Anatomy

MT got its name from its anatomical location in the owl monkey, a New World primate (Allman and Kaas, 1971). The inappropriateness of this name for its location in the macaque brain (between the occipital and parietal lobes; Fig. 81.1) leads to its alternative, more neutral name, V5. Nonetheless, I will use the original term, MT, throughout, though V5 is entirely equivalent. There is little dispute at present that a homologous area is present in many species of primates and even prosimians (Krubitzer and Kaas, 1990). The area is jointly defined by two anatomical features: dense myelination and direct reciprocal connections with area V1 (Ungerleider and Mishkin, 1979; Van Essen et al., 1981; Zeki, 1974b). Figure 81.1 also schematizes some of the more important cortical and subcortical connections of MT. In addition to its input from V1, MT receives ascending input from V2, V3, and the lateral subdivision of the pulvinar complex. MT is connected with a wide variety of other cortical areas in the superior temporal sulcus (FST, STP, MST), the parietal lobe (VIP, LIP, 7a), and the frontal lobe areas (area 46, FEF, SEF) and by descending connections to the brainstem (dorsolateral pontine nuclei, DTN, and NOT) and midbrain (superior colliculus). In addition, it has extensive connections with the cerebellum.

Figure 81.1..  

Summary of location and main connections of area MT. The left panel shows the superior temporal sulcus opened up to reveal the areas inside, including MT highlighted in yellow. The right panel shows the main connections of MT; subcortical structures are indicated by oval symbols. DTN, dorsal terminal nucleus; FEF, frontal eye fields; FST, fundus of the superior temporal sulcus; MST, medial superior temporal; NOT, nucleus of the optic tract; Pll, pulvinar nucleus, pars lateralis; SC, superior colliculus; SEF, supplementary eye fields; STP, superior temporal polysensory; VIP, ventral intraparietal. (Left panel from Maunsell and Newsome, 1987, with permission.)

This suite of connections places MT near the middle of a cortical hierarchy for motion processing sometimes called the motion system or motion pathway (Fig. 81.1). It starts in V1, where directionally selective neurons first appear, and heads toward posterior parietal cortex, where many of the structures participate in planning upcoming movements (Andersen et al., 1997). On this pathway, MT is the last area to have a clear retinotopy (Maunsell and Van Essen, 1987) and the first to be strongly connected to explicitly premotor structures. However, MT is not a bottleneck in this pathway by any means; V1 also connects to target areas in posterior parietal and frontal cortices by parallel routes that bypass MT.

One of the hallmark features of MT's anatomical organization is its retinotopy: it contains a fairly orderly map of contralateral visual space. The map varies from individual to individual, is often incomplete, and often contains spatial irregularities (Maunsell, 1986). The coordinates of the map are fairly consistent, with the fovea represented laterally and the vertical meridian representation running along the area boundaries. In addition to this overall retinotopy, MT contains organized maps for several different physiological response properties, discussed below.