Evidence for the Involvement of the Posterior Parietal Cortex in Coordination of Fingertip Forces for Grasp Stability in Manipulation

1 Neuropediatric Research Unit, Department of Woman and Child Health, Karolinska Institutet, SE-17176 Stockholm, Sweden 2 Division of Human Brain Research, Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden 3 Physiology Section, Department of Integrative Medical Biology, Umeå University, SE-90187 Umeå, Sweden Submitted 24 October 2002; accepted in final form 10 June 2003 Grasp stability during object manipulation is achieved by the grip forces applied normal to the grasped surfaces increasing and decreasing in phase with increases and decreases of destabilizing load forces applied tangential to the grasped surfaces. This force coordination requires that the CNS anticipates the grip forces that match the requirements imposed by the self-generated load forces. Here, we use functional MRI (fMRI) to study neural correlates of the grip-load force coordination in a grip-load force task in which six healthy humans attempted to lift an immovable test object held between the tips of the right index finger and thumb. The recorded brain activity was compared with the brain activity obtained in two control tasks in which the same pair of digits generated forces with similar time courses and magnitudes; i.e., a grip force task where the subjects only pinched the object and did not apply load forces, and a load force task, in which the subjects applied vertical forces to the object without generating grip forces. Thus neither the load force task nor the grip force task involved coordinated grip-load forces, but together they involved the same grip force and load force output. We found that the grip-load force task was specifically associated with activation of a section of the right intraparietal cortex, which is the first evidence for involvement of the posterior parietal cortex in the sensorimotor control of coordinated grip and load forces in manipulation. We suggest that this area might represents a node in the network of cortical and subcortical regions that implement anticipatory control of fingertip forces for grasp stability. Address for reprint requests and other correspondence: H. H. Ehrsson, Functional Imaging Lab., Wellcome Dept. of Imaging Neuroscience, 12 Queen Square, London WC1N 3BG, UK (E-mail: h.ehrsson{at}fil.ion.ucl.ac.uk ).