The role of parietal cortex in the online control of hand movement trajectory: an inactivation and a case report study

The role of the Superior Parietal Lobule (SPL) of the primate brain in the control of hand and eye movements has been investigated through two experiments. In one experiment two macaque monkeys were trained to perform arm movements under two task conditions. In the first the animal performed direct reaches from a central location to one of 4 peripheral visual targets. In a second condition (50% of the trials) a sudden change of target location occurred, either during the hand reaction-time (RT) or at movement-time (MT) onset. The animals were required to adjust as fast as possible their hand trajectory to reach the second target location. A behavioral testing was performed before and after SPL inactivation. The inactivated area had previously been studied in the same task by recording the activity of 225 cells that showed modulation by hand position, speed and movement direction, as well as by saccadic signals. In separate sessions, unilateral and bilateral injections of the GABA-A agonist muscimol were performed within area 5 (PE/PEc) of the SPL. As control, physiological saline was injected in the same loci. Bilateral muscimol injections caused an increase of the hand RT and MT toward the first target in the direct reaches, and to both the first and second target in the corrected ones. In the latter, this resulted in an increase of the time necessary for the correction of the hand trajectory and in an elongation of the hand-path toward the first target location. During corrected reaches, an elongation of the eye RT to both first and second target was also observed, together with a change of eye-hand coupling, which could partially explain the hand reaching disorder. These results identify SPL as a crucial node in the on-line control of hand and eye movement, and highlight the role of an eye impairment in the emergence of the movement disorder characteristic of Optic Ataxia (OA). In a second experiment a patient with OA from unilateral tumor lesion of the right SPL and a group of 8 age-matched normal control subjects were asked to perform reaching movements from a central position to peripheral visual targets presented on a touch-screen. The subjects were also requested to perform a similar center-out task under isometric condition, where a force had to be applied to an isometric manipulandum in order to move a visual cursor from the center of the workspace to peripheral visual targets. Both tasks were 3 performed with and without central fixation (extrafoveal and foveal condition, respectively). The results show no major impairments in the temporal aspects of the movement (such as eye and hand RT and MT). On the contrary, in the extrafoveal condition the patient showed larger constant errors (CE) of the movement endpoints than controls in both the reaching and isometric task. In the foveal condition statistically higher CEs were observed only in the isometric task. The patient also showed a higher variability in the endpoints' position as compared to controls across all tested conditions. These results show that OA can emerge not only when a hand movement is performed, but also when only a force pulse of desired strength and direction has to be generated.