RESEARCH ARTICLE Unitary haptic perception: integrating moving tactile inputs from anatomically adjacent and non-adjacent digits Marius V. Peelen • Jack Rogers • Alan M. Wing • Paul E. Downing • R. Martyn Bracewell Received: 25 April 2009 / Accepted: 17 May 2010 / Published online: 8 June 2010 Ó Springer-Verlag 2010 Abstract How do we achieve unitary perception of an object when it touches two parts of the sensory epithelium that are not contiguous? We investigated this problem with a simple psychophysical task, which we then used in an fMRI experiment. Two wooden rods were moved over two digits positioned to be spatially adjacent. The digits were either from one foot (or hand) or one digit was from either foot (or hand). When the rods were moving in phase, one object was reliably perceived. By contrast, when the rods were moving out of phase, two objects were reliably perceived. fMRI revealed four cortical areas where activity was higher when the moving rods were perceived as one object relative to when they were perceived as two separate objects. Areas in the right inferior parietal lobule, the left inferior temporal sulcus and the left middle frontal gyrus were activated for this contrast regardless of the anatomical configuration of the stimulated sensory epithelia. By contrast, the left intrapari- etal sulcus was activated specifically when integration across the midline was required, irrespective of whether the stim- ulation was applied to the hands or feet. These results reveal a network of brain areas involved in generating a unified percept of the presence of an object that comes into contact with different parts of the body surface. Keywords Tactile integration Á fMRI Á Haptic perception Á Binding Á Intraparietal sulcus Introduction In order to recognize objects, the brain typically integrates information from multiple sensory sources. For example, when manually exploring an object, spatio-temporally consistent tactile information from multiple fingers informs us that this information, although coming from different fingers, is indeed coming from the same object. This is reminiscent of the Gestalt ‘common fate’ principle in the visual modality: dots moving with the same velocity will be reliably perceived as belonging to the same object, whereas dots moving independently will not cohere into a whole (Braddick 1980). Indeed one might consider these examples of what Helmholtz termed ‘unconscious infer- ence’, the notion that our perceptions are based on both a knowledge of the world and the data our sensory organs provide. A direct demonstration of tactile integration across digits was provided by Kitada et al. (2003), who showed that two wooden rods moving in phase on the volar surface of the right second and third fingers are reliably perceived as a single object. Furthermore, a subsequent fMRI study M. V. Peelen (&) Center for Mind/Brain Sciences (CIMeC), University of Trento, Via delle Regole 101, 38100 Matterello, TN, Italy e-mail: marius.peelen@unitn.it M. V. Peelen Department of Psychology, Harvard University, Cambridge, MA, USA M. V. Peelen Á J. Rogers Á P. E. Downing Á R. M. Bracewell School of Psychology, Bangor University, Gwynedd LL57 2AS, UK e-mail: m.bracewell@bangor.ac.uk J. Rogers MRC Cognition and Brain Sciences Unit, Cambridge, UK A. M. Wing Á R. M. Bracewell Behavioural Brain Sciences Centre, School of Psychology, University of Birmingham, Birmingham, UK R. M. Bracewell School of Medical Sciences, Bangor University, Wales, UK 123 Exp Brain Res (2010) 204:457–464 DOI 10.1007/s00221-010-2306-3