TMS investigations into the task-dependent functional interplay between human posterior parietal and motor cortex

Abstract

Transcranial magnetic stimulation (TMS) can be used in two different ways to investigate the contribution of cortical areas involved in grasp/reach movements in humans. It can produce “virtual lesions” that interfere with activity in particular cortical areas at specific times during a task, or it can be used in a twin coil design to test the excitability of cortical projections to M1 at different times during a task. The former method has described how cortical structures such as the ventral premotor cortex (PMv), dorsal premotor cortex (PMd) and the anterior intraparietal sulcus (aIPS) are important for specific aspects of reaching, grasping and lifting objects. In the latter method, a conditioning stimulus (CS) is first used to activate putative pathways to the motor cortex from, for example, posterior parietal cortex (PPC) or PMd, while a second, test stimulus (TS), delivered over the primary motor cortex a few ms later probes any changes in excitability that are produced by the input. Thus changes in the effectiveness of the conditioning pulse give an indication of how the excitability of the connection changes over time and during a specific task. Here we review studies describing the time course of operation of parallel intracortical circuits and cortico-cortical connections between the PMd, PMv, PPC and M1, thus demonstrating that functional interplay between these areas and the primary motor cortices is not fixed, but can change in a highly task-, condition- and time-dependent manner.

Introduction

Although a common action such as reaching and grasping an apple in order to bring it to the mouth seems an almost automatic process, it requires a complex interaction between different cortical areas. Non-primary motor regions such as the premotor and the posterior parietal cortex elaborate crucial information as to the optimal motor plan that has to be performed. 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 synchronised with the opening of the hand so that the object can be grasped as effectively and quickly as possible [1], [2]. This information then has to be conveyed to the primary motor cortex and converted into the ultimate motor command.

Although fMRI studies indicate that an extensive bilateral network of frontoparietal areas, including the posterior parietal cortex (PPC), the ventral (PMv) and the dorsal (PMd) premotor cortex, become active during reach/grasp movements [2], they give little insight into the respective functional role of each area into particular components of the task. Lesion techniques in monkeys [3], [4]) or observations in stroke patients [5], [6], [7] can fill some of these gaps in knowledge by revealing the motor deficits that occur after dysfunction of particular brain areas. For example, functional imaging studies in healthy subjects show activation of the anterior intraparietal sulcus (aIPS) a sub-region of the PPC in association with visually guided grasping [8], [9]. Complementary studies of patients with lesions in the same area reveal marked deficits in hand preshaping during visually guided reach-to-grasp movements, whereas reaching remains relatively intact [5].

However, studies in human patients often involve large and complex lesions, and together with the existence of possible compensation mechanisms can often make behavioural results difficult to interpret. As we indicate below, transcranial magnetic stimulation (TMS) in healthy subjects can be applied in such a way as to interfere temporarily with function in particular cortical areas. It has therefore become a very useful additional technique to investigate the functional role of parietal and frontal areas in reach to grasp movements.