FOOLING THE SENSE OF TOUCH: NEW EVIDENCE ON THE ROLE OF BODY REPRESENTATIONS IN SHAPING SOMATIC PERCEPTION

In everyday life, we use our hands to explore or manipulate objects. The processing of somatosensory inputs involves different representations of the body, and the resulting subjective experience of touch is strongly influenced by the type of body representation used for sensory processing. In the first instance tactile stimuli are processed within somatotopic maps that preserve the topographical organization of the physical body. Successively, the somatosensory inputs are further processed within the body schema which takes track of the postural changes of the body. In this framework, the current thesis is aimed at investigating the specific role of somatotopic maps and body schema in modulating the multi-digit tactile and thermo-tactile perception. This purpose has been achieved by “fooling” the sense of touch through two somatosensory illusions: Aristotle’s illusion and the Thermal Grill Illusion. In the first study we investigated the tactile processing occurring within the somatotopic maps. We approached to this issue by studying the multi-digit tactile perception as influenced by alterations in somatosensory maps. To address this specific aim, focal hand dystonia was chosen as a model of altered somatotopic maps. We used the Aristotle’s illusion to specifically investigate the functional meaning of the interplay between different fingers in tactile perception. Since, behavioural and neurophysiological studies indicate that the mechanisms underlying this illusion involve primary somatosensory cortex, and since a number of FHD neuroimaging studies have widely demonstrated anatomical alterations in the primary somatosensory cortex of FHD patients, we would expect that the Aristotle’s illusion is compromised in focal hand dystonia. This result would demonstrate a case of interdigit functional somatosensory alterations as specific counterpart of the widely reported anatomical alteration of somatosensory maps. However, FHD shares common somatosensory alterations features with other forms of focal dystonia and with PD. This is probably due to a common underlying factor: the abnormal basal ganglia activity. In order to exclude any possible role of basal ganglia in potentially modulation of Aristotle’s illusion in FHD we performed two experiments in which we compared the Aristotle’s illusion in FHD with other forms of focal dystonia and PD. Altogether, the two experiments: 1) confirm that fingers representation is specifically altered in FHD, and 2) demonstrate that abnormal alterations in fingers representation determine distorted tactile perception of objects simultaneously touched by different digits. The second study investigated the role the of body schema in processing thermotactile stimuli. More precisely, we studied how multiple thermotactile innocuous stimuli are combined in an unusual paradoxical painful sensation, termed Thermal Grill Illusion (TGI). In the classical TGI, innocuous warm and cold stimuli are arranged in a warm-cold-warm fashion. Intriguingly, this type of stimulation evokes a feeling of burning heat. In the second study, we applied the warm-cold-warm pattern on the first three digits (index, middle, and ring) and we observed whether the TGI was modulated by changing the relative spatial position of the fingers, that is by crossing or uncrossing the middle over the index. Using this simple method we found that the perceptual experience of paradoxical pain might arise from the integration of multiple thermal inputs based on the relative position of the stimulated fingers according to external frame of reference. Indeed, in the crossed position, the paradoxical heat sensation was reduced when the cooled finger was the middle, and increased when the cooled finger was the index. This interesting result indicates that the brain takes into account the relative spatial position of each stimulus to produce the paradoxical heat sensation termed TGI.