The cortical network for eye-hand coordination and its relevance to understanding motor disorders of parietal patients.

Cortical neurons in both superior (SPL) and inferior (IPL) parietal lobules are modulated by a variety of signals concerning planning and execution of eye and hand movement. Thanks to these properties, parietal neurons are ideally suited for eye-hand coordination during reaching. In SPL, a fundamental feature of neurons is the invariance of their directional tuning properties across tasks that require different forms of spatial relationships between the eye and the hand. In such conditions, the orientation of the preferred directions (PDs) of individual SPL cells cluster within a limited sector of space, the global tuning field (GTF), to be regarded as an ideal frame to dynamically match eye and hand signals on the basis of the orientation of their PDs. At the population level, the mean vectors of the GTF cover the direction continuum in a uniform fashion. These neurons are part of a parietal network richly interconnected with the premotor and motor areas of the frontal lobe. Thus, the reaching disorders of patients with optic ataxia might be interpreted as a consequence of the breakdown of the combinatorial mechanisms of the GTF of parietal neurons, and of their interplay with premotor cortex. In IPL, the main feature of eye and/or hand related neurons is the uneven distribution of their PDs, that mostly point toward the contralateral space. This anisotropy of the representation of directional motor space might explain the movement disorders that characterize directional hypokinesia in neglect patients. In conclusion, the study of the dynamic properties of parietal neurons and of their relationships with the premotor cortex via cortico-cortical connections provides a basis for an interpretation of movement disorders of parietal patients from a neurophysiological perspective.