Grip forces isolated from knowledge about object properties following a left parietal lesion.

When lifting two objects with equal weight but different size, we judge the smaller object to be heavier. This size-weight illusion has been intensively tested by the recruitment of fingertip grip forces during precision lifting. Previous findings have suggested that perceptual (object size) prediction can influence sensorimotor prediction (anticipatory grip force scaling to the object size) but these predictions could be processed independently. This study investigates whether the anticipatory scaling of the grip forces according to object properties critically depends on the integrity of the posterior parietal cortex (PPC) and how a deficit may affect the perceptual size-weight illusion. Here, we report the case of a patient, F.S., with a large left temporal parietal lesion intruding into the temporal cortex and limb apraxia, who did not show anticipatory scaling of fingertip grip force to object size whereas matched controls did. However, the patient's perception of the size-weight illusion was only impaired during his ipsi-lesional hand lifting. Our findings suggest that left parietal cortex may be particularly responsible for the anticipatory grip force scaling of both hands and the perceptual process of size-weight illusion involving ipsi-lesional hand motion.