primate cerebral cortex
Melvyn A. Goodale
University of Western Ontario, London, Canada
Behavioral and electrophysiological evidence suggests a new interpretation of the division of labor between the ventral and dorsal streams of visual processing in primate cerebral cortex. It is suggested that the ventral stream mediates the perception of objects while the dorsal stream mediates the on-line control of skilled actions directed at those objects.
Ungerleider and Mishkin  first suggested the possibility that the visual pathways projecting from striate cortex to other cortical regions could be divided into two relatively independent 'streams' of visual processing. According to their original account, a ventral stream, arising in striate cortex and eventually terminating in the inferotemporal region, plays a critical role in the identification and recognition of objects, while a dorsal stream, projecting from striate cortex to the posterior parietal cortex, mediates the localization of those same objects. Since its introduction just over ten years ago, this distinction between object vision and spatial vision ('what' versus 'where') has become one of the most familiar functional dichotomies in visual neuroscience.
With the discovery that the retinal projections to the parvocellular (P) and magnocellular (M) layers of the dorsal lateral geniculate nucleus (LGNd) have different response properties and that these cytologically-defined channels remain segregated at the level of primary visual cortex and perhaps beyond (for review, see [2,3˙˙]), attempts were made [4,5] to relate the differences in the properties of these channels to the distinction between two streams of processing made earlier by Ungerleider and Mishkin. According to at least one account , the P pathway, which had been linked to color and form vision, was thought to be the main contributor to the ventral stream (and thus object vision), while the M pathway, which was believed to be responsible for motion perception and stereopsis, was thought to provide the main input to the dorsal stream (and thus spatial vision). If such an account were true, then the primate visual system (within the geniculostriate route, at least) would consist of two independent and parallel sets of inputs that arise in the retina and remain segregated even at the highest levels of visual processing.
Although empirical support for this idea was initially quite compelling, recent evidence from a broad range of studies has challenged this simple account of visual processing. The results of psychophysical studies in monkeys with discrete lesions of the parvo- and magnocellular layers of LGNd [6-8] suggest that the distinctions between the kinds of visual processing carried out by these two pathways are not as sharp as was originally thought (with the exception of color vision which appears to depend almost entirely on the P pathway [6,7]). In fact, even the earlier electrophysiological studies of the responses of ganglion and geniculate cells had shown considerable overlap in the range of effective stimuli (for review, see [3˙˙]). The current consensus is that the difference between the two channels is largely one of relative sensitivity within the temporal and spatial domains, with the M pathway favoring high temporal and low spatial frequencies, and the P pathway, low temporal and high spatial frequencies. In summary, the specializations of the parvo and magno pathways are perhaps best understood in terms of a trade-off amongst the different requirements of spatial, wavelength, and temporal processing [2,3˙˙] and thus it is difficult to argue that one channel is specialized for object vision and the other for spatial vision.
Recent electrophysiological studies by John Maunsell and his colleagues [9,10˙˙,11] suggest that the anatomical segregation of the magno and parvo inputs to the dorsal and ventral streams is also much less clear-cut than was originally thought. Thus, while inactivation of the magnocellular layers of the LGNd almost always reduces the responsivity of cells in the middle temporal area (MT), some cells in this dorsal stream area are also affected by inactivation of the parvocelluar layers, although to a lesser degree . In V4, however, which is part of the ventral stream, most neurons are affected equally by inactivation of either the parvocellular or magnocellular layers of the LGNd [10˙˙], a result which is consistent with the earlier observation that inputs from both of these cytological subdivisions are present in the blob and interblob regions of V1 . In summary then, both the ventral and____________________
LGNd--dorsal lateral geniculate nucleus; LIP--lateral intraparietal sulcus; M--magnocellular; MST--medial superior temporal area; MT--middle temporal area; P--parvocellular.