The question of how the brain represents the spatial relationship between the own body and external objects is fundamental. Here we investigate the neural correlates of the somatic perception of bimanual interaction with an external object. A novel bodily illusion was used in conjunction with functional magnetic resonance imaging (fMRI). During fMRI scanning, seven blindfolded right-handed participants held a cylinder between the palms of the two hands while the tendon of the right wrist

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... r muscle was vibrated. This elicited a kinesthetic illusion that the right hand was flexing and that the hand-held cylinder was shrinking from the right side. As controls, we vibrated the skin surface over the nearby bone beside the tendon or vibrated the tendon when the hands were not holding the object. Neither control condition elicited this illusion. The significance of the illusion was also confirmed in supplementary experiments outside the scanner on another 16 participants. The "bimanual shrinking-object illusion" activated anterior parts of the superior parietal lobule (SPL) bilaterally. This region has never been activated in previous studies on unimanual hand or hand-object illusion. The illusion also activated left-hemispheric brain structures including area 2 and inferior parietal lobule, an area related to illusory unimanual hand-object interaction between a vibrated hand and a touched object in our previous study. The anterior SPL seems to be involved in the somatic perception of bimanual interaction with an external object probably by computing the spatial relationship between the two hands and a hand-held object. The responses of human muscle spindle endings to vibration of non-contracting muscles. J Physiol 261: 673-693, 1976. Caspers S, Geyer S, Schleicher A, Mohlberg H, Amunts K, Zilles K. The human inferior parietal cortex: cytoarchitectonic parcellation and interindividual variability. Neuroimage 33: 430 -448, 2006. Cavada C, Goldman-Rakic PS. Posterior parietal cortex in rhesus monkey. I. Parcellation of areas based on distinctive limbic and sensory corticocortical connections. J Comp Neurol 287: 393-421, 1989. Connolly JD, Andersen RA, Goodale MA. FMRI evidence for a "parietal reach region" in the human brain. Exp Brain Res 153: 140 -145, 2003. Cramer SC, Weisskoff RM, Schaechter JD, Nelles G, Foley M, Finklestein SP, Rosen BR. Motor cortex activation is related to force of squeezing. Hum Brain Mapp 16: 197-205, 2002. Dai TH, Liu JZ, Sahgal V, Brown RW, Yue GH. Relationship between muscle output and functional MRI-measured brain activation. Exp Brain Res 140: 290 -300, 2001. Darian-Smith C, Darian-Smith I, Burman K, Ratcliffe N. Ipsilateral cortical projections to areas 3a, 3b, and 4 in the macaque monkey. J Comp Neurol 335: 200 -213, 1993. Dettmers C, Connelly A, Stephan KM, Turner R, Friston KJ, Frackowiak RS, Gadian DG. Quantitative comparison of functional magnetic resonance imaging with positron emission tomography using a force-related paradigm. Neuroimage 4: 201-209, 1996. Dettmers C, Fink GR, Lemon RN, Stephan KM, Passingham RE, Silbersweig D, Holmes A, Ridding MC, Brooks DJ, Frackowiak RS. Relation between cerebral activity and force in the motor areas of the human brain. J Neurophysiol 74: 802-815, 1995. Ehrsson HH, Fagergren A, Forssberg F. Differential fronto-parietal activation depending on the force used in a precision grip task: an fMRI study.