Action and perception
The visual control of actions -- from saccadic eye movements to skilled grasping movements of the hand -- depends on visual mechanisms that are functionally and neurally independent from those mediating our conscious perception of the world. Goodale and Milner (1992) have proposed that the distinction between vision-for-action and vision-for-perception can be mapped onto the two streams of visual projections arising from early visual areas in primate cerebral cortex: a dorsal stream projecting to the posterior parietal cortex and a ventral stream projecting to inferotemporal cortex. According to this account, for which Goodale and Milner amassed an impressive array of anatomical, neuropsychological, electrophysiological, and behavioural evidence, the dorsal ‘action’ stream provides transforms incoming visual information into the required coordinates for skilled motor behaviour whereas the ventral ‘perceptual’ stream provides the rich and detailed representation of the visual world required for cognitive operations.
Two Visual Streams
It seems self-evident the actions we perform on visible objects make use of the same visual representation that allows us to perceive those objects. This idea, which is commonly accepted by many philosophers and scientists, is sometimes referred to as the Assumption of Experience-based Control (Clark, 2002). According to this view, the visual system creates a single ‘general-purpose’ representation of the external world that provides a platform for both cognitive operations and the real time control of goal-directed actions. There are good reasons to believe, however, that such a monolithic account is incorrect.
The retina sends direct projections to more than a dozen separate networks in the primate brain, in which the processing has been shaped by the particular output mechanisms that each network serves. One of the most prominent pathways runs from the eye to the dorsal part of the lateral geniculate nucleus in the thalamus and from there to an area in the occipital lobe known variously as primary visual cortex, striate cortex, area 17, or V1. Beyond V1, visual information is conveyed to a complex network of areas extending from the occipital lobe into the parietal and temporal lobes. Despite the complexity of the interconnections between these different areas, two broad "streams" of visual projections from area V1 and other early visual areas were identified in the primate brain over thirty years ago: a ventral stream projecting eventually to the inferior part of the temporal lobe and a dorsal stream projecting to the posterior part of the parietal lobe (Ungerleider & Mishkin, 1982). The two streams are not only intimately interconnected but the different areas within them send prominent projections back to area V1. Moreover, both streams also receive inputs from a number of other subcortical visual structures, such as the superior colliculus in the midbrain, which projects to areas in the dorsal stream (via the thalamus). Although most of what we know about the organization of these pathways in the primate brain is derived from neurophysiological and neuroanatomical studies in the monkey, the advent of neuroimaging, particularly functional magnetic resonance imaging (fMRI) has revealed that the projections from area V1 to extra-striate regions in the human brain can be separated into ventral and dorsal streams similar to those seen in the monkey (Tootell, Tsao, & Vanduffel, 2003).
The Division of Labour Between the Two Streams
Why did two separate streams of visual processing evolve in the primate brain? To put it another way, what are the functional differences between the two streams? In theoretical paper published more than twenty years ago, Goodale and Milner (1992) proposed that the ventral stream plays the major role in constructing the perceptual representation of the visual world and the objects within it, while the dorsal stream mediates the visual control of actions directed at those objects. Note that this is not the distinction between ‘what’ and ‘where’ (object vision and spatial vision) that was originally put forward by Ungerleider and Mishkin (1982), who first described the two streams in the monkey. In the Goodale and Milner scheme, the structural and spatial attributes of an object are processed by both streams, but for different purposes. In the ventral stream, the transformations deliver the enduring characteristics of objects and their relations, permitting the formation of long-term perceptual representations that constitute the contents of our visual consciousness. Such representations play an essential role in the identification of objects and enable us to classify objects and events, attach meaning and significance to them, and establish their causal relations. Such operations are essential for accumulating a knowledge base about the world, communicating with others, and planning future courses of action. In contrast, the transformations carried out by the dorsal stream deal with the moment-to-moment information about the size, geometrical structure, location, and disposition of a goal object -- thereby mediating the visual control of skilled actions, such as reaching out and grasping that object. The dorsal stream can be regarded as a cortical extension of the dedicated visuomotor modules in the midbrain and brainstem that mediate visually guided movements in all vertebrates. The two streams of visual processing work together in the production of adaptive behaviour. The perceptual representations constructed by the ventral stream interact with various high-level cognitive mechanisms and enable an organism to select a particular course of action with respect to objects in the world while the visuomotor networks in the dorsal stream (and associated cortical and subcortical pathways) are responsible for the programming and on-line control of the particular movements that action entails. For a detailed discussion of the differences between the functions of the two streams, see Milner and Goodale (2006).
References
Clark A. (2002). Is seeing all it seems? Action, reason and the grand illusion. Journal of Consciousness Studies, 9, 181-202.
Goodale, M.A. & Milner, A.D. (1992). Separate visual pathways for perception and action. Trends in Neurosciences, 15, 20-25.
Milner, A.D., & Goodale, M.A. (2006). The Visual Brain in Action: 2nd Edition. Oxford: Oxford University Press, 297 pp.
Tootell, R.B., Tsao, D.Y., & Vanduffel, W. (2003). Neuroimaging weighs in: humans meet macaques in “primate” visual cortex. Journal of Neuroscience, 23, 3981-3989.
Ungerleider, L.G. & Mishkin, M. (1982). Two cortical visual systems. In D.J. Ingle, M.A. Goodale, & R. Mansfield (eds.). Analysis of Visual Behavior, 549-586. Cambridge MA: MIT Press.
