Spatial representations for action in parietal cortex

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Introduction

We live in an ever-changing sensory world. As we move our eyes and move through the environment, new images are continuously presented to the brain. Given such constantly changing input, it is remarkable how easily we are able to keep track of where things are. We can reach for an object, or look at it, or even kick it without making a conscious effort to assess its location in space.

The traditional view of spatial perception, strongly supported by subjective experience, is that we know where things are in some absolute, world-based frame of reference and use this spatial information to guide our movements. In this standard and intuitively plausible view, spatial perception is a monolithic process: the brain forms a single spatial representation of each object regardless of what action is going to be performed in relation to that object.

A new and somewhat counterintuitive view is that the brain represents the spatial location of an object many times over in different cortical areas and each representation is suited to certain kinds of behavioral responses 7, 15, 23. Neurophysiological research carried out in monkeys indicates that in areas responsible for controlling head movements, visually sensitive neurons encode the location of an object relative to the head 7, 8, 9, 31. In contrast, areas involved in limb movement have neurons which encode object location relative to limb position 16, 17, 18, 22, 32 and, in areas controlling eye movements, visually sensitive neurons encode the location of an object relative to the center of gaze 14, 20, 21.

Three cortical areas with distinctive spatial representations have been discovered in the intraparietal sulcus of rhesus monkeys [7]. Neurons in each area are selective for particular stimulus dimensions and particular regions of space. They signal not only where an object is but how to act on it. The representation of space in a given area reflects a particular motor output by which a stimulus can be acquired or avoided. These results suggest that parietal cortex contains multiple action-based spatial representations. In the scheme emerging from these studies, spatial perception is a modular process. A single object may be multiply represented in terms of the actions that can be performed on it. While this new view seems implausible at first glance, it is analogous to current views of visual perception. When we see a red bouncing ball, we perceive a single object, even though its shape, color and direction of motion are analyzed separately. Likewise, beneath the apparent unity of subjective spatial experience, may lie a diversity of spatial representations, each with specific knowledge of how to act on an object.

Section snippets

Visual areas of the intraparietal sulcus

Posterior parietal cortex is divided into a number of separate areas. Unlike lower level extrastriate visual areas, these parietal areas do not typically contain simple retinotopic maps and their borders cannot be defined with reference to ventral or horizontal meridian representations. Parietal areas have instead been initially identified on the basis of their connections with other cortical areas 1, 2, 10, 29. Connections alone, however, are not sufficient to define an area. For instance,

Spatial representation in area LIP

Neurons in the lateral intraparietal area are active in relation to both visual and oculomotor events 8, 9, 19, 21. They discharge when a visual stimulus appears in the receptive field and, for about half the population, discharge again when the monkey executes a saccade to the location cued by the stimulus (Fig. 2). The strength of the visual response is modulated by the behavioral set induced by the task. When the monkey must attend to the stimulus, the amplitude of the visual response is

Spatial representation in area VIP

The ventral intraparietal area is located in the fundus of the intraparietal sulcus and has been defined on the basis of its distinctive visual response properties [9]. Cortex dorsal to VIP in the anterior portion of the medial bank is purely somatosensory with an emphasis on hand representation (Fig. 1). Near the fundus of the sulcus, there is a sudden transition to a region of strong visual responsiveness. This visual area extends from the medial bank across the fundus to the lateral bank.

Spatial representation in area MIP

The medial intraparietal area (MIP) is located in the posterior portion of the medial bank. As shown in Fig. 1, a regular sequence of different cell types is encountered in penetrations parallel to the cortical surface. Near the shoulder of the sulcus, adjacent to convexity cortex, neurons respond well to passive somatosensory stimulation. Receptive fields are generally found on the limbs and are commonly restricted to the contralateral hand. Slightly deeper in the sulcus, cells are most active

Summary

Parietal cortex contains multiple representations of visual space. Single neurons in area LIP encode attended locations relative to the fovea, while some VIP neurons encode stimulus location relative to the head and some MIP neurons may encode location relative to the arm. These multiple representations are tailored to guide specific kinds of actions: eye movements, head movements and arm movements, respectively. The function of parietal cortex is to signal the location of attended objects

Acknowledgements

We thank Dr. Michael E. Goldberg, with whom these experiments were done and these concepts developed.

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