The over-representation of contralateral space in parietal cortex: a positive image of directional motor components of neglect?

The activity of single cells was recorded in behaving monkeys while they performed several eye-hand directional motor tasks. The results revealed that in parietal area 7a there exists a directional representation of eye and hand motor space that, contrary to that of superior parietal, premotor and motor cortex, is highly skewed toward the contralateral workspace. In man, the loss of this representation after parietal lesions might explain the emergence of the directional movement disorders of neglect. In fact, although unilateral neglect is consequence of damage to different brain structures, it is more common and enduring after right inferior parietal cortex lesions. Neglect patients ignore and avoid interacting with events occurring in the contralesional part of their physical and mental space. Current theories distinguish perceptual from motor components of neglect. One key feature of the latter is directional hypokinesia, an impaired representation of space for action, evident as difficulty to plan hand movements toward the contralesional part of egocentric space. An impairment of a similar nature is also observed for eye movements. In this study, we offer an interpretation of directional movement disorders of neglect from a physiological perspective, i.e. by focusing on the mechanisms underlying the representation of visuomotor space in parietal cortex.