Two different groups discovered the middle temporal visual area (MT, or V5) in the owl monkey (Allman & Kaas, 1971) and rhesus monkey (Dubner & Zeki, 1971) around the same time in 1971. MT was described as a visual cortical area
containing a preponderance of neurons with selective responses to motion directions (Baker et al., 1981; Felleman & Kaas, 1984; Maunsell & Van Essen, 1983a, 1983b; Van Essen et al., 1981; Zeki, 1974, 1980), which plays an important role in the initiation of slow, smooth-pursuit eye movements (Lisberger et al., 1987). Area MT receives inputs from various cortical and subcortical areas including LGN, V1, V2, V3, and etc. and projects to downstream of MT in the dorsal pathway, such as MST and VIP (Figure 5) (Maunsell & Van Essen, 1983c).
Figure 4 | Organization of direction preferences in a region of ferret V1 using optical imaging: direction preferences are color-coded. The direction of arrows overlaid on the color map indicates the preferred directions of cells and length of arrows shows the magnitude of direction selectivity. The figure is adapted from Weliky et al., 1996.
MT inputs are dominated by direct projections originating from direction selective (Movshon & Newsome, 1996), speed tuned (Orban et al., 1986) neurons in V1, which show preferences for binocular disparity (Prince et al., 2000). As much as 90% of V1 projections to MT originate from the layer 4B (Maunsell &
Van Essen, 1983c; Shipp & Zeki, 1989; Tigges et al., 1981). The cells in the layer 4B of V1 sending axons to MT have distinct characteristics. These cells are large and have dense dendritic trees located close to the bottom of the layer (Nassi &
Callaway, 2009). It has been shown that V1 inactivation (Girard et al., 1992) or its removal (Rodman et al., 1989) impairs both responsiveness, and to less extent, direction-selectivity of MT neurons. Connections between the superior colliculus (SC) and MT and callosal connections are thought to account for the MT residual direction-selective responses after inactivation or removal of V1(Born & Bradley, 2005; Girard et al., 1992; Movshon & Newsome, 1996; Rodman et al., 1990).
Several studies showed columnar organization of direction selectivity (Albright et al., 1984; Dubner & Zeki, 1971; Geesaman et al., 1997), binocular
disparity2 (DeAngelis & Newsome, 1999), and to some extent, speed preference (Liu & Newsome, 2003) in area MT of macaque monkeys. MT cells are reported to be tuned for not only motion direction but also motion speed and binocular disparity (Baker et al., 1981; Felleman & Kaas, 1984; Maunsell & Van Essen, 1983a). Several reports, however, indicated that stimulus shape (Albright, 1984) and color (Zeki, 1983) do not have noticeable effects on MT responses. By functional binocular alignment, a recent research by Czuba et al. (Czuba et al., 2014) rule out the contributions of static disparity tuning to the 3D motion tuning and proposes that MT cells encode the information of 3D motion.
Moving along the dorsal hierarchy, the receptive fields become larger, e.g.
an MT cell with RF centered at 10o eccentricities may have a receptive field size of 10o diameter, whereas the RF of V1 neuron centered at the same location may be around 1o diameter (Andersen, 1997; Born & Bradley, 2005). About half of the MT cells have RFs with center-surround antagonism, which means that a moving stimulus located in the center region of the RF (classical RF, or stimulation field) maximally drives the cell and following the invading of stimulus into the surrounding region (suppressive field) responses become suppressed (Allman et al., 1985; Born, 2000; Bradley & Andersen, 1998; DeAngelis & Uka, 2003;
Raiguel et al., 1995; Tanaka et al., 1986). The highest suppression occurs when the stimuli in the center and surround regions have the same direction and disparity, meaning that the driven response of the neuron depends on the saliency of the stimulus in the center relative to the surround stimulus (Bradley
& Andersen, 1998). The other half of the MT neurons have receptive fields with reinforcing surrounds, meaning that they optimally respond to wide-field motion (Born, 2000).
2 Difference between the image location of a visual stimulus on the two retinae, which plays an important role in stereoscopic depth perception
Figure 5 | Major inputs into area MT: thickness of arrow is roughly proportional to the magnitude of the inputs. Thickest arrows indicate direct cortical pathways. The figure is adapted from Born & Bradley, 2005.
Responses of cells in area MT are affected by several nonretinal sources, such as attention, smooth pursuit eye movements, and saccadic eye movements (extraretinal effects). In early studies of attentional effects on MT responses, Treue & Maunsell (Treue & Maunsell, 1996) and Seidemann & Newsome (Seidemann & Newsome, 1999) showed that attention enhances the responses of MT cells to the stimulus inside the RF. Smooth pursuit eye movements are the movements of the eye enabling us to track the moving of visual targets. These eye movements allow us to have a stabilized image of the moving objects on or near the fovea (Ono, 2015). It has been shown that responses of MT cells play a critical role in initiating the pursuit eye movements. Newsome et al. (DeAngelis et al., 1998; Parker & Newsome, 1998) have indicated that pursuit eye movements are initiated following the discharge of a subset of direction-selective MT cells, pursuit cells. The discharge of pursuit cells is reduced once the eye velocity reaches the target. These cells have foveal RFs and their responses are modulated by pursuit eye movements. Saccadic suppression is a phenomenon where the perception of image motion on the retina is dependent on its origin: the image motion induced by saccade (rapid gaze shift) is not perceived while the image motion induced by an external stimulus is perceived. Thiele and colleagues
(Thiele et al., 2002) reported a subset of direction-selective MT cells, ‘saccadic suppressive cells’, with responses silenced during saccadic image motion whereas responded well to an identical external image motion. They proposed that existence of saccadic suppressive cells accounts for the saccadic suppression. (A more recent study demonstrates that V1 cells also exhibit different responses to identical retinal motion depending it is internally or externally generated (Troncoso et al., 2015).)