In humans, functional imaging studies have found a homolog of the macaque motion complex, MT+, which is suggested to contain both the middle temporal (MT) and medial superior temporal (MST) areas in the ascending limb of the inferior temporal sulcus. In the macaque, the motion-sensitive MT and MST areas are adjacent in the superior temporal sulcus. Electrophysiology has identified several motion-selective regions in the superior temporal sulcus (STS) of the macaque. Two of the best-studied areas include the MT and MST areas. The MT area has strong projections to the adjacent MST area and is typically subdivided into the dorsal (MSTd) and lateral (MSTl) subregions. While MT encodes the basic elements of motion, MST has higher-order motion-processing abilities and has been implicated in the perception of both object motion and self motion. The macaque MST area has been shown to have considerably larger receptive fields than the MT area. The receptive fields of MT cells typically extend only a few degrees into the ipsilateral visual field, while MST neurons have receptive fields that extend well into the ipsilateral visual field. This study tentatively identifies these subregions as the human homologs of the macaque MT and MST areas, respectively (Fig. 1). Putative human MT and MST areas were typically located on the posterior/ventral and anterior/dorsal banks of a dorsal/posterior limb of the inferior temporal sulcus. These locations are similar to their relative positions in the macaque superior temporal sulcus.
TopIntroduction
The human visual area V5, also known as the MT (middle temporal) visual area, is a region of the extrastriate visual cortex that is thought to play a major role in motion perception, the integration of local motion signals into global percepts and the guidance of some eye movements. MT is connected to a wide array of cortical and subcortical brain areas. Its inputs include the visual cortical areas V1, V2 and dorsal V3 (dorsomedial area), the koniocellular regions of the LGN and the inferior pulvinar. The pattern of projections to the MT area changes somewhat between the representations of the foveal and peripheral visual fields, with the latter receiving inputs from areas located in the midline cortex and retrosplenial region.
A standard view holds that V1 provides the “most important” input to the MT area. Nonetheless, several studies have demonstrated that neurons in the MT are capable of responding to visual information, often in a direction-selective manner, even after the V1 area has been destroyed or inactivated. Moreover, studies by Semir Zeki and collaborators have suggested that certain types of visual information may reach the MT area before they reach the V1 area. The MT area sends its major outputs to areas located in the cortex immediately surrounding it, including the FST, MST and V4t (the middle temporal crescent) areas. Other MT projections target the eye movement-related areas of the frontal and parietal lobes (frontal eye field and lateral intraparietal area).
In primates, a motion-sensitive area in the occipitotemporal visual cortex was identified both functionally and anatomically. It was named the V5 area or MT area, after its middle temporal location in the owl monkey (Duffy and Wurtz, 1991b). Recently, it has been renamed the MT+ area, indicating that it probably comprises functionally segregated subregions (Albright and Desimone, 1987; Ha¨ndel et al, 2007). In the macaque, it is now accepted that the preference of V5/MT+ for motion stimuli is rooted in the receptive field properties of retinal M ganglion cells, which project exclusively to neurons of the magnocellular subdivisions of the dorsal lateral geniculate nucleus (Huk et al, 2002). This magnocelluar pathway has been shown to project to V5/MT+ (Huk et al, 2002). With the advent of non-invasive brain imaging tools, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), human motion processing has been investigated more directly than was possible in the past (Tianyi et al, 2009).
Several neuroimaging studies have localized the human homolog of the monkey motion complex that is often referred to as MT+ and includes the middle temporal (MT) and other adjacent motion-sensitive areas, such as the medial superior temporal (MST). The monkey MST area has been shown to have considerably larger receptive fields than the MT area. The receptive fields of MT cells typically extend only a few degrees into the ipsilateral visual field, while MST neurons have receptive fields that extend well into the ipsilateral visual field (Tootell et al, 1995). Raiguel et al. (1997) recorded neurons in the MST whose receptive fields extended 30–40° into the ipsilateral field (Ha¨ndel et al, 2007), whereas MT receptive fields protruded only 10–15° into the ipsilateral field.
No study has been able to distinguish the MT area from the MST area in humans. Although previous experiments have assessed ipsilateral responses within human MT and thus offer some evidence for large receptive fields within a region of the MT area, our experiments are unique in that they provide conclusive evidence for a double-dissociation of the human MT and MST areas. A previous study of MT subdivisions defined a putative MT area as the region of MT that did not exhibit ipsilateral responses (Smith et al, 2006; Boussaoud et al, 1992). Our experiments use two complementary measurements, one measuring relatively large receptive fields and the other measuring relatively small receptive fields. In addition to providing positive evidence for the existence of human MT and MST areas, our measurements revealed a retinotopic organization in human MT that was similar to proposals that have been previously documented in macaque MT. This evidence further strengthens the case for homology between these cortical motion-processing structures in humans and macaques. Here, we show that, similar to the results found in monkeys, the two areas are adjacent and can be functionally separated in humans.