Haptically guided grasping. FMRI shows right-hemisphere parietal stimulus encoding, and bilateral dorso-ventral parietal gradients of object- and action-related processing during grasp execution

The neural bases of haptically-guided grasp planning and execution are largely unknown, especially for stimuli having no visualrepresentations. Therefore, we used functional magnetic resonance imaging (fMRI) to monitor brain activity during hapticexploration of novel 3D complex objects, subsequent grasp planning, and the execution of the pre-planned grasps. Haptic objectexploration, involving extraction of shape, orientation and length of the to-be-grasped targets, was associated with the frontoparietal,temporo-occipital, and insular cortex activity. Yet, only the anterior divisions of the posterior parietal cortex (PPC) ofthe right hemisphere were significantly more engaged in exploration of complex objects (vs. simple control disks). None of theseregions were re-recruited during the planning phase. Even more surprisingly, the left-hemisphere intraparietal, temporal, andoccipital areas that were significantly invoked for grasp planning did not show sensitivity to object features. Finally, graspexecution, involving the re-recruitment of the critical right-hemisphere PPC clusters, was also significantly associated with twokinds of bilateral parieto-frontal processes. The first represents transformations of grasp-relevant target features and is linked tothe dorso-dorsal (lateral and medial) parieto-frontal networks. The second monitors grasp kinematics and belongs to the ventrodorsalnetworks. Indeed, signal modulations associated with these distinct functions follow dorso-ventral gradients, with left aIPSshowing significant sensitivity to both target features and the characteristics of the required grasp. Thus, our results from thehaptic domain are consistent with the notion that the parietal processing for action guidance reflects primarily transformationsfrom object-related to effector-related coding, and these mechanisms are rather independent of sensory input modality.