The Parietal Visual System and some Aspects of Visuospatial Perception

A complex function of highly developed brains is their capacity to construct and update veridical images of surrounding space and to identify accurately the spatial relations between objects in that space. These perceptual and control functions are executed ordinarily at preconscious levels, as witness the powerful but usually unnoticed role of the visual flow fields in guiding locomotion. These central images of the immediate surround can be brought immediately to conscious perception by directed attention. Studies of humans and other primates indicate that the parietal lobe system plays an important role in these aspects of visuospatial perception and in the visual guidance of pointing, reaching, and locomotion. Evidence obtained in neurophysiological studies in waking monkeys conforms with these general statements, for the functional properties of the several major classes of neurons identified in the posterior parietal homotypical cortex of the monkey mimic many of the functions attributed to the parietal system in man, and mirror the defects seen in man and other primates after parietal lobe lesions (Mountcastle et al. 1975; Lynch et al. 1977).