Cone receptors contain light-sensitive photo- pigment allowing them to respond to light. According to trichromatic (three-coloured) theory, there are three different kinds of cone receptors. One type of cone receptor is most sensitive to short-wavelength light, and gener- ally responds most to stimuli perceived as blue. A second type of cone receptor is most sensitive to medium-wavelength light, and responds greatly to stimuli generally seen as yellow- green. The third type of cone receptor responds most to long-wavelength light such as that coming from stimuli perceived as orange-red.
How do we see other colours? According to the theory, most stimuli activate two or all three cone types. The colour we perceive is deter- mined by the relative levels of stimulation of Third, the emphasis within the theory is
on the separate contributions of the dorsal and ventral streams to vision and action. In fact, however, the two visual systems typic- ally interact with each other. Kourtzi et al. (2008) discussed some of these interactions. For example, Kourtzi and Kanwisher (2000) found that photographs of an athlete running produced strong responses in human MT/ MST (specialised for motion processing) in the dorsal stream. Thus, visual perception can have a direct impact on pro cessing in the dorsal stream. Much additional research provides evidence that there are numerous reciprocal connections between the two visual streams (Mather, 2009).
Fourth, the notion that dorsal and ventral streams process very different kinds of informa- tion is too extreme. As we saw earlier, there is evidence that motion-relevant information can reach the ventral stream without previously having been processed within the dorsal stream. Some of the complex interactions between the two processing streams can be inferred from Figure 2.5.
Fifth, it is often diffi cult to make fi rm predictions from the theory. This is because most visual tasks require the use of both processing streams, and there are individual differences in the strategies used to perform these tasks.
Sixth, there has been some scepticism (e.g., Pisella, Binkofski, Lasek, Toni, & Rossetti 2006) as to whether clear double dissociations between optic ataxia and visual agnosia have been demonstrated. For example, patients with optic ataxia are supposed theoret ically to have impaired reaching for visual objects but intact visual perception. However, some of them have impaired visual perception for stimuli presented to peri pheral vision (see Pisella et al., 2006, for a review).
Seventh, there is much exciting research to be done by studying visual illusions in brain- damaged patients. Such research has hardly started, but early fi ndings (e.g., Coello et al., 2007) seem somewhat inconsistent with predic- tions of perception–action theory.
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periphery of the human retina (Kuchenbecker, Sahay, Tait, Neitz, & Neitz, 2008). Since long- wavelength cones are maximally responsive to stimuli perceived as red, this may help to explain why matadors use red capes while engaged in bull-fi ghting.
Many forms of colour defi ciency are consis- tent with trichromacy theory. Most individuals with colour defi ciency have dichromacy, in which one cone class is missing. In deuter- anomaly, the medium-wavelength (green) cones are missing; in protanomaly, the long-wavelength (red) cones are missing; and in tritanopia, the short-wavelength (blue) cones are missing. each cone type, with activation of all three cone
types leading to the perception of whiteness. Bowmaker and Dartnall (1980) obtained support for trichromatic theory using micro-
spectrophotometry, a technique permitting
measurement of the light absorbed at different wavelengths by individual cone receptors. This revealed three types of cones or receptors responding maximally to different wavelengths (see Figure 2.15). Each cone type absorbs a wide range of wavelengths, and so it would be wrong to equate one cone type directly with perception of blue, one with yellow-green, and one with orange-red. There are about 4 million long-wavelength cones, over 2 million medium- wavelength cones, and under 1 million short- wavelength cones (Cicerone & Nerger, 1989). Roorda and Williams (1999) found that all three types of cone are distributed fairly ran- domly within the human eye. However, there are few cones responsive to short-wavelength light within the fovea or central part of the retina. More recent research has indicated that the ratio of long-wavelength to medium-wavelength cones increases dramatically in the extreme
Figure 2.15 Three types of colour receptors or cones identifi ed by microspectrophotometry. From Bowmaker and Dartnell (1980). Reprinted with permission of Wiley- Blackwell. 1.00 0.75 0.50 0.25 0.00 Relative absorbance 400 500 600 700
Wavelength of light (nanometres) Sensitive to short wavelength
Sensitive to medium wavelength
Sensitive to long wavelength
microspectrophotometry: a technique that
allows measurement of the amount of light absorbed at various wavelengths by individual cone receptors.
dichromacy: a defi ciency in colour vision in
which one of the three basic colour mechanisms is not functioning.
K E Y T E R M S
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can be seen. That is precisely what Abramov and Gordon (1994) found when observers indicated the percentage of blue, green, yellow, and red they perceived when presented with single wavelengths.
Opponent-process theory explains nega- tive afterimages. Prolonged viewing of a given colour (e.g., red) produces one extreme of activity in the relevant opponent process. When attention is then directed to a white surface, the opponent process moves to its other extreme, thus producing the negative afterimage.
The theory is of relevance in explaining some types of colour defi ciency. Red-green defi ciency (the most common form of colour blindness) occurs when the high- or medium- wavelength cones are damaged or missing, and so the red–green channel cannot be used. Blue- yellow defi ciency occurs when individuals lack- ing the short-wavelength cones cannot make effective use of the blue–yellow channel.