Figure 212

Two-stage theory of colour vision.

between the wavelength composition of the light reaching the eye from every point on a surface and the colour of that point.

Why do we show colour constancy? One factor is chromatic adaptation, in which sensitivity to light of any given colour decreases over time. For example, if you are standing outside after dark, you may be struck by the yellowness of the artificial lights in people's houses. However, if you have been in a room illuminated by artificial light for some time, the light does not seem yellow. Chromatic adaptation has the effect of reducing the distorting effects of any given illumination on colour constancy.

One reason why we show colour constancy is because of familiarity. We know that letter-boxes are bright red, and so they look the same colour whether they are illuminated by the sun or by artificial street lighting. For example, Delk and Fillenbaum (1965) presented various shapes cut out of the same orange-red cardboard. The shapes of objects that are typically red (e.g., heart; apple) were perceived as slightly redder than the shapes of other objects (e.g., mushrooms). However, it is hard with such evidence to distinguish between genuine perceptual effects and response or reporting bias.

These findings do not explain colour constancy for unfamiliar objects. Some insight into the factors involved in colour constancy was obtained by Land (1977). He presented his participants with two displays (known as Mondrians) consisting of rectangular shapes of different colours. He then adjusted the lighting of the displays so that two differently coloured rectangles (one from each display) reflected exactly the same wavelengths of light. However, the two rectangles were seen by Land's participants in their actual colours, showing strong evidence of colour constancy in the absence of familiarity. Finally, Land found that the two rectangles looked exactly the same (and so colour constancy broke down) when everything else in the two displays was blocked out.

What was happening in Land's study? According to Land's (1977, 1986) retinex theory, we decide the colour of a surface by comparing its ability to reflect short, medium, and long wavelengths against that of adjacent surfaces. Colour constancy breaks down when such comparisons cannot be made. More specifically, it is assumed within retinex theory that "the logarithm of the ratio of the light of a given wavelength reflected from a surface (the numerator), and the average of light of the same wavelength reflected from its surround (the denominator) is taken...The process is done independently three times for the three wavelengths [red, green, and blue light]" (Tovee, 1996, p. 107).

Zeki (1983) identified part of the physiological system involved in colour constancy. He found in a study on monkeys that certain cells in area V4 (discussed in the section entitled "Brain systems") responded strongly to a red patch in a multi-coloured display illuminated mainly by red light. These cells did not respond when the red patch was replaced by a green, blue, or white patches, even though the dominant reflected wavelength was red. Thus, these cells seem to respond to the actual colour of a surface rather than simply to the wavelengths reflected from it.

Evaluation

As is predicted by retinex theory, the perception of an object's colour depends on some kind of comparison of the wavelengths of light reflected from that object and from other objects in the visual field. However, retinex theory does not provide a complete account of colour perception and colour constancy. For example, the theory does not indicate the precise ways in which neurons such as colour-opponent cells might be involved in colour perception. In addition, it does not directly address the role of familiar colour in influencing colour constancy.

It would seem to be predicted by retinex theory that colour constancy will be complete provided that observers can see the surroundings of a shape or object. However, that is often not the case. As Bramwell and Hurlbert (1996) pointed out, the extent to which colour constancy is obtained varies across studies from about 20% to 130%. One reason why colour constancy is often far from complete is because of limitations in the method of asymmetric matching by adjustment that is generally used. With this method, participants view two scenes under different lighting conditions, and adjust the colour of part of one scene to match that of the other scene. This is an unnatural task, because in everyday life we tend simply to decide whether a colour is the same as, or different from, that seen under different lighting conditions. Bramwell and Hurlbert (1996) devised a more natural task involving same-different judgements of colour, and found greater colour constancy than is normally found. However, it was still not perfect.

Further evidence that retinex theory provides an incomplete account of colour constancy was produced by Jakobsson et al. (1997). They presented two-dimensional visual displays consisting of vertical stripes in two shades of grey. There was yellow-orange illumination of the upper half of the display, and bluish illumination of the lower half. What was seen by observers alternated between two different three-dimensional percepts of the two-dimensional display:

1. Horizontally folded percept: the central horizontal border between the two illuminations seemed to push out towards the observer, and there was almost complete colour constancy.

2. Vertically folded percept: the display seemed to be folded along the edges between successive grey stripes. There was no colour constancy, because the top half of the display looked yellow-orange and the bottom half looked blue. In other words, colour differences that were due to lighting were wrongly attributed to the display itself.

How can we explain these peculiar results? Jakobsson et al. (1997) argued that what they called the AMBEGUJAS phenomenon occurs because the observers make the wrong assumption that there is a single illuminant. The crucial point, however, is that Land's (1977, 1986) retinex theory cannot account for the findings. According to that

Stop Anxiety Attacks

Stop Anxiety Attacks

Here's How You Could End Anxiety and Panic Attacks For Good Prevent Anxiety in Your Golden Years Without Harmful Prescription Drugs. If You Give Me 15 minutes, I Will Show You a Breakthrough That Will Change The Way You Think About Anxiety and Panic Attacks Forever! If you are still suffering because your doctor can't help you, here's some great news...!

Get My Free Ebook


Post a comment