The Dorsal Streams

The intraparietal sulcus is, evolutionary speaking, a very ancient sulcus. This sulcus, that is already present in prosimians, represents a fundamental parietal landmark. It subdivides the posterior parietal lobe into two main sectors: the superior parietal lobule (SPL) and the inferior parietal lobule (IPL). These two sectors receive different cortical inputs and have different connections with the motor cortex and frontal lobe. Early experiments in monkey showed that the superior parietal lobule (SPL) is part of the somatosensory system. It receives information from the primary sensory cortex, and in particular from those areas that code proprioception, and sends inputs to the primary motor cortex (F1) and to the dorsal premotor cortex (area F2). Recent neurophysiological data showed that SPL receives also visual inputs. Neurons activated by visual stimuli have been described in its caudal part (Galletti et al., 1996; Caminiti et al., 1996) and, in particular, in areas V6A and MIP. Both these areas are connected with frontal motor areas. Their main target is the dorsal premotor cortex (area F2) (Caminiti et al., 1996; Matelli et al., 1998).

Like SPL, also IPL (especially its rostral sector) receives somatosensory afferents. In addition, IPL is the main site of convergence of the pathways from the extrastriate visual areas of the dorsal stream. It projects to areas of the ventral premotor cortex (areas F4 and F5) and to the prefrontal lobe. The functional properties of IPL are in accord with the pattern of anatomical connections. IPL neurons are often bimodal responding both to visual and somatosensory stimuli (see also below). Taken together, these findings indicate that the parietal cortex performs two separate analysis of incoming sensory information. The analysis carried out in SPL (dorso-dorsal stream) concerns mostly proprioceptive input, but with an important visual contribution. The analysis performed in IPL (ventro-dorsal stream) consists in the integration of visual, auditory and somatosensory stimuli for action on the external world.

Before reviewing the organization of the ventro-dorsal stream in detail, an important point about the homology between human and other primates parietal lobe organization should be clarified. Human posterior parietal lobe as that of the macaque monkey is formed by two lobules - SPL and IPL -separated by the intraparietal sulcus. As shown by Forster (1936), contrary to the original parcellation of Brodmann, SPL is basically co-extensive with area 5 (5a + 5b), while IPL with area 7 (7a + 7b).

This means that the cytoarchitectonic organization of the posterior parietal lobe is similar in monkeys and humans. This view was confirmed by von Bonin and Bailey (1947) who, adopting the terminology of von

Economo (1929), found that in monkeys as in humans SPL is formed in large part by area PE (area 5) and the IPL by areas PF (7b) and PG (7a).

Figure 17-2. A. The inferior parietal lobule: monkey-human homology. Lateral view of the macaque monkey brain showing the cytoarchitectonic parcellation of the superior and inferior parietal lobules according to Von Bonin and Bailey (1947). B. Lateral view of the human brain showing the cytoarchitectonic parcellation of the superior and inferior parietal lobules according to Von Economo (1929).

Thus, when monkey data on area 7 are used to discuss functional properties of human parietal lobe, they should be used in reference to human IPL and not SPL as one can be tempted to do on the basis of the (wrong) Brodmann map. Similarly, the data on monkey area 5 should be used in reference to human SPL.

In the following sections I will describe in detail two parallel parietal-premotor neural circuits, both components of the ventro-dorsal stream: the VIP-F4 network and the PF/PFG-F5 network. It will be shown that these networks are involved in the organization of action in space and space perception and in action understanding, respectively.

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