As we have just seen, neurons from the P and M pathways mainly project to V1 in the primary visual cortex. What happens after V1? The answer is given in Figure 2.3. In order to understand this Figure, note that V3 is generally assumed to be involved in form processing, V4 in colour processing and V5/MT in motion processing (all discussed in more detail shortly). The P pathway associates with the ventral or “what” pathway that proceeds to the inferotemporal cortex. In contrast, the M pathway associates with the dorsal or “how” pathway (previously described as the “where” pathway) that
proceeds to the posterior parietal cortex. The assertions in the last two sentences are very approximate reflections of a complex reality. For example, some parvocellular neurons project into dorsal visual areas (Parker, 2007).
We will consider the P and M pathways in more detail later. For now, bear three points in mind:
1 The ventral or “what” pathway culminating in the inferotemporal cortex is mainly concerned with form and colour processing and with object recognition (see Chapter 3). In contrast, the dorsal or “how” pathway culminating in the parietal cortex is more concerned with movement processing.
2 There is no rigid distinction between the types of information processed by the two streams. For example, Gilaie-Dotan et al. (2013b) studied patients with brain damage limited to the ventral or “what” pathway. These patients had widespread impairments in motion perception even though visual motion perception is primarily associated with the dorsal or “how” pathway.
3 The two pathways are not totally segregated. There are numerous interconnections between the ventral and dorsal pathways or streams (Felleman &
Van Essen, 1991; Pisella et al., 2009). For example, both streams project to the primary motor cortex (Rossetti & Pisella, 2002).
As already indicated, Figure 2.3 provides only a rough sketchmap of visual processing in the brain. A more complex picture is presented in Figure 2.4, which reveals three important points. First, the interconnections among the various visual cortical areas are complicated. Second, the brain areas within the ventral pathway or stream are more than twice as large as those within the dorsal pathway. Third, cells in the lateral geniculate nucleus respond fastest when a visual stimulus is presented followed by activation of cells in V1. However, cells are activated in several other areas (V3/V3A, MT, MST) very shortly thereafter.
Finally, note that Figure 2.3 is limited in other important ways. Kravitz et al. (2013) proposed a contemporary account of the ventral pathway (Figure 2.5). The traditional view was that the ventral pathway involved a serial hierarchy proceeding from simple to complex. In contrast, Kravitz argued that the ventral pathway actually consists of several overlapping recurrent networks. Of key importance, there are connections in both directions between the components of the networks.
V1 and V2
We start with three general points. First, to understand visual processing in primary visual cortex (V1; also described as BA17) and secondary visual cortex (V2; also described as BA18), we must consider the notion of receptive field. The receptive field for any given neuron is that region of the retina in which light affects its activity. Receptive field can also refer to visual space because it is mapped in a one-to-one manner on to the retinal surface.
Second, neurons often influence each other. For example, there is lateral inhibition, in which a reduction of activity in one neuron is caused by activity in a neighbouring neuron. Lateral inhibition is useful because it increases the contrast at the edges of objects, making it easier to identify the dividing line between objects. The phenomenon of simultaneous contrast depends on lateral inhibition (see Figure 2.6). The two central squares are physically identical but the one on the left appears lighter than the one on the right. This difference is due to simultaneous contrast produced because the left surround is much darker than the right surround.
KEY TERMS
Receptive field
The region of the retina in which light influences the activity of a particular neuron.
Lateral inhibition
Reduction of activity in one neuron caused by activity in a neighbouring neuron.
Figure 2.3
The ventral (what) and dorsal (where or how) pathways involved in vision have their origins in primary visual cortex (V1).
From Gazzaniga et al. (2008). Copyright © 2009, 2002, 1998. Used by permission of W.W. Norton & Company, Inc.
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The visual cortex
Figure 2.4
Some distinctive features of the largest visual cortical areas. The relative size of the boxes reflects the relative area of different regions. The arrows labelled with percentages show the proportion of fibres in each projection pathway. The vertical position of each box represents the response latency of cells in each area, as measured in single-unit recording studies. IT = inferotemporal cortex; MT = medial or middle temporal cortex; MST = medial superior temporal cortex. All areas are discussed in detail in the text.
From Mather (2009). Copyright © 2009 George Mather. Reproduced with permission.
Figure 2.5
Connectivity within the ventral pathway on the lateral surface of the macaque brain. Brain areas involved include V1, V2, V3, V4, the middle temporal (MT)/medial superior temporal (MST) complex, the superior temporal sulcus (STS) and the inferior temporal cortex (TE).
From Kravitz et al. (2013). Reprinted with permission of Elsevier.
Third, the primary visual cortex (V1) and secondary visual cortex (V2) occupy relatively large areas (see Figure 2.4). Early visual processing in these areas is fairly extensive. Hegdé and Van Essen (2000) found in macaques that one-third of V2 cells responded to complex shapes and to differences in orientation and size.
V1 and V2 are both involved in the early stages of visual processing. However, that is not the complete story. There is an initial “feedforward sweep”
proceeding through the visual areas starting with V1 and then V2. In addition, there is a second phase of processing (recurrent pro cessing) in which processing proceeds in the opposite direction (Lamme, 2006). Some recurrent processing can occur in V1 within 120 ms of stimulus onset. Boehler et al.
(2008) found greater visual awareness of the stimulus when recurrent processing was strongly present (see Chapter 16).
Figure 2.6
The square on the right looks darker than the identical square on the left because of simultaneous contrast involving lateral inhibition.
From Lehar (2008). Reproduced with permission of the author.