Mental rotation of objects versus hands: Neural mechanisms revealed by positron emission tomography STEPHEN M. KOSSLYN, a,b GREGORY J. DIGIROLAMO, a WILLIAM L. THOMPSON, a and NATHANIEL M. ALPERT c a Department of Psychology, Harvard University, Cambridge, MA, USA b Department of Neurology, Massachusetts General Hospital, Boston, USA c Department of Radiology, Massachusetts General Hospital, Boston, USA Abstract Twelve right-handed men participated in two mental rotation tasks as their regional cerebral blood flow ~rCBF! was monitored using positron emission tomography. In one task, participants mentally rotated and compared figures com- posed of angular branching forms; in the other task, participants mentally rotated and compared drawings of human hands. In both cases, rCBF was compared with a baseline condition that used identical stimuli and required the same comparison, but in which rotation was not required. Mental rotation of branching objects engendered activation in the parietal lobe and Area 19. In contrast, mental rotation of hands engendered activation in the precentral gyrus ~ M1!, superior and inferior parietal lobes, primary visual cortex, insula, and frontal Areas 6 and 9. The results suggest that at least two different mechanisms can be used in mental rotation, one mechanism that recruits processes that prepare motor movements and another mechanism that does not. Descriptors: Mental rotation, Mental imagery, Positron emission tomography, Cognitive neuroscience Objects in visual mental images can be manipulated much like actual objects, which can help one to reason about the conse- quences of the corresponding physical manipulation ~see Shep- ard & Cooper, 1982!. Indeed, Shepard and Metzler ~1971! found that when people compared two similar objects at different ori- entations, an increment of time is required for each degree of angular disparity between the objects. These and similar findings ~for reviews, see Kosslyn, 1980, 1994; Shepard & Cooper, 1982! indicate that people perform such tasks by “mentally rotating” an object as if it were moving through the intermediate positions along a trajectory, as would occur if the object were physically rotated. But there is a mystery here: objects are constrained by the laws of physics to move along trajectories, whereas internal representations are not. The laws of physics do not prevent a mental image from undergoing instantaneous translation from one position to the next. The present study was designed to test one possible account for the fact that people visualize objects rotating through trajectories. One type of account we considered was inspired in part by two very different sets of findings. First, Parsons ~1987, 1994! found that people can rotate images of body parts more easily if the parts move in natural ways; for example, it is easier to visualize a hand rotating if the rotation corresponds to a comfortable movement than if it does not ~see also Cooper & Shepard, 1975; Sekiyama, 1983!. Second, Georgopoulos, Lurito, Petrides, Schwartz and Mas- sey ~1989! found that neurons in the motor strip discharge before a monkey begins to shift a lever in a specific arc and that there is an orderly sequence of activity over time: Neurons that are tuned for orientations near the starting point of the lever fire first, neu- rons that are tuned for slightly displaced orientations fire next, and so on. Both sets of results suggest that motor processes play a role in mental rotation. Kosslyn ~1994! offered a theory of how such a mechanism might operate: He suggested that visual mental images arise via the same mechanisms that “prime” the representations of expected objects during perception, but during imagery an indi- vidual anticipates seeing an object so strongly that its visual rep- resentation is activated from memory and a spatial pattern is reconstructed in topographically mapped visual cortex ~cf. Neisser, 1976!. According to this theory, an imaged object rotates through a trajectory because the person is anticipating what he or she would see if the object were physically manipulated, and objects are physically constrained to move along trajectories. A recent positron emission tomography ~ PET! result appears to be consistent with this view. Alivisatos and Petrides ~1997! asked participants to decide whether alphanumeric characters faced nor- mally or were mirror-reversed in two conditions: in one, the letters were upright; in the other, the letters were tilted various amounts This research was supported by Office of Naval Research Grant N00014- 94-1-0180. We thank Adam K. Anderson, Christopher F. Chabris,Avis Loring, and Steve Weise for technical assistance. Gregory J. DiGirolamo’s present address: Department of Psychology, University of Oregon, Eugene, OR 97403, USA. Address reprint requests to: S. M. Kosslyn, Department of Psychology, Harvard University, 830 William James Hall, 33 Kirkland Street, Cam- bridge, MA 02138, USA. E-mail: smk@wjh.harvard.edu. Psychophysiology, 35 ~1998!, 151 – 161. Cambridge University Press. Printed in the USA. Copyright © 1998 Society for Psychophysiological Research 151