McCullough effect

The McCollough effect is a phenomenon of human visual perception in which colourless gratings appear coloured depending on (contingent on) the orientation of the gratings. It is an aftereffect requiring a period adaptation to produce it, but unlike typical aftereffects, the McCollough effect is perceivable for many hours after adaptation. For example, if someone alternately looks at a red vertical grating and a green horizontal grating for a few minutes, a black-and-white vertical grating will then look greenish and a black-and-white horizontal grating will then look pink. The effect was discovered by Celeste McCollough in 1965.

Properties of the effect
The McCollough effect is remarkable because it is very long lasting (e.g., Jones & Holding, 1975, found that 10 minutes of induction can lead to the effect lasting 24 hours), because it depends on retinal orientation (tilting the head by 45 degrees makes the colours disappear; tilting the head by 90 degrees makes the colours reappear such that the gravitationally vertical grating now looks red), and because inducing the effect with one eye leads to no effect being seen with the other eye (but see White, Petry, Riggs, & Miller, 1978, for some evidence of binocular interactions). The effect is different from coloured afterimages, which appear superimposed on whatever is seen and which are quite brief.

Any aftereffect requires a period of induction (or adaptation) with an induction stimulus (or, in the case of the McCollough effect, induction stimuli). It then requires a test stimulus on which the aftereffect can be seen. In the McCollough effect as described above, the induction stimuli are the red vertical grating and the green horizontal grating. A typical test stimulus might show adjacent patches of black-and-white vertical and horizontal gratings. The McCollough-effect colours are more desaturated than the induction colours.

The induction stimuli can have any different colours. The effect is strongest, however, when the colours are complementary, such as red and green, and blue and yellow. A related version of the McCollough also occurs with a single colour and orientation. For example, induction with only a red vertical grating makes a black-and-white vertical test grating appear greenish leaving a black-and-white horizontal test grating as colourless (although there is some argument about that). Stromeyer (1978) called these non-redundant effects. According to him, the classic effect with induction from two different orientations and colours simply makes the illusory colours more noticeable via colour contrast.

The effect is specific to the region of the retina that is exposed to the induction stimuli. This has been shown by inducing opposite effects in adjacent regions of the retina (i.e., from one region of the retina verticals appear pink and horizontals appear greenish; from an adjacent region of the retina, verticals appear greenish and horizontals appear pink). Nevertheless, if a small region of the retina is exposed to the induction stimuli, and the test contours run through this region, the effect spreads along those test contours. Of course, if the induced area is in the fovea (central vision) and the eyes are allowed to move, then the effect will appear everywhere in the visual scene visited by the fovea.

The effect is also optimal when the thickness of the bars in the induction stimulus match those in the test stimulus (i.e, the effect is tuned, albeit broadly, to spatial frequency). This property led to non-redundant effects being reported by people who had used computer monitors with uniformly coloured phosphors to do word processing. These monitors were popular in the 1980s, and commonly showed text as green on black. People noticed later when reading text of the same spatial frequency, in a book say, that it looked pink. Also a horizontal grating of the same spatial frequency as the horizontal lines of the induction text (such as the horizontal stripes on the letters "IBM" on the envelope for early floppy disks) looked pink.

Explanations of the effect
McCollough's paper sparked hundreds of scientific papers on the effect (e.g., see reviews by Stromeyer, 1978 and by McCollough, 2000). The effect has been variously attributed to adaptation of cells in the lateral geniculate nucleus designed to correct for chromatic aberration of the eye, to adaptation of cells in the visual cortex jointly responsive to colour and orientation (this was McCollough's explanation), to processing within higher centres of the brain (including the frontal lobes as held by Barnes et al., 1999), and to learning and memory. In 2006, the explanation of the effect was still the subject of debate, although there was a consensus in favour of McCollough's original explanation.