Unitary haptic perception: integrating moving tactile inputs from anatomically adjacent and non-adjacent digits.

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 intraparietal sulcus was activated specifically when integration across the midline was required, irrespective of whether the stimulation 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.