Stimulation of the Posterior Parietal Cortex Interferes with Arm Trajectory Adjustments during the Learning of New Dynamics

Abstract

Substantial neurophysiological evidence points to the posterior parietal cortex (PPC) as playing a key role in the coordinate transformation necessary for visually guided reaching. Our goal was to examine the role of PPC in the context of learning new dynamics of arm movements. We assessed this possibility by stimulating PPC with transcranial magnetic stimulation (TMS) while subjects learned to make reaching movements with their right hand in a velocity-dependent force field. We reasoned that, if PPC is necessary to adjust the trajectory of the arm as it interacts with a novel mechanical system, interfering with the functioning of PPC would impair adaptation. Single pulses of TMS were applied over the left PPC 40 msec after the onset of movement during adaptation. As a control, another group of subjects was stimulated over the visual cortex. During early stages of learning, the magnitude of the error (measured as the deviation of the hand paths) was similar across groups. By the end of the learning period, however, error magnitudes decreased to baseline levels for controls but remained significantly larger for the group stimulated over PPC. Our findings are consistent with a role of PPC in the adjustment of motor commands necessary for adapting to a novel mechanical environment.

A key region in the human parietal cortex for processing proprioceptive hand feedback during reaching movements

“…The combined results of this and other studies (Chib et al, 2009;Della-Maggiore et al, 2004;Vesia et al, 2010) identify this region as a key area for processing proprioceptive information for motor control. Furthermore, this region can be clearly distinguished from an area around the anterior IPS, which is crucial for online sensorimotor processing in the presence of visual feedback about the hand position.…”

“…This finding complements the studies based on which we included mIPS posterior into the current study. Della-Maggiore et al (2004) have shown that TMS applied over this area also interferes with the learning of novel dynamics induced by a force field, and Chib et al (2009) have demonstrated that this area is important for tracking the movement of a manipulandum in the absence of visual feedback. In both studies, processing of proprioceptive information was the key for successful task execution as in our proprioception-only condition.…”

“…Additionally, TMS over the sites aIPS, SMG superior , and SMG middle yielded significant disturbance of the correction to visual perturbations in that study. mIPS posterior was included because two TMS studies successfully used this stimulation site to interfere with the processing of proprioceptive hand/arm information during a motor task (Chib et al, 2009;Della-Maggiore et al, 2004). Finally, aIPS posterior and SMG inferior were included to complement the grid.…”

“…The tested sites constitute a compromise between feasibility of the study with respect to the testing time and covering the portions of the PPC that might be involved in online processing of sensory information about the reaching hand/ arm. We did not include more medial-posterior areas as they have mainly been associated with processing information about the target of reach (Bernier and Grafton, 2010;Diedrichsen et al, 2005;Gallivan et al, 2009;Vesia et al, 2010 (Reichenbach et al, 2011) previous TMS studies investigating online control or proprioceptive processing during reaching (Chib et al, 2009;Della-Maggiore et al, 2004;Reichenbach et al, 2011) (Table 1). With these sites, we covered a broad region of the left PPC for which sensorimotor processing related to our questions had been localized previously.…”

Cortical Processing during Dynamic Motor Adaptation

“…This imaging study also found that adaptation involved cerebral cortical structures, including primary motor and somatosensory cortices and posterior parietal cortex (PPC). The latter cortical area has also been investigated by Della-Maggiore et al (2004), who found that single-pulse TMS applied over the PPC after movement onset impaired adaptation to a curl field relative to controls. Therefore, the evidence from the human literature suggests that dynamic motor adaptation involves a cerebello-thalamocortical circuit.…”