Observers were initially presented with six uniformly coloured papers that were similar in colour and learned to name them. After that, the observers tried to identify individual papers presented at various indoor and outdoor whether blue or yellow is seen. Finally, the red–
green channel represents the difference between activity levels in the medium- and long-wavelength cones. The direction of this dif ference deter- mines whether red or green is perceived.
Evaluation
As we have seen, there is plentiful support for the dual-process theory. However, it is becom- ing increasingly clear that it is oversimplifi ed (see Solomon & Lennie, 2007, for a review). For example, Solomon and Lennie identify two fi ndings that are puzzling from the perspective of dual-process theory. First, the proportions of different cone types vary considerably across individuals, but this has very little effect on colour perception. Second, the arrangement of cone types in the eye is fairly random (e.g., Roorda & Williams, 1999). This seems odd because it presumably makes it diffi cult for colour-opponent mechanisms to work effectively. What such fi ndings suggest is that the early processes involved in colour vision are much more complicated than was previously believed to be the case. Solomon and Lennie discuss some of these complications in their review article.
Colour constancy
Colour constancy is the tendency for a surface
or object to be perceived as having the same
Figure 2.16 Schematic diagram of the early stages of neural colour processing. Three cone classes (red = long; green = medium; blue = short) supply three “channels”. The achromatic (light–dark) channel receives nonspectrally opponent input from long and medium cone classes. The two chromatic channels receive spectrally opponent inputs to create the red–green and blue–yellow channels. From Mather (2009), Copyright © 2009, George Mather. Reproduced with permission.
Red – Green Blue – Yellow Light – Dark
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colour constancy: the tendency for any given
object to be perceived as having the same colour under widely varying viewing conditions.
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conditions could be predicted on the basis of cone-excitation ratios.
Reeves, Amano, and Foster (2008) argued that it is important to distinguish between our subjective experience and our judgements about the world. We can see the difference clearly if we consider feelings of warmth. As you walk towards a fi re, it feels subjectively to get progressively hotter, but how hot the fi re is judged to be is unlikely to change. Reeves et al. found high levels of colour constancy when observers made judgements about the
objective similarity of two stimuli seen under
different illuminants. Observers were also very good at deciding whether differences between two stimuli resulted from a change in material or a change in illumination. However, low levels of colour constancy were obtained when observers rated the subjective similarity of the hue and saturation of two stimuli. Colour con- stancy was high when observers took account of the context to distinguish between the effects of material change and illumination change, but it was low when they focused only on the stimuli themselves. More generally, the fi ndings show that we can use our visual system in very
fl exible ways.
locations differing substantially in term of light- ing conditions. The key fi nding was that 55% of the papers were identifi ed correctly. This may not sound very impressive, but represents a good level of performance given the similarities among the papers and the large differences in viewing conditions.
A crucial problem we have when identifying the colour of an object is that the wavelengths of light refl ected from it are greatly infl uenced by the nature of the illuminant. Indeed, if you observe a piece of paper in isolation, you cannot tell the extent to which the wavelengths of light refl ected from it are due to the illuminant. Many factors are involved in allowing us to show reasonable colour constancy most of the time in spite of this problem. However, what is of central importance is context – according to Land’s (1977, 1986) retinex theory, we decide the colour of a surface by comparing its ability to refl ect short, medium, and long wavelengths against that of adjacent surfaces. Land argued that colour constancy breaks down when such comparisons cannot be made effectively.
Foster and Nascimento (1994) developed some of Land’s ideas into an infl uential theory based on cone-excitation ratios. They worked out cone excitations from various surfaces viewed under different conditions of illumina- tion. We can see what their big discovery was by considering a simple example. Suppose there were two illuminants and two surfaces. If sur- face 1 led to the long-wavelength or red cones responding three times as much with illuminant 1 as illuminant 2, then the same threefold difference was also found with surface 2. Thus, the ratio of cone responses was essentially invariant with different illuminants, and thus displayed reasonably high constancy. As a result, we can use information about cone-excitation ratios to eliminate the effects of the illuminant and so assess object colour accurately.
There is considerable support for the notion that cone-excitation ratios are important. Nascimento, De Almeida, Fiadeiro, and Foster (2004) obtained evidence suggesting that the level of colour constancy shown in different
Shadows create apparent colour changes, yet we interpret the colour as remaining constant under a variety of conditions despite this. In this example, we perceive a continuous green wall with a sun streak, rather than a wall painted in different colours.
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Top-down infl uences (e.g., knowledge, familiar colour) can have a strong effect on colour constancy. Suppose that light from a strongly coloured surface refl ects onto a nearby white surface. We all know that will affect the light refl ected from the white surface, and take that into account when judging the colour of the white surface. Bloj, Kersten, and Hurlbert (1999) set up a visual display in which observ- ers judged the colour of a white surface. In one condition, observers were presented with a three-dimensional display that created the false impression that a strongly coloured surface refl ected onto that white surface. This misled the observers and produced a substantial reduc- tion in colour constancy.
Colour constancy is infl uenced by our know- ledge of the familiar colours of objects (e.g., bananas are yellow; tomatoes are red). This was shown in a study by Hansen, Olkkonen, Walter, and Gegenfurtner (2006). Observers viewed digitised photographs of fruits and adjusted their colour until they appeared grey. The key fi nding was a general over-adjustment. For example, a banana still looked yellowish to the observers when it was actually grey, causing them to adjust its colour to a slightly bluish hue. Thus, objects tend to be perceived in their typical colour.
Zeki (1983) found in monkeys that cells in area V4 (specialised for colour processing) responded strongly to a red patch illuminated by red light. However, these cells did not respond when the red patch was replaced by a green, blue, or white patch, even though the dominant refl ected wavelength would generally be perceived as red. Thus, these cells responded to the actual colour of a surface rather than simply to the wavelengths refl ected from it. In similar fashion, Kusunoki, Moutoussis, and Zeki (2006) found that cells in V4 continued
Other factors
One of the reasons we show colour constancy is because of chromatic adaptation, in which sensitivity to light of any given colour or hue decreases over time. If you stand outside after dark, you may be struck by the yellowness of the artifi cial light in people’s houses. However, if you have been in a room illuminated by artifi cial light for some time, the light does not seem yellow. Thus, chromatic adaptation can enhance colour constancy. Uchikawa, Uchikawa, and Boynton (1989) carried out a study in which observers looked at isolated patches of coloured paper. When the observer and the paper were both illuminated by red light, there was chro- matic adaptation – the perceived colour of the paper only shifted slightly towards red. The fi ndings were different when the observer was illuminated by white light and the paper by red light. In this condition, there was little chromatic adaptation, and the perceived colour of the paper shifted considerably towards red.
Kraft and Brainard (1999) set up a visual environment in a box. It included a tube wrapped in tin foil, a pyramid, a cube, and a Mondrian stimulus (square shapes of different colours). When all the objects were visible, colour constancy was as high as 83% even with large changes in illumination. However, it decreased when the various cues were progressively elimi- nated. The most important factor in colour constancy was local contrast, which involves comparing the retinal cone responses from the target surface with those from the immediate background (cone-excitation ratios). When local contrast could not be used, colour constancy dropped from 83 to 53%. Another important factor was global contrast, in which retinal cone responses from the target surface are com- pared with the average cone responses across the entire visual scene. When the observers could not use global contrast, colour constancy dropped from 53 to 39%. When all the non-target objects were removed, the observers were denied valuable information in the form of refl ected highlights from glossy surfaces (e.g., tube wrapped in tin foil). This caused colour constancy to drop to 11%.
chromatic adaptation: reduced sensitivity
to light of a given colour or hue after lengthy exposure.
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sive theory of how the various factors combine to produce colour constancy. Second, there is much to be discovered about the brain mech- anisms involved in colour perception and colour constancy. For example, we do not have a clear understanding of why the cone types in the eye are distributed fairly randomly rather than systematically. Third, there is evidence (e.g., Reeves et al., 2008) indicating that the extent to which we show colour constancy depends greatly on the precise instructions used. Little is known of the factors producing these large differences.