Exploring bodily representations: spatial maps around, in, and on the body.

My Doctoral Thesis investigated different aspects of bodily and spatial representations, how they are modulated by multisensory stimulation, and some physiological correlates of their manipulation. Chapter #1, “The space around the body”, reports three studies on prism adaptation, a technique that takes advantage of brain plasticity in the generation and modification of spatial bodily maps. Specifically, Study #1 investigates how the vision of the limb during two different versions of prism adaptation modulates their aftereffects: larger aftereffects take place with a concurrent vision of the limb after prism adaptation achieved through “ecological” visuo-motor activities; conversely, the vision of the very last part of the movement brings about larger aftereffects after the repeated pointing task. Study #2 examines the effect of a multisensory stimulation during prism adaptation, showing that the pointing error reduction is obtained with fewer pointing movements when the target is a visual-acoustic (multisensory), rather than unisensory stimulus. Finally, Study #3, which was performed in a brain-damaged patient, showed that the integrity of the left parieto-cerebellar circuit is required for an appropriate spatial remapping of proprioceptive maps to occur, and that the modulation through transcranial Direct Current Stimulation of these cortical areas temporarily restores the aftereffects. Chapter #2, “The space in the body”, reports studies on the link between bodily spatial representations and homeostatic regulation. Skin temperature has been recently considered as a physiological parameter of disembodiment, and can be modulated by the manipulation of bodily representations. Three experiments in which spatial bodily maps were manipulated by means of different techniques, inducing direction-specific and lateralized effects, are presented. Specifically, Study #4 showed a reduction of hands’ skin temperature after adaptation to right shifting, but not to left shifting, optical prisms. In Study #5, a modulation of temperature during leftward, but not rightward, optokinetic stimulation was found. Preliminary results from Study #6 show that the sole lateral shift of visual attention is not sufficient to induce a specific skin temperature modulation. Chapter #3, “The space on the body”, concerns the perception of tactile distances, namely, the spatial relationships between single objects that simultaneously touch the skin. In Study #7 a sensory adaptation-aftereffects paradigm was used to show that a tactile distance aftereffect can be induced; this tactile aftereffect shares many lower-level characteristics of classic visual aftereffects, such as orientation and location specificity. These findings suggest that the processing of spatial relationships among tactile events takes place at an early stage of somatosensation. Overall these results suggest the following: firstly, some bodily and spatial representations are susceptible to multisensory stimulations, especially those underpinning the sense of location of the body, sustained by the high-order posterior parietal cortex; secondly, modulation of skin temperature may be considered as an index of modifications of the multisensory representation of the body; thirdly, other bodily maps, such as those providing information about its metric, used in order to process tactile spatial relationships, are lower-level, likely arising at early stages of somatosensory processing.