P1: ARS/ary P2: ARS/plb QC: ARS/anil T1: ARSDecember 15, 1998 12:35 Annual Reviews AR076-14Annu. Rev. Neurosci. 1999. 22:319–49SPACE AND ATTENTIONIN PARIETAL CORTEXCarol L. ColbyDepartment of Neuroscience and Center for the Neural Basis of Cognition,University of Pittsburgh, Pittsburgh, Pennsylvania 15260Michael E. GoldbergLaboratory of Sensorimotor Research, National Eye Institute, Bethesda, MD 20892and Department of Neurology, Georgetown University School of Medicine,Washington, DC 20007KEY WORDS: saccadic eye movements, spatial representation, spatial perceptionABSTRACTThe space around us is represented not once but many times in parietal cor-tex. These multiple representations encode locations and objects of interest inseveral egocentric reference frames. Stimulus representations are transformedfrom the coordinates of receptor surfaces, such as the retina or the cochlea,into the coordinates of effectors, such as the eye, head, or hand. The trans-formation is accomplished by dynamic updating of spatial representations inconjunction with voluntary movements. This direct sensory-to-motor coordinatetransformation obviates the need for a single representation of space in envi-ronmental coordinates. In addition to representing object locations in motoriccoordinates, parietal neurons exhibit strong modulation by attention. Both top-down and bottom-up mechanisms of attention contribute to the enhancement ofvisualresponses.Thesalianceofastimulusistheprimaryfactorindeterminingtheneural response to it. Although parietal neurons represent objects in motor coordi-nates, visual responses are independent of the intention to perform specific motoracts.The US Government has the right to retain a nonexclusive, royalty-free license in and to anycopyright covering this paper.319Annu. Rev. Neurosci. 1999.22:319-349. Downloaded from arjournals.annualreviews.orgby University of Texas - Austin on 09/10/07. For personal use only.P1: ARS/ary P2: ARS/plb QC: ARS/anil T1: ARSDecember 15, 1998 12:35 Annual Reviews AR076-14320 COLBY & GOLDBERGINTRODUCTIONSpace is a supramodal construct not limited to a specific sensation. We canestimate an object’s spatial location from many different sensory cues, but onlywith our own movement can the accuracy of that estimation be determined.Although space must be calculated from sensation and calibrated by movement,it is not necessarily linked to any specific sensation or movement. Thus we canidentify a spatial location by sight, sound, or touch, and we can look at it orreach for it with either hand. Even Shakespeare understood this: In King LearReganinvitesthe newly blinded Gloucesterto“smell his way to Dover”(ActIII,scene vii). The nature of the brain’s spatial representation has been the subjectof much debate (see Stein 1992), though the parietal lobe is considered crucialfor that representation (Critchley 1953). In this review, we consider how spaceis represented in the parietal cortex of humans and monkeys, and how thatrepresentation is related to the generation of movement and to the more generalprocess of visuospatial attention.MULTIPLE REPRESENTATIONS OF SPACEIN PARIETAL CORTEXThe traditional view of the representation of space, supported by subjectiveexperience, is that we construct a single spatial map of the world in whichobjects and actions are represented in a unitary framework. The alternativeview holds that the brain constructs multiple spatial representations with eachrepresentation linked to a differentaction or region of space (Arbib 1991, Colby& Duhamel 1991, 1996; Fogassi et al 1996; Graziano et al 1994; Jeannerodet al 1995). While each spatial representation may be related to a specific classof actions, it is not clear whether the parietal lobe actually plans those actions(Snyder et al 1997) or only provides the sensory substrate for them (Goodale& Milner 1992).Behavioral and neuropsychological studies in humans indicate that multiplespatial reference frames are used to guide behavior and that parietal cortex iscentral to the construction of these representations. Patients with lesions of theparietal lobe exhibit a variety of spatial deficits. The most striking of these isneglect, the tendency to ignore objects in the half of space opposite the side ofthe lesion (Bisiach& Vallar 1988, Heilman 1979). Apatient with a right parietallobe lesion may fail to notice or respond to objects on the left, including foodon the left side of a plate or words on the left side of a page. Neglect occursin all sensory modalities and can be expressed relative to any of several spa-tial reference frames, including retinocentric, body-centered, and environment-centered. These deficits in spatial perception are matched by correspondingAnnu. Rev. Neurosci. 1999.22:319-349. Downloaded from arjournals.annualreviews.orgby University of Texas - Austin on 09/10/07. For personal use only.P1: ARS/ary P2: ARS/plb QC: ARS/anil T1: ARSDecember 15, 1998 12:35 Annual Reviews AR076-14SPACE AND ATTENTION 321deficits in the generation of spatially directed actions. For example, neglect canbe specific for stimuli presented at particular distances. Some patients tend toignore stimuli presented near the body, in peripersonal space, while respondingnormally to distant stimuli, or vice versa (Bisiach et al 1986, Cowey et al 1994,Halligan & Marshall 1991). Distance-specific neglect may be apparent onlywhen the subject must produce a motor response to the stimulus, and not whenspatial perception alone is tested (Pizzamiglio et al 1989).The variety of deficits observed following parietal lobe damage suggests thatparietal cortexmust contain more than one kind of spatial representation. Phys-iological and anatomical studies in monkeys have provided direct evidence thatparietal cortex contains several separate functional areas and multiple represen-tations of space (Andersen et al 1997; Caminiti et al 1996; Colby & Duhamel1991, 1996; Colby et al 1988; Jeannerod et al 1995; Lacquaniti et al 1995;Rizzolatti et al 1997; Stein 1992) (Figures 1 and 2).The anatomical connections between parietal areas and their cortical targetsprovide some insight into the spatial reference frames served. Parietal corticalareasarestronglylinkedwithareasofprefrontalcortex, premotorcortex,andthefrontal and supplementary eye fields, which themselvesencode object locationsin relation to a variety of reference frames (Gentilucci et al 1983; Graziano et al1994, 1997; Rizzolatti et al 1981b). These spatial
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