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Although the role of the V2 area as it relates to colour processing is still debated, it is safe to conclude that to some degree, colour information is processed in the V2, noted in the study by Friedman, Zhou, and von der Heydt (2003). In an even more recent review by Sincich and Horton (2005), it has become increasingly clear that V1 and V2 layers are anatomically connected and that multiple layers in V1 project into the V2. This not only suggest that a substantial amount of colour information is projected into the V2, but that other

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types of visual properties – motion, depth, are sent to this area for further processing. Information leaving V1 eventually makes its way through either the dorsal (or parietal pathway) or ventral pathway (or temporal pathway), as it was originally discovered by Mishkin and Ungerleider (1982).

Much of the human primate’s ability to experience the colour of familiar objects in the physical world is greatly indebted to V4. As a result of this, considerable damage in this region can, in certain conditions, lead to the inability to correctly perceive the colour of a given object. The complete damage in this region, which has been hypothesized to severely impair colour vison, is commonly referred to as cerebral achromatopsia (Zeki, 1990). In the classical study conducted by Zeki (1973), using single unit recording techniques on non-human primates (i.e. Rhesus macaque), Zeki discovered that almost all of the cells in V4 area were colour sensitiveas they typically responded to the presentation of different colours of light and not to achromatic illuminants (Zeki, 1973). The study further noted that these cells were often observed to occupy different regions in V4 area and were not grouped together in a certain area of the region. As a result of this, this study concluded that V4 area was devoted exclusively to processing colour information about the physical world. Another study by Zeki (1983,), using psychophysical and single unit recording techniques on non-human primates, discovered that many of the cells in V1 were wavelength- sensitive cells, typically responding to different wavelength types (S- ,M-, L-), while cells in V4 area were colour-sensitive and responded to the actual colours of a physical object (Zeki 1983). When colours of the same wavelength type (e.g. orange and red) were presented, wavelengths-sensitive cells in V1 responded in the same manner, as they could not distinguish between the colours of the wavelengths. However, when the same stimulus was presented to the cells in V4 area, the colour-sensitive cells frequently responded in fundamentally different patterns. This lead to the conclusion that V1 is primarily responsible for processing wavelengths while V4 is responsible for translating wavelength information, allowing the human primate to understand, in the form of his or her day to day experience, colours in the physical world.

With growing doubt as to whether V4 area is of such great importance to the human primate’s experience of colour, a very convincing study was conducted by Zeki, Watson, Lueck, Friston, Kennard and Frackowiak (1991) using Positron emission tomography (PET) to measure the following regions of interest (ROI) : V1, V2, V4, and the V5 of the human primate. When a moving black and white Mondrian patch was presented to the observers, significant levels of cortical activation were observed in V1, V2 and V5 area of the human primate. However, when observers were presented with static coloured Mondrian patches, the activation in the V5 considerably decreased, while V1, V2, and V4 area was significantly activated (Zeki, Watson, Lueck, Friston, Kennard and Frackowiak 1991). This study, as the first of its kind, not only suggested that V4 area is in fact a major colour region in the brain of the human primate, but that there is a strong anatomical connection between V1, V2, and V4, as well as between V1, V2 and V5, giving birth to the idea commonly known as parallel processing (for a complete review on parallel processing see Zeki, 1998). To get a more complete picture of V4’s role in the human primate’s day to day experience not just on colour but on familiar coloured objects - CDOs, another study by Zeki & Marini, (1998) using fMRI on human primates, was conducted. In this study observers were presented CDOs in their typical, atypical, and achromatic versions while the following regions of interests (RIO) - V1, V2, and V4 - were measured. The study observed that when achromatic versions of these objects were presented, only V1 and some

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parts of the V2 area were significantly activated. When observers were presented with the object in their typical hue, V1, V2, and V4 areas were significantly activated, whereas when observers were presented the incorrect atypical hue version (colours in which objects are rarely or never experienced in the physical world), activation in V4 area significantly decreased (Zeki & Marini, 1998). This led to the conclusion that V1 is primarily responsible for processing the wavelength of light associated with a given object, while V2 is responsible for refining the intensity associated with a given wavelength, leaving V4 primarily responsible for translating colour information associated with familiar objects that are often experienced in the physical world.

Figure 2.11: The Figure is unavailable due to copyright restrictions.

Despite the evidence which supports that V4 area is quite important to the human primate’s experience of colour, the role of V4 area is still questioned by many. The lesion study on non-human primates, conducted by Heywood, Gadotti, and Coweyl (1992), found that despite the absence of V4 area, non-human primates’ (i.e. Rhesus macaque) ability to discriminate between grey and a range of other coloured stimuli was roughly similar (Heywood, Gadotti, and Coweyl 1992). This result leads to the conclusion that V4 area may not be the only area responsible for colour processing. This also led to the idea that V4 area in non-human primates (e.g. macaque monkeys) may be anatomically different from V4 area of the human primate, further casting doubt as to the role of V4 area in the human primates’ day to day experience of colour. In another study by Yasuda, Banno, and Komatsu (2010) recording the posterior inferotemporal (PIT) region using multicellular recording techniques in non-human primates, researchers found colour and shape selective cells in the PIT region (Yasuda, Banno, Komatsu, 2010). Such studies further suggest that colour translation may be occurring within various cortical regions, despite V4 area being the main area in which colour information is processed.