A colored line flanking a darker border will appear to assimilate its color onto the enclosed white area over distances of up to 45 deg (the Watercolor Effect). This coloration is uniform and complete within 100 ms. We found that thin (6 arcmin), winding inducing lines with different contrasts to the ground are generally more effective than thick, straight, and equiluminant lines. Blue and red lines induce the strongest effects, but watercolor spreading may also be seen with green and yellow. On a white background, color spreading is stronger than on chromatic, gray or black backgrounds. Little or no color is perceived when a narrow white zone (gap) is inserted in between the two inducing lines. However, chains of colored dots instead of continuous lines suffice to produce spreading. Edge-induced color is also observed when the two colored lines are presented dichoptically, suggesting a cortical origin. The Watercolor Effect described here may serve to enhance figure-ground segregation by imparting surface color onto the enclosed area, and to promote grouping between distant stimulus elements. As a grouping factor, watercolor coloration wins over proximity. Assimilative color spreading may arise in two steps: First, weakening of the contour by lateral inhibition between differentially activated edge cells (local diffusion); and second, unbarriered flow of color onto the enclosed area (global diffusion).

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When a dark chromatic contour delineating a figure is flanked on the inside by a brighter chromatic contour, the brighter color will spread into the entire enclosed area. This is known as the watercolor effect (WCE). Here we quantified the effect of color spreading using both color-matching and hue-cancellation tasks. Over a wide range of stimulus chromaticities, there was a reliable shift in color appearance that closely followed the direction of the inducing contour. When the contours were equated in luminance, the WCE was still present, but weak. The magnitude of the color spreading increased with increases in luminance contrast between the two contours. Additionally, as the luminance contrast between the contours increased, the chromaticity of the induced color more closely resembled that of the inside contour. The results support the hypothesis that the WCE is mediated by luminance-dependent mechanisms of long-range color assimilation.

There are many celebrated examples of ambiguous perceptual configurations such as the Necker cube that abruptly and repeatedly ''switch'' among possible perceptual states. When such ambiguous configurations are presented intermittently, observers tend to see the same perceptual state on successive trials. The outcome of each trial apparently serves to ''prime'' the outcome of the following. We sought to determine how long the influence of a past trial persists by using ambiguous motion quartets as stimuli. We found large, significant effects of all four most recent trials, but the results were not consistent with any priming model. The results could be explained instead as perceptual completion of two kinds of temporal patterns, repeating and alternating. We conclude that the visual system does not passively remember perceptual state: it analyzes recent perceptual history and attempts to predict what will come next. These predictions can alter what is seen.ambiguous figures ͉ apparent motion ͉ hysteresis ͉ priming V isual perception under ordinary circumstances is an ongoing process. Current visual information is integrated with past information as part of a perceptual cycle (1), and it is not surprising that, for example, the outcome of a given trial in a psychophysical experiment is affected by what has occurred in recent trials. The observer's response time on a particular trial, for example, is significantly affected by recent task history (2-8). This trial-to-trial effect of the past on the present is particularly pronounced in the perception of motion quartets, a commonly used apparent motion stimulus. A motion quartet consists of a brief display of two tokens presented at opposite ends of a diameter of an invisible circle followed a short time later by presentation of two other tokens on a possibly different diameter. With proper choice of timing, the observer sees apparent motion carrying one token of each pair to a token of the other (Fig. 1). The direction of perceived motion implies a correspondence between each token in the first pair and one of the tokens in the second. This pairing of tokens represents the visual system's solution to the motion correspondence problem (9 -11).The perceived motion is compelling, but it can also be ambiguous. When the tokens are all identical and the angle between the diameters is Ϸ90°, many observers are as likely to see movement in the clockwise direction as they are in the counterclockwise direction. By varying , the experimenter can vary the probability that the observer will perceive movement in one direction or the other. When is near 180°, motion is almost always seen as counterclockwise, and when it is near 0°, motion is almost always seen as clockwise.The perceived direction of motion is affected by proximity, the similarity between potentially corresponding tokens (9-14), and the direction of motion perceived during recent trials. Ramachandran and Anstis (15) found that the tendency for the observer to perceive the same direction of motion persisted even wit...