Figure 214

Neurons from the P layers and from the M layers mainly project to the primary visual cortex or V1 (see Figure 2.13). The P and M pathways are not totally segregated, because there seems to be an input from the M pathway into the P pathway (Nealey & Maunsell, 1994). There is good evidence that the P pathway has two divisions. When cytochrome oxidase is applied to the surface of V1, it becomes concentrated in areas of high metabolic activity. The areas associated with high metabolic activity are called blobs, whereas the areas of lower activity are called interblobs. These areas correspond to separate divisions within the P pathway. Cells in all three pathways (the M pathway; blob regions of the P pathway; interblob regions of the P pathway) respond strongly to contrast. Cells in the M pathway also respond strongly to motion, those in the blob regions of the P pathway respond strongly to colour, and those in the interblob regions respond strongly to location and orientation (Figure 2.14).

There seem to be three repeating substructures in area V2, consisting of thick stripes, thin stripes, and interstripes. The thick stripes represent the continuation of the M pathway, the thin stripes are a continuation of the P-blob pathway, and the interstripes are an extension of the P-interblob pathway.

After V2, there are two visual pathways proceeding further into the cortex, with these pathways corresponding to the magno and parvo layers. These are the parietal and temporal pathways, respectively (see Figure 2.13). We will be considering these pathways in more detail shortly and in the next chapter. For now, it should be noted that the parietal pathway is mainly concerned with movement processing, whereas the temporal pathway is concerned with colour and form processing.

The research of cognitive neuroscientists on the visual system was summarised by Zeki (1992, 1993). According to his functional specialisation theory, different parts of the cortex are specialised for different visual functions. This contrasts with the traditional view, according to which there was a unitary visual processing system.

Some of the main areas of the visual cortex in the macaque monkey are shown in Figure 2.15. The retina connects primarily to what is known as the primary visual cortex or area V1. The importance of area V1 is shown by the fact that lesions at any point along the pathway to it from the retina lead to total blindness within the affected part of V1. However, areas V2 to V5 are also of major significance in visual perception. Here are the main functions that Zeki (1992, 1993) ascribed to these areas:

• V1 and V2: these areas are involved at an early stage of visual perception. They contain different groups of cells responsive to colour and form, and may be said to "contain pigeonholes into which the different signals are assembled before being relayed to the specialised visual areas" (Zeki, 1992, p. 47). Research on cells in V1 by Hubel and Wiesel (e.g., 1968) is discussed in Chapter 4.

• V3 and V3A: cells in these areas are responsive to form (especially the shapes of objects in motion) but not to colour.

• V4: the overwhelming majority of cells in this area are responsive to colour; many are also responsive to line orientation.

• V5: this area is specialised for visual motion (Zeki found in studies with macaque monkeys that all the cells in this area are responsive to motion, but are not responsive to colour).

A central assumption made by Zeki (1992, 1993) was that colour, form, and motion are processed in anatomically separate parts of the visual cortex. Much of the original evidence came from studies of monkeys. However, there is now considerable evidence from humans that Zeki's assumption is broadly correct, although form processing occurs in several different areas. Some of this evidence is considered next.

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