Evidence from Monkey Studies

The weight of evidence from single-unit studies of the dorsal stream in monkeys suggests that the spatial coding of visual information is primarily in egocentric coordinates. Although most visually sensitive cells in the posterior parietal cortex show spatial tuning, they are modulated by the concurrent motor behavior of the animal (e.g., Hyvarinen and Poranen 1974; Mountcastle et al. 1975). In reviewing the electrophysiological studies which have been carried out on the posterior parietal cortex, Andersen (1987) concluded that most neurons in these areas "exhibit both sensory-related and movement-related activity." The activity of some visually driven cells in this region has been shown to be linked to saccadic eye movements; the activity of others, to whether or not the animal is fixating a stimulus; and the activity of still other cells, to whether or not the animal is engaged in visual pursuit or is making goal-directed reaching movements (e.g., Snyder et al. 1997).

In other words, as far as spatial coding in the dorsal stream is concerned, the current electro-physiological evidence indicates that egocentric frames of reference predominate. Indeed, there is no clear evidence that allocentric spatial coding —insofar as this refers to the position of an object with respect to other objects—exists in the posterior parietal cortex. Some coding of object size and of configurational properties within single objects is required for guiding actions such as grasping, and evidence is accumulating that subsets of neurons in monkey parietal cortex are able to code some of these object properties (for review, see Sakata et al. 1998). At the same time, there is evidence that cells which are primarily related to limb movements also show subtle modulations by eye and head position (Andersen 1997). But while these mechanisms might permit different levels of abstraction in the coding of location, the coding would nonetheless remain intrinsically egocentric.

Lesion studies of the posterior parietal cortex in monkeys have also revealed deficits in egocentric spatial coding. Misreaching in space is the most immediately obvious visual effect of large lesions of the posterior parietal cortex in monkeys, just as it is in humans with large lesions in the superior regions of the posterior parietal cortex (Ettlinger 1977). And just as is the case in the human patient with optic ataxia, the misreaching in the monkey is a visuomotor, not a purely visuospatial, deficit. For example, after unilateral lesions, the reaching disorder is invariably restricted to the monkey's contra-lesional arm. In other words, the monkey has no problem reaching to the same visual targets with its ipsilesional arm (Ettlinger and Kalsbeck 1962; Hartje and Ettlinger 1974; Faugier-Grimaud et al. 1978; Lamotte and Acufia 1978). Yet at the same time, a simple motor disorder is ruled out by the fact that the monkey is able, once it has grasped a food object, to bring it efficiently and accurately to its mouth.

Smaller lesions in the posterior parietal cortex can result in more specific deficits. For example, they can result in deficits that are restricted to saccadic eye movements (Lynch and McLaren 1989), ocular pursuit (Dursteler and Wurtz 1988), or grasping (Gallese et al. 1994). Indeed, except for the well-known landmark studies discussed earlier (Pohl 1973; Ungerleider and Brody 1977), there is little evidence for deficits in the allocentric coding of spatial relations. In fact, the observed deficits in the landmark task (and other maze and route-finding tasks) appear to be more related to deficits in the visual control of eye, hand, or lower limb movements than to a fundamental deficit in allocentric coding of position (for a review, see Goodale 1995; Milner and Goodale 1995).

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