ReviewNeuroimaging of cognitive functions in human parietal cortex
Introduction
Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have provided powerful tools for mapping the human brain. Neuroimaging has been particularly successful in mapping cortical visual areas in the human occipital [1] and temporal [2] lobes. The human parietal lobes (excluding somatosensory regions, which are not discussed here), which traditionally fall into the category of ‘association cortex’ because of their complex, multimodal responses, provide one of the next challenges for neuroimaging.
Regions of parietal cortex form a major component of the ‘dorsal stream’, which is thought to be involved fundamentally in spatial localization [3] and the control of action [4] (in contrast to the ventral stream, which is thought to be more involved in perceptual recognition). In patients with parietal damage, human neuropsychology has identified a host of deficits, including attentional disorders (such as hemispatial neglect and simultanagnosia), spatial localization disorders and sensorimotor coordination problems (optic ataxia and apraxia) [5]. Single-neuron recording in macaques has demonstrated numerous regions in parietal cortex that perform highly specialized spatial and sensorimotor functions (Fig. 1a) 6., 7..
Although monkey physiology and human neuropsychology have provided invaluable insights, these techniques have important limitations in providing an understanding of human parietal function. Comparisons of brain maps between humans and other primates show striking differences even in early sensory areas 8., 9., and one-to-one homologies are even less likely in higher-tier areas. Furthermore, the densely packed areas found in macaque parietal cortex are generally too small to be distinguished by the large lesions typical of most human neuropsychological studies. Neuroimaging thus holds promise for the mapping of human parietal cortex in greater detail than previously possible. What has it delivered so far?
In keeping with findings from human neuropsychology and monkey neurophysiology, numerous articles over the past decade have shown that the parietal lobes are activated in tasks involving visuomotor control, attention and eye movements. Here, we review the main new results in these areas, and also mention some of the other tasks that have been reported to activate parietal cortex. To facilitate localization and cross-species comparisons, Fig. 1 illustrates key functional and anatomical areas in macaque cortex (Fig. 1a) and the best estimates of homologous regions in human cortex based on the current literature (Fig. 1b).
Section snippets
Comparisons of human and monkey parietal cortex
Monkey neurophysiology has identified a number of parietal areas within the intraparietal sulcus (IPS) that respond during specific visuomotor processes. Briefly, these include areas specialized for saccades (lateral intraparietal area [LIP]) [10], reaching (parietal reach region [PRR], which includes both area V6A and the medial intraparietal area [MIP]) 11., 12., grasping (anterior intraparietal area [AIP]) [13], processing of shape and orientation (caudal IPS [cIPS]) [14], and movements
Lateral intraparietal area
Numerous areas within the IPS (e.g. the junction of the IPS and transverse occipital sulci [IPTO], which may include visual areas V7 and/or V3A; posterior IPS; and anterior IPS) are activated by both saccades and attention [22]. One of these areas may be the homologue of monkey LIP [23••], which is also strongly driven by saccades and attention [10]. The most likely candidate region lies in the mid-posterior IPS, responds strongly even during predictable saccades (which have reduced attentional
Parietal reach region
Neuroimaging studies have reported activation in the IPS during reaching movements [26]. It is not yet clear whether this region is distinct from other parietal areas. Reach activity was reported anterior to saccade activity in one study [27]. A more recent study using pointing (directing the finger towards a target without reaching to it) found, however, that although pointing and saccade regions overlapped, pointing-related activation was more medial [28]. Interestingly, a reach-related
Anterior intraparietal area
The human anterior IPS is activated during visually guided grasping 32., 33., although grasping activity appears to overlap completely with reach-related activity [34]. This area is a probable homologue of monkey AIP, which contains neurons that respond to the visual and motor components of the grasp and that are tuned to specific shapes to be grasped [35]. The human area is also activated by the tactile manipulation of objects 36., 37., by the observation of others’ hand movements [38], and
Caudal intraparietal sulcus
Human neuroimaging has identified a region in the caudal end of the IPS that is activated during object matching and grasping [32], as well as during discriminations of object size and orientation [40]. This area may be a homologue of monkey cIPS, an area that contains neurons selective to binocular disparity, shape and three-dimensional orientation, and that may send projections to AIP to provide information for the visual guidance of hand action 14., 41.. The relationship of cIPS to other
Ventral intraparietal area
Preliminary data suggest an area in human IPS that may correspond to monkey VIP. Like the monkey area, putative VIP in humans responds to visual motion towards the face as well as tactile stimulation of the face (SP Dukelow et al., unpublished data) and has multimodal responses [42••].
Attention and eye movements
Few would challenge the claim that the parietal lobes play an important role in visual attention 6., 43., the mechanism that enables us to direct our processing resources to a subset of the available information. Most physiological research on attention has focused on area 7 in the monkey inferior parietal lobule (IPL), which is believed to be homologous with area 7 in the human superior parietal lobule (SPL; Fig. 1) [44]. In the human, attention-related activation has been reported throughout
Other functions
In addition to the functions reviewed above, parietal activation has also been reported for a stunningly diverse range of stimuli and tasks. These include motion processing 52•., 66•., 67., 68., stereo vision [69], spatial 70., 71. and non-spatial working memory (which shows considerable overlap with visual attention activation [72••]), mental imagery [73], mental rotation [74], response inhibition 75., 76., task switching [77], alertness [78], calculation 79., 80., and even functions not
Why is parietal activation so general?
The most striking finding in a review such as this is the heterogeneity of stimuli and tasks that produce parietal activation. Why is parietal activation so general? We propose several possible explanations.
First, the parietal lobes may really be purely ‘association cortex’, a zone in which many related functions such as attention, spatial representation, working memory, eye movements and the guidance of actions come together. Although these topics have been treated traditionally as separate
Acknowledgements
We are grateful to Carol Colby, David Carey, Ewa Wojciulik and Mel Goodale for commenting on the manuscript. The authors are supported by grants from the McDonnell-Pew Program in Cognitive Neuroscience to JC Culham and from the National Eye Institute (EY Neuroscience to JC Culham and from the National Eye Institute (EY 13455) to NG Kanwisher.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
•of special interest
••of outstanding interest
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