Space coding in primate posterior parietal cortex

Neuropsychological studies of patients with lesions of right frontal (premotor) or posterior parietal cortex often show severe impairments of attentive sensorimotor behavior. Such patients frequently manifest symptoms like hemispatial neglect or extinction. Interestingly, these behavioral deficits occur across different sensory modalities and are often organized in head- or body-centered coordinates. These neuropsychological data provide evidence for the existence of a network of polymodal areas in (primate) premotor and inferior parietal cortex representing visual spatial information in a nonretinocentric frame of reference. In the monkey, a highly modular structural and functional specialization has been demonstrated especially within posterior parietal cortex. One such functionally specialized area is the ventral intraparietal area (VIP). This area is located in the fundus of the intraparietal sulcus and contains many neurons that show polymodal directionally selective discharges, i.e., these neurons respond to moving visual, tactile, vestibular, or auditory stimuli. Many of these neurons also encode sensory information from different modalities in a common, probably head-centered, frame of reference. Functional imaging data on humans reveal a network of cortical areas that respond to polymodal stimuli conveying motion information. One of these regions of activation is located in the depth of human intraparietal sulcus. Accordingly, it is suggested that this area constitutes the human equivalent of monkey area VIP. The functional role of area VIP for polymodal spatial perception in normals as well as the functional implications of lesions of area VIP in parietal patients needs to be established in further experiments.