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McCollough effects, a group of visual contingent aftereffects involving color and contour, have been the subject of a recent body of literature. While a number of investigations have simply reported variations of the original phenomenon, others have closely examined characteristics of these effects and the conditions under which they can be optimally produced. Indications that their spatial specificity is similar to that of single cortical cells investigated electrophysiologically in animals have inspired several models; however, the extraordinary persistence of the effects is difficult to explain at a single-unit level. -Findings which suggest that McCollough effects have characteristics like those of learned responses may help to resolve this paradox.

An experiment was conducted to determine whether the decay rate of the McCollough effect could be differentially influenced by the type of visual stimulation that followed its induction. After acquiring this effect, Os were exposed to achromatic gratings, homogeneous chromatic fields, natural visual stimulation, or complete darkness. Exposure to achromatic gratings caused a marked fading of the effect; the other types of stimulation were associated with similar and much less rapid decay.In 1965, McCollough discovered a new color aftereffect that had an exceptionally long duration. She exposed Ss to a grating of vertical black stripes on an orange background which alternated every few seconds with a horizontal grating on a blue background. After several minutes of exposure, Ss presented with horizontal and vertical black and white' "test" gratings reported that the patterns appeared tinted with the hue complementary to that which had been paired with each orientation during adaptation.Since McCollough's report (1965), it has been established that the color of the McCollough effect (ME) is dependent on spatial characteristics of the grating patterns used for both induction of the effect and testing for its presence; for example, in order for the ME to appear, test patterns must be close in orientation and spatial frequency to the patterns used for adaptation (Teft & Clark, 1968). This dependence might reflect some aspect of the. way in which color and contour are processed neurophysiologically, and there have been single-ceIl-level models formulated which describe the ME in terms of "fatigued" units (e.g., Murch, 1972a). However, the persistence of this effect-it has been estimated to last days or even weeks (Stromeyer & Mansfield, 1970; Stromeyer, 1971)-is difficult to explain within a simple neurophysiological framework.It has been remarked that aftereffects like the ME have characteristics which suggest a perceptual learning mechanism (Mayhew & Anstis, 1972). We approached the question of whether this effect might be maintained over long periods as a learned response by asking how its rate of decay could be influenced by the type of visual stimulation following its induction. For example, a learned response should be retained in the absence of stimulation (complete darkness), whereas fatigued units might be expected to recover relatively quickly under such a condition. METHODSOur basic procedure was to adapt each of three trained Os to a ME adaptation sequence and then expose them to one of several types of postadaptation stimulation (PAS). The ME was induced 47 by alternately viewing a vertical and a horizontal grating (5 cycles/dog) projected on either a green (Wratten No. 53) or violet (Wratten No. 34A) background. The two gratings were alternated every 5 sec for a 10-min period. The filter/orientation combinations were changed from session to session for each O. Each ME adaptation sequence was followed by 50 min of exposure to one of the following PAS conditions: (l) alternating vertical and horizontal achr...

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