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Abstract
The small visual area known as MT or V5 has played a major role in our understanding of the primate cerebral cortex. This area has been historically important in the concept of cortical processing streams and the idea that different visual areas constitute highly specialized representations of visual information. MT has also proven to be a fertile culture dish-full of direction- and disparity-selective neurons-exploited by many labs to study the neural circuits underlying computations of motion and depth and to examine the relationship between neural activity and perception. Here we attempt a synthetic overview of the rich literature on MT with the goal of answering the question, What does MT do?
Key Words
extrastriate, motion perception, center-surround antagonism, magnocellular, structure-from-motion, aperture problem
INTRODUCTION
The middle temporal visual area (MT or VS) of the macaque monkey possesses a number of attributes that have made it particularly attractive to systems neuroscientists. This region is typical of extrastriate cortex but is still readily identifiable both anatomically and functionally. Though extrastriate, it is still quite close to the retina-its principle inputs as few as five synapses from the photoreceptors-a feature which means, among other things, that the mechanisms by which its receptive field properties arise can be profitably studied. And, although MT neurons are near enough to the inputs to be mechanistically tractable, they are also close enough to some outputs-in particular, those involved in eye movementsto provide an easily measurable, continuous readout of computations performed in this pathway. Finally, MT neurons are concerned with visual motion, which is of obvious ethological importance, which has been extensively characterized psychophysically, and for which there are well-defined mathematical descriptions. Much of the work on MT has focused on its role in visual motion processing, though, as we hope to make clear in what follows, MT plays a richer and more varied role in vision.
MT WAS A KEY PART OF THE EARLY EXPLORATION OF EXTRASTRIATE CORTEX
Part of MT's significance is historical; it played an important role in the discovery of new extrastriate visual areas (Felleman & Van Essen 1991) and in the idea that they constitute specialized representations of the visual world (Zeki 1978, Barlow 1986).
At the beginning of the twentieth century, primate visual cortex was thought...