Summary

Transcranial magnetic stimulation (TMS) has emerged as a suitable technique to investigate the network of cortical areas involved in human grasp/reach movements. Applied over the primary motor cortex (M1), TMS reveals the pattern of activation of different muscles during complex reaching-to-grasp tasks. Repetitive TMS (rTMS) used to induce “virtual lesions” of other cortical areas has allowed investigation of other cortical structures such as the ventral premotor cortex (PMv), dorsal premotor cortex (PMd) and the anterior intraparietal sulcus (aIPS). Each of these appears to contribute to specific aspects of reaching, grasping and lifting objects. Finally, twin-coil TMS studies can illustrate the time course of operation of parallel intracortical circuits that mediate functional connectivity between the PMd, PMv, the posterior parietal cortex and the primary motor cortices.

Introduction

The ease with which we can make reach-to-grasp movements conceals a good deal of the underlying complexity of the task. Thus, the target of the reach must be located in space; a decision must be made about the most appropriate type and orientation of grasp according to the weight and shape of the object; and the timing of the reaching movement of the arm must be synchronized with the opening of the hand so that the object can be grasped as effectively and quickly as possible (for a review see Castiello, 2005; see also Chapters 2 and 10).