Area MT contains a complete representation of the contralateral visual field. Receptive field size increases linearly with eccentricity and is about ten times larger than in striate cortex, suggesting a significant convergence of inputs. The area is organized in a columnar fashion with blocks of cortical tissue containing similar directional selectivity. Projections from V1 originate from layer4B and 6, while projections from V2 originate from the cytochrome oxidase thick strips, that in turn receive a direct projection from 4B. Hence whether directly or indirectly through V2, the input to MT seems to be dominated by the M pathway.
Strong physiological evidence for an almost exclusive M contribution in MT was recently provided by experiments, in which the responses of MT units were studied while selectively blocking the activity in the magnocellular or parvocellular layers of dorsal lateral geniculate(dLGN)(Maunsell, Nealey. & DePriest, 1990). Whereas parvocellular blockade had minor effects on the activity of MT neurons, inactivation of magnocellular produced complete or nearly complete block of MT responses suggesting a predominance of the M pathway in the input of visual information to this area.
MT projects to various subcortical structures such as the ventral geniculate nucleus, the reticular nucleus of the thalamus, the claustrum the putamen, the caudate nucleus and the pontine nucleus MT is also connected to the inferior and lateral pulvinar nuclei. Extensive reciprocal connection has been shown between many cortical areas, which include the primary visual cortex,V2 V3, V3A, V4, the MST, the lateral intra-parietal(LIP, VIP, the FST and frontal eye fields (Maunsell &Van Essen, 1983).
A striking characteristic of MT is its high proportion of neurons that are selective to stimulus motion. More than 90% of the cells of this area are directionally selective, and most of them show selectivity to the stimulus orientation when tested with stationery, flashed bars (Maunsell &Van Essen, 1983). The response of the neurons to their preferred direction can be up to 10 times that to their non-preferred direction. Neurons in MT respond best to similar directions of motion. Neurons that respond to the same direction of motion cooperate, amplifying one another’s response, while neurons that respond to very different directions compete and inhibit one another. This results in a small amount of motion in a single direction being amplified and the effect of motion in various random directions being decreased (Sekular, R. & Blake, R. 1994).
Area MST is located medial to MT occupying most of the bottom half of the upper bank of the superior temporal sulcus (STS) and a small proportion of the adjacent floor, while FST lies anterior to MT (Logothetis, 1994). Both areas receive a direct projection from MT and project to posterior parietal cortex.
MST has a crude topographical organization, with cells that have large receptive fields including the fovea. Most are strongly direction selective, and they respond best to large moving stimuli. In addition over 90% of MST neurons were found to be sensitive to the disparity of the visual stimulus( i.e. how unique or uncommon it is). Some neurons respond strongly to rotating or isotropically expanding of contracting patterns of motion, as well as frontal parallel planar motion, and their response is invariant with respect to the size shape or contrast of the pattern. Such neurons might contribute to the analysis of the large optic flow stimulation generated as an observer moves through the visual environment (Logothetis 1994).
Area FST has no retinotopical organization. Receptive field size is large, usually including the fovea. About 32% of the cells in FST show directional selectivity mainly responding to complex object motion.
Area VIP, a projection zone described by Maunsell and Van Essen(1983), contains neurons with response properties very similar to those reported for area MT. In fact about 80% of VIP neurons respond at least twice and on average nine times as well to stimulus movement in the preferred direction as compared to stimulus movement tin the non-preferred direction. Many neurons in VIP are tuned to speed.
Lesion Studies
The primate cortex began to replace the cat as the primary object of study and the discovery of functional specialization in extra-striate visual cortex, including the motion area MT (V5) was a major advance. Area MT (V5) in monkeys is anatomically and physiologically distinctive and has been studied in length and great detail. Areas homologous to macaque MT appear to exist in all non-human primates, including Old World Monkeys, New World Monkeys and groups of prosimians (Tootell, Dale, Sereno, & Malach, 1996)).
Macaque and human vision is remarkably similar psychophysically and anatomically. This being the case human homologs of monkey middle temporal (MT) are reasonably well accepted. Topographical dimensions in human visual cortex have been found to be approximately double the size of the corresponding features in macaque. MT in humans appears homologous to macaque MT. Based on topographical and functional properties human pMSTd is also probably homologous to the dorsal division of area MST (MSTd) in macaques(Tootell, Dale, Sereno & Malach 1996).
Normally termed the middle temporal area (MT) due to its initial description in new world monkeys MT it is actually located in n the superior temporal sulcus of Old Word Monkeys and is referred to as V5. The Old World monkey provides important evidence for investigating motion perception in humans as is supposedly the most similar in visual function to that of humans.
Newsome and Pare(1988) trained macaque monkeys to discriminate the direction of a set of dots. To measure the animals’ ability at this task a number of the moving dots could be given a random direction of motion, thus producing patterns in which the movement was only partially correlated frame to frame. In normal circumstances the animal is very good at this task and only requires as little as 3% correlation to discriminate one direction from its opposite. When a small part of MT was destroyed, then the animal had great difficulties with the task if the stimulus was presented to the part of the visual field served by the lesioned section of MT. However the animal performed normally’ if the stimulus was presented in another part of the visual field. When other visual functions were tested such as contrast sensitivity and color perception it was found that the lesion did not affect vision. The results of this study support the notion that area MT is an area to which motion perception is localized for it is not that the animal cannot see the pattern, it simply cannot tell us in which direction the pattern moved.
Zihl et al. (1983) reported on patient LM, a woman unable to see objects move. LM is unable to see water flowing, cars moving and when she walks everything moves up and down. The loss of her ability to sense motion followed a bilateral cerebral vascular lesion. The patient performed normally in terms of acuity, visual fields, flicker detection color vision and contrast sensitivity. Testing of the patient has revealed that her deficit strongly resembled that of MT lesioned monkeys seen in Newsome et al.
Lesions in area MST produce deficits in pursuit of eye movements. Monkeys with MST lesions were unable to match eye speed to target speed or to correctly estimate the amplitude of the saccae needed to acquire the target to compensate for target motion, nor were they able to pursue smoothly after the target was foveated (Logothetis 1994). Area MST plays an important role in both the initiation and the maintenance or smooth pursuit. The fact that a subset of visually responsive, directional cells discharge during smooth pursuit of a small target in an otherwise dark room, suggests that MST also receives signals from extraretinal sources (Newsome et al., 1988).
Some of the most suggestive evidence for functional localization still comes from one of the oldest methods, lesion studies. Ablation and lesion studies examine what functions are lost when part of the cortex is removed in order to establish a causal relationship between the part of the brain removed and the function lost. However cause-effect conclusions are not always reliable. In a complex system why should removing one part explain a certain function if the entire system needs to be entact. That is, we have no idea of the design principles involved in the architecture of the brain.
Imaging studies
An alternative to ablation is the mapping of input/output functions and stages. Also other sophisticated anatomical and electrophysiological techniques have supported the claim that different parts of the cortex are indeed specialised. (Phillips, Zeki & Barlow, 1984). This does not however rule out lesion studies as an important method of investigating brain function as these different techniques are used in mutual support of each other. The use of multiple convergent methodologies is becoming increasingly important in order to eliminate the effect of method properties and help highlight brain functions
Imaging techniques including PET and fMRI function by reflecting local changes in blood flow or oxygenation or both that occur very near the neurons activated by a given experimental paradigm. FMRI is the newer of the two techniques and has better spatial and temporal resolution and permit unlimited scanning without risks from radioactive toxicity. This means that certain types of functional imaging studies not previously possible can now be attempted (Tootell, Dale, Sereno & Malach, 1996).
It has proved easier to reveal an apparent human area MT functionally in PET and fMRI experiments produced by moving as opposed to stationery stimuli. Roughly half of the cells in MT should respond to stimuli moving in a given local direction compared with control stimuli that are stationary and non-flickering. Human MT is located in a somewhat inferior position in humans than it is in monkeys. Human MT is closer to the temporal lobe than to the parietal lobe. It also usually lies on the inferior temporal sulcus. The posterior location of human MT in relation to its location in macaques is consistent with the locations of other human visual areas(Tootell, Dale, Sereno, & Malach, 1996)
Despite localization differences human MT functions like macaque MT. Results from fMRI studies suggest that human MT neurons are direction selective and motion- selective. They have very high contrast sensitivity (Tootell, Dale, Sereno, & Malach, 1996).
Other studies have noted motion selective activity located superior and posterior to the MT focus. From its general location PET studies have suggested that this area might correspond to area V3 of macaques, in which half the cells are motion and direction selective (Tootell, Dale, Sereno, & Malach, 1996). In macaques and other monkeys area MT is bordered by several small satellite areas the share strong interconnections and refine the motion selective information available from MT. Perhaps the more closely studied of these is MST, particularly the dorsal subdivision (MSTd). PET studies have suggested that certain foci activated in higher order vision motion processing tests might correspond to human MST area.
The oldest method for relating brain events to motion perception, lesion studies, has helped specify the anatomical locus of neurons crucially involved in motion perception. There are limitations however to the conclusions that can be drawn from lesion studies. And so we have seen how fMRI and PET studies examine patterns of activation in the brain in relation to detection of motion. The implication is clear. Within the human cortex there is a fractionation of the visual image(Snowden 1994). These fractionated parts of the image are processed by specialised centres localised in specific parts of the brain. Imaging studies have recently clarified the functional properties and retinotopy and determined anatomists have described the histology using autopsy material of these specialized centers. The main gap in our knowledge of the human visual cortex map concerns the cortical connections between these specialized centers.
Bibliography
Badcock, D. R. & Khuu, S. K. (2001) Independent fist- and second-order motion energy analyses of optic flow. Psychological Research.Psychological Research. 65, pp. 50-56
Gregory, R.L. (1974) Concepts and mechanisms of perception. Chapter 49 The brain as an engineering problem. Duckworth, London, pp. 547-565.
Logothetis, N. K. in Visual Detection of Motion (ed. Smith, A. T. & Snowden, R. J. ) 219-252 Harcourt Brace & Co., 1994
Maunsell, J. H. R., Nealey, T. A. & DePriest, D. D. (1990). Magnocellular and parvocellular contributions to responses in the middle temporal visual area(MT) of the macaque monkey. Neuroscience, Vol. 10, 3323-3334.
Maunsell, J. H. R. & Van Essen, D. C. (1983). Functional properties of neurons in middle temporal area of the macaque monkeyNeurophysiology, Vol. 49, 1127-1167.
Newsome, W.T. & Pare, E. B. (1988). A selective impairment of motion perception following lesions of the middle temporal visual area(MT). Neuroscience, Vol. 8, 2201-2211.
Phillips, C.G, Zeki,S. & Barlow, H.B. (1984). Localization of function in the cerebral cortex: past, present and future. Brain, 107, 327-361.
Sekular, R. & Blake, R. (1994) Perception. pp. 276-290
Snowden, R. J. in Visual Detection of Motion (ed. Smith, A. T. & Snowden, R. J. ) 51-84 Harcourt Brace & Co., 1994
Tootell, R. B., Dale, A. M., Sereno, M.I. & Malach, R. (1996). New images from the human visual cortex. Trends in Neuroscience. 19, pp. 481-489
Zihl, J., Cramon, D. V. & Mai, N. (1983). Selective disturbance of movement vision after bilateral brain damage. Brain, 106, 313-340.