Selective Inhibition of Volitional Hand Movements after Stimulation of the Dorsoposterior Parietal Cortex in Humans.

Inhibition is a central component of motor control. Although current models emphasize the involvement of frontal networks [1, 2], indirect evidence suggests a potential contribution of the posterior parietal cortex (PPC). This region is active during inhibition of upper-limb movements to undesired targets [3], and its stimulation with single magnetic pulses can depress motor-evoked potentials [4, 5]. Also, it has been speculated that alien hand movements caused by focal parietal lesions reflect a release of inhibition from PPC to M1 [6]. Considering these observations, we instructed 16 patients undergoing awake brain surgery to perform continuous hand movements while electrical stimulation was applied over PPC. Within a restricted dorsoposterior area, we identified focal sites where stimulation prevented movement initiation and instantly inhibited ongoing responses (which restarted promptly at stimulation offset). Inhibition was selective of the instructed response. It did not affect speech, hand movements passively generated through muscle electrical stimulation, or the ability to initiate spontaneous actions with other body segments (e.g., the feet). When a patient inadvertently performed a bilateral movement, a bilateral inhibition was found. When asked to produce unilateral movements, this patient presented a contralesional but not ipsilateral inhibition. This selectivity contrasted sharply with the unspecific inhibitions reported by previous studies within frontal regions, where speech and all limbs are typically affected (as we here confirm in a subset of patients) [7-10]. These results provide direct evidence that a specific area in the dorsoposterior parietal cortex can inhibit volitional upper-limb responses with high selectivity.