Neural substrates of knowledge of hand postures for object grasping and functional object use: evidence from fMRI.

A number of lines of evidence suggest that computation of hand posture differs for object grasping as compared to functional object use. Hand shaping for grasping appears to rely strongly upon calculations of current object location and volume, whereas hand shaping for object use additionally requires access to stored knowledge about the skilled manipulation specific to a given object. In addition, the particular hand postures employed for functional object use may be either prehensile (clenching, pinching) or non-prehensile (e.g., palming, poking), in contrast to the prehensile postures that are obligatory for grasping. In this fMRI study, we assessed the hypothesis that a left-hemisphere-lateralized system including the inferior parietal lobe is specifically recruited for the computation and recognition of hand postures for functional object use. Fifteen subjects viewed pictures of manipulable objects and determined whether they would be grasped with a pinch or clench (Grasp condition), functionally used with a pinch or clench (Prehensile Use condition), or functionally used with a palm or poke hand posture (Non-prehensile Use condition). Despite the fact that the conditions were equated for behavioral difficulty, significantly greater activations were observed in the left inferior frontal gyrus (IFG), posterior superior temporal gyrus (STG), and inferior parietal lobule (IPL) in Non-prehensile Use trials as compared to Grasp trials. Comparison of Non-prehensile Use and Prehensile Use activations revealed significant differences only in the left IPL. These data confirm the importance of the left IPL in storing knowledge of hand postures for functional object use, and have implications for understanding the interaction of dorsal and ventral visual processing systems.

On beyond mirror neurons: internal representations subserving imitation and recognition of skilled object-related actions in humans.

A considerable recent literature argues that the same representations, encoded by inferior prefrontal and parietal cells known as "mirror neurons", may be activated in both production and recognition of object-related actions. Here, we test several predictions derived from the contemporary literature on the parity between production and recognition and the putative emergence of the mirror neuron system from a system coding hand-object interactions. Forty-four patients with left-hemisphere stroke, 21 of whom exhibited ideomotor apraxia, performed a number of pantomime imitation and recognition tasks, and performance was scored with respect to hand posture, arm posture, amplitude, and timing. Consistent with predictions, there were strong relationships between object-related pantomime imitation and object-related pantomime recognition, and between imitation and recognition of the hand posture component of object-related actions. Skilled object-related gesture representations are likely to be closely tied to evolutionarily more primitive systems controlling object grasping, to emerge from a mapping between object and action information coded by ventral and dorsal streams, and to be lateralized to the left hemisphere in humans.