Abstract
In the present article, we investigated the possibility of inducing phantom tactile sensations in healthy individuals similar to those that we observed in patients after stroke. On the basis of previous research, we assumed that manipulating visual feedbacks may guide and influence, under certain conditions, the phenomenal experience of touch. To this aim, we used the Tactile Quadrant Stimulation (TQS) test in which subjects, in the crucial condition, must indicate whether and where they perceive a double tactile stimulation applied simultaneously in different quadrants of the two hands (asymmetrical Double Simultaneous Stimulation trial, Asym-DSS). The task was performed with the left-hand out of sight and the right-hand reflected in a mirror so that the right-hand reflected in the mirror looks like the own left-hand. We found that in the Asym-DSS trial, the vision of the right-hand reflected in the mirror and stimulated by a tactile stimulus elicited on the left-hand the sensation of having been touched in the same quadrant as the right-hand. In other words, we found in healthy subjects the same phantom touch effect that we previously found in patients. We interpreted these results as modulation of tactile representation by bottom-up (multisensory integration of stimuli coming from the right real and the right reflected hand) and possibly top-down (body ownership distortion) processing triggered by our experimental setup, unveiling bilateral representation of touch.
Highlights
Tactile processing is a fundamental aspect of body ownership construction
Comparisons within each condition between types of error for each hand showed that, for the left-hand, in the Mirror Condition-Silhouette (MC-S), synchiric extinction was significantly greater than mislocalization [z = −4.630; p < 0.0001; r = 0.59] and extinction [z = −4.606; p < 0.0001; r = 0.59], and mislocalization was greater than extinction [z = −2.803; p < 0.01; r = 0.36]
We evaluated if it was possible to induce phantom tactile sensations in healthy subjects similar to those observed in patients after stroke, based on the assumption that vision can, under certain circumstances, guide and influence tactile perception
Summary
Tactile processing is a fundamental aspect of body ownership construction. It is characterized by both operations whose product remains implicit (i.e., linked to processes that do not reach the subject’s consciousness, e.g., Berti et al, 1999) and operations whose product becomes explicit and reported by the subject as conscious experience. Results showed that in DSS trials, at least 50% of the patients, ‘‘correctly’’ reporting a bilateral tactile experience, erroneously pointed, on the contralesional hand, to the quadrant corresponding to the one stimulated on the intact hand. We interpreted these findings as a manifestation of pathological neuroplastic mechanisms, triggered by the brain lesion, unmasking bilateral touch representation following unilateral stimulation (Noachtar et al, 1997; Hansson and Brismar, 1999; Tamè et al, 2012, 2016) that would be inhibited in the healthy brain (Medina and Coslett, 2016). The above mechanisms would be responsible for synchiria, when abnormal activation of homotopic representations are suprathreshold, or synchiric extinction, with sub-threshold homotopic representations requiring to be enhanced by stimulation of the affected hand
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