Contributions of the PPC to online control of visually guided reaching movements assessed with fMRI-Guided TMS

The posterior parietal cortex (PPC) plays an important role in controlling voluntary movements by continuously integrating sensory information about body state and the environment. We tested which subregions of the PPC contribute to the processing of target- and body-related visual information while reaching for an object, using a reaching paradigm with 2 types of visual perturbation: displacement of the visual target and displacement of the visual feedback about the hand position. Initially, functional magnetic resonance imaging (fMRI) was used to localize putative target areas involved in online corrections of movements in response to perturbations. The causal contribution of these areas to online correction was tested in subsequent neuronavigated transcranial magnetic stimulation (TMS) experiments. Robust TMS effects occurred at distinct anatomical sites along the anterior intraparietal sulcus (aIPS) and the anterior part of the supramarginal gyrus for both perturbations. TMS over neighboring sites did not affect online control. Our results support the hypothesis that the aIPS is more generally involved in visually guided control of movements, independent of body effectors and nature of the visual information. Furthermore, they suggest that the human network of PPC subregions controlling goal-directed visuomotor processes extends more inferiorly than previously thought. Our results also point toward a good spatial specificity of the TMS effects. © 2010 The Author

Reduced parietal connectivity with a premotor writing area in writer's cramp

Item does not contain fulltextWriter's cramp is a task-specific form of dystonia with symptoms characterized by abnormal movements and postures of the hand and arm evident only during writing. Its pathophysiology has been related to faulty sensorimotor integration, abnormal sensory processing, and impaired motor planning. Its symptoms might appear when the computational load of writing pushes a tonically altered circuit outside its operational range. Using resting-state fMRI, we tested whether writer's cramp patients have altered intrinsic functional connectivity in the premotorparietal circuit. Sixteen patients with right-sided writer's cramp and 19 control subjects were studied. We show that writer's cramp patients have reduced connectivity between the superior parietal lobule and a dorsal precentral region that controls writing movements. This difference between patients and controls occurred in the absence of writing and only in the hemisphere contralateral to the affected hand. This finding adds a novel element to the pathophysiological substrate for writer's cramp, namely, task-independent alterations within a writing-related circuit

Selective modulation of interactions between ventral premotor cortex and primary motor cortex during precision grasping in humans

In humans, the rostral part of the ventral premotor cortex (PMv), the homologue of F5 in monkeys, is known to be critically involved in shaping the hand to grasp objects. How does information about hand posture, that is processed in PMv, give rise to appropriate motor commands for transmission to spinal circuits controlling the hand? Whereas PMv is crucial for skilled visuomotor control of the hand, PMv sends relatively few direct corticospinal projections to spinal segments innervating hand muscles and the most likely route for PMv to contribute to the control of hand shape is through cortico-cortical connections with primary motor cortex (M1). If this is the case, we predicted that PMv–M1 interactions should be modulated specifically during precision grasping in humans. To address this issue, we investigated PMv–M1 connections by means of paired-pulse transcranial magnetic stimulation (TMS) and compared whether they were differentially modulated at rest, and during precision versus power grip. To do so, TMS was applied over M1 either in isolation or after a conditioning stimulus delivered, at different delays, over the ipsilateral PMv. For the parameters of TMS tested, we found that, at rest, PMv exerted a net inhibitory influence on M1 whereas, during power grip, this inhibition disappeared and was converted into a net facilitation during precision grip. The finding that, in humans, PMv–M1 interactions are selectively modulated during specific types of grasp provides further evidence that these connections play an important role in control of the hand