has contributed to establish the role of action in the perception of space-related attributes like orientation, curvature, size, or depth and it is now common to think that space perception involves adaptable sensorimotor mechanisms. On the other hand, the influence of action on color perception has never been successfully addressed experimentally. Classical attempts were made to modify the perceived color as a function of gaze direction, that is, not properly on action but on proprioception. To this purpose, Kohler (
1962) had one subjects wear left-field yellow/right-field blue goggles for several weeks and obtained the desired shift: white adjustments differed depending on gaze direction (left versus right). However, his results failed to be replicated by better-controlled attempts (McCullough,
1965), even with longer exposure (Harrington,
1965). However, the consequences of an involvement of action—essentially eye movements—in color perception have been discussed at several occasions in philosophical work (Broackes,
1992; Hurley,
1998; Myin,
2001) and artificial vision (Clark & O'Regan,
2000; Skaff, Arbel, & Clark
2002), in particular as a way to achieve color constancy using the nonuniformity of retinal sampling.
In a recent series of experiments, Bompas and O'Regan (
2006) also had subjects wear left-field yellow/right-field blue spectacles but for a different purpose. Half-split colored spectacles introduce a correlation between gaze direction and color, but as McCullough (
1965) showed, no contingent adaptation linking these two kinds of sensory inputs can be obtained. However, the change introduced by the spectacles can be described in a slightly different way: they create a contingency between eye saccades—from one visual hemifield to the other—and color changes. Contrary to the first-mentioned contingency, absolute and static, this second one, relative and dynamic, proved to be easily subject to adaptation (Bompas & O'Regan,
2006). The test stage of these recent experiments involved the successive presentation, on each side of a computer screen, of two patches whose color the subjects had to compare using an eye saccade. Forty minutes of exposure with the spectacles proved to be enough to measure consistent shifts in subjective equality. The shifts were in opposite directions for left/right and right/left eye saccades. A control experiment confirmed, with the same 40-min exposure stage with the spectacles, the failure to measure shifts in absolute color judgments, that is, on single patches presented on the left or on the right. These results showed that Kohler (
1962) was wrong considering that adaptation to the split-field spectacles could be adequately described according to the “situational conditioning” hypothesis, as a form of color adaptation contingent on gaze direction or position in external space. Instead, the effect of the spectacles seems to be better described as a sensorimotor adaptation. This suggests that, in this particular example, the visual system is more sensitive to sensorimotor contingencies than to sensory correlations.
In Bompas and O'Regan (
2006), as in Kohler's (
1962), a large part of the visual field was tinted yellow when the subject gazed leftward and tinted blue when the gaze was directed rightward. A problem could have come from the fact that whereas the spectacles allowed the central retina to be equally adapted to blue and yellow, this was not the case for periphery: for example, when the eyes gazed leftward, a small part of the blue glass was still visible on the right side of the visual field and could not be removed. This had the consequence that the right periphery was more adapted to blue, and the left periphery to yellow, causing an asymmetry that may have been at the root of some part of the observed effects. In the present experiment, we avoided the problem of nonhomogeneity of adaptation of the peripheral retina by using a computer-controlled adaptation method. In this, we presented only a single colored patch at a time: one colored patch when the eyes gazed leftward and another one when the eyes gazed rightward. This procedure permits more precise control of retinal stimulation. The method used is also simpler, can be easily replicated, and allows more flexibility in further study of the effect.
Experiment 1 is a computer-controlled replication—with red and green—of our earlier spectacle experiment. The adaptation stage involves repeated eye movements between red patches on the left and green patches on the right. The test trials involve the same time sequence and positions on the screen as in adaptation trials.
Experiment 2 is a control experiment where (a) in the adaptation stage, the eyes are kept fixed in the center of the screen during the appearance of the colored patches in periphery. The test trials are identical to those in
Experiment 1. The experiment is intended to show that nonhomogeneities in retinal adaptation cannot explain the effect found in
Experiment 1. Indeed, such uncontrolled nonhomogeneities could have occurred in
Experiment 1 as a consequence of performing the eye saccades slightly before the disappearance of the first patch or slightly after the appearance of the second patch.
Experiment 3 is another control experiment where the adaptation stage is identical to that in
Experiment 1 and test trials are made from the center to the periphery. The purpose of this experiment is to show that the effect depends on movement and not on position. It involves testing the effect with a different pair of departure and arrival positions of the eye saccade than used during adaptation. The experiment aims to show that the effect cannot be understood as the learning of a regularity between colors and sides of the screen (conditioning) or as the adaptation to the correlation between color and position in space or even between colors and position of the eyes in the orbits (sensori-sensorial adaptation).