Exp Brain Res (1996) 112:103-111 9 Springer-Verlag 1996 RESEARCH ARTICLE Scott T. Grafton 9 Michael A. Arbib Luciano Fadiga 9 Giacomo Rizzolatti Localization of grasp representations in humans by positron emission tomography 2. Observation compared with imagination Received: 5 January 1996/Accepted: 21 May 1996 Abstract Positron emission tomography imaging of ce- rebral blood flow was used to localize brain areas in- volved in the representation of hand grasping move- ments. Seven normal subjects were scanned under three conditions. In the first, they observed precision grasping of common objects performed by the examiner. In the second, they imagined themselves grasping the objects without actually moving the hand. These two tasks were compared with a control task of object viewing. Grasp observation activated the left rostral superior temporal sulcus, left inferior frontal cortex (area 45), left rostral inferior parietal cortex (area 40), the rostral part of left supplementary motor area (SMA-proper), and the right dorsal premotor cortex. Imagined grasping activated the left inferior frontal (area 44) and middle frontal cortex, left caudal inferior parietal cortex (area 40), a more ex- tensive response in left rostral SMA-proper, and left dor- sal premotor cortex. The two conditions activated differ- ent areas of the right posterior cerebellar cortex. We pro- pose that the areas active during grasping observation may form a circuit for recognition of hand-object inter- actions, whereas the areas active during imagined grasp- ing may be a putative human homologue of a circuit for hand grasping movements recently defined in nonhuman primates. The location of responses in SMA-proper con- firms the rostrocaudal segregation of this area for imag- s. T. Grafton (ES~) l The USC PET Imaging Sciences Center, Departments of Radiology and Neurology, University of Southern California, Los Angeles, Calif., USA M. A. Arbib Center for Neural Engineering, University of Southern California, Los Angeles, Calif., USA L. Fadiga. G. Rizzolatti Istituto Di Fisiologia Umana, Universita Degli Studi Di Parma, Italy Present address." I Departments of Neurology and Radiology, Emory University PO Drawer V, 1639 Pierce Drive, 6000 Woodruff Building, Atlanta, GA 30322, USA; Fax: +1 404-727-3732, e-marl: sgrafto@emory.edu ined and real movement. A similar segregation is also present in the cerebellum, with imagined and observed grasping movements activating different parts of the pos- terior lobe and real movements activating the anterior lobe. Key words Motor control 9 Positron emission tomography. Cerebral blood flow 9 Grasp 9 Imagined movements 9 Human Introduction Previous electrophysiological studies in nonhuman pri- mates as well as functional neuroimaging studies in hu- mans define a diversified set of cortical areas that are ac- tive during goal-directed grasping movements. Multiple frontal motor areas involved in hand control can be iden- tified in nonhuman primates (Dum and Strick 1991; Gentilucci et al. 1988; He et al. 1993, 1995; Hepp-Rey- mond et al. 1994; Kurata 1993; Kurata and Tanji 1986; Luppino et al. 1991; Matelli et al. 1986; Matsuzaka et al. 1992; Matsumura and Kubota 1979; Muakkassa and Strick 1994; Rizzolatti et al. 1988, 1981). Additional sites in parietal and temporal cortex are likely to be es- sential for spatial representation, visuomotor transforma- tion, and the pragmatic manipulation of objects (see Jeannerod et al. 1995; Milner and Goodale 1995; and Wise and Desimone 1988 for recent reviews). Functional imaging studies are beginning to identify putative human homologues to some of the areas defined in monkey brain (Grafton et al. 1996, 1992; Haxby et al. 1991; Par- sons et al. 1995; Tyszka et al. 1994; Watson et al. 1993). An enduring challenge is to define the functionality that each of these cortical sites contributes to movement. Of particular interest in the present study is the problem of localizing where entire movements or gestures are repre- sented in the motor system. Within the constraints of functional neuroimaging, the localization of areas where movements or gestures are represented can be investigated with at least three ap-