DEFINITIONS
Simultaneous Colour Contrast
When different hues are placed next to each other, physiological inhibitory interactions occur between them in the visual system which then cause particular colours (hues) to appear in adjacent parts of the visual field where they do not really exist. This phenomenon is related to simultaneous brightness contrast, or simply brightness contrast.
Afterimage
A transient visual sensation which appears after an intense or prolonged exposure to a visual stimulus.
INSTRUCTIONS
In FIGURE 1 below there is a large, blue square surrounding a smaller gray square. Observe that the gray square appears to have a slightly yellow tinge, although it has no objective yellow colouration. Placing the mouse pointer over the blue square causes it to disappear, leaving only the SAME small gray square, but now it appears to have LOST its yellow hue. When the mouse removes the blue square, the yellow tinge appears to fade out. Try turning the yellow off by quickly moving the mouse pointer on and off the blue background area. Notice how this phenomenon illustrates the definition of simultaneous colour contrast given above.
To create an afterimage keep the mouse away from the area of the blue background in FIGURE 1B and just focus on the gray square for a minute or two. Now make the blue square disappear with the mouse and stare intently at the gray square by itself. The observer should now perceive, in "reverse video," a yellow background in place of the blue one and a light blue square should appear to float over the small gray square.
FIGURE 1
INSTRUCTIONS
In FIGURE 2 below, there is a large, yellow square surrounding the exact same smaller gray square as above. This time, the gray square appears to have a delicate light-blue tinge, although it has no objective blue colouration. The light-blue tinge also becomes more obvious by turning the yellow surround on and off in quick succession.
Once again, create an afterimage by staring at FIGURE 2 for a minute or two. Now remove the yellow background with the mouse and fix your gaze on the remaining gray square for a minute. The colours are in "reverse video" again, but note that the background square of the afterimage should be a very light, blue tint, and a pale yellow should slowly begin to appear to hover over the gray square. The blue in the background of the afterimage is the same blue tint that makes the gray square in FIGURE 2B look different than the gray square by itself. In addition, this afterimage has the same colours as the original objective stimulus in FIGURE 1B, although they do not appear to be as deep. The systematically complementary relationship between blue and yellow should now be apparent.
FIGURE 2
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COMMENTARY
Blue and yellow, and red and blue-green (cyan) are complementary colours. By definition, colours are complementary if they produce white light when added together as lights (additive colour mixing), but NOT as pigments (subtractive colour mixing). It is also a characteristic of complementary colours that selected pairs of them reciprocally inhibit each other in a predictable manner: blue versus yellow and red versus cyan, thus producing the phenomena of simultaneous colour contrast and the complementary afterimages, and both produced under conditions such as depicted in FIGURES 1 and 2 above. The reader may wish to practice afterimages with both pairs of complementary colours by viewing FIGURE 3 below. The reader focuses on each of the images in turn for a minute and then shifts his gaze to the dot in the white background space. The colours observed in the afterimage sensation are complementary to the objective physical stimulus.
FIGURE 3
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B
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That a given colour is induced from its complement, yellow from blue or blue from yellow, is at least partly explained theoretically by a particular cell type in the visual cortex of the brain. Double opponent cells, as they are called, "see" the world via photoreceptors on the retina which send them colour information. These photoreceptors are of three types corresponding to three thirds of the visual spectrum: red green, and blue. FIGURE 4 below shows the pattern of photoreceptors on the retina through which double-opponent cells "view" the world. In diagram A of FIGURE 4, for example, the double opponent cell "sees" the world through green and red cones (to make yellow) which are exciting it in the centre of its "field of vision" on the retina. Blue receptors in the ring surrounding the centre add to the excitability of the central green and red receptors through lateral pathways. At the same time, the double opponent cell is inhibited by blue receptors in the centre and by green and red receptors in the surround. Thus this double opponent cell responds best to a yellow spot in a blue background.
Double-opponent cells can explain simultaneous colour contrast. When the double opponent cell which responds best to a yellow spot in a blue background "looks" through its retinal photoreceptors at a gray square on a yellow background, it "sees" the gray square through its green and red cones in the centre, and it "sees" the blue background through the blue receptors around them. The surrounding blue cones excite the green and red cones in the centre of its field of vision through lateral pathways, even though these green and red cones are not being excited by any objective yellow in the gray square on which they focus.
How is the yellow after image depicted in FIGURE 1, and described in the instructions above, explained? The white background reflects the entire spectrum to the retina, and therefore all three primary colours of red, green and blue. When the observer turns his focus to the white background after focusing on FIGURE 1, the blue receptors are saturated and therefore much less sensitive to the less intense "blue" wavelengths coming from the white. The opponent green and red receptors (yellow) in the neighbourhood, being fresh, become relatively more active than normal and in turn activate, by lateral pathways other opponent cells which see blue in their centres. In effect, double-opponent cells "opposed" to the objective stimulus which the observer originally brought to focus are activated. See FIGURE 4B below.
FIGURE 4
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B
sorry images didn't transfer. Source:
www.langara.bc.ca/psychology/IllusionsIndex.htm
