Effects of Rubber Hand Illusion and Excitatory Theta Burst Stimulation on Tactile Sensation: A Pilot Study

Abstract

Synchronous visuotactile stimulation on the own hidden hand and a visible fake limb can alter bodily self-perception and influence spontaneous neuroplasticity. The rubber hand illusion (RHI) paradigm experimentally produces an illusion of rubber hand ownership and arm shift by simultaneously stroking a rubber hand in view and a participant’s visually occluded hand. The aim of this cross-over, placebo-controlled, single-blind study was to assess whether RHI, in combination with high-frequency repetitive transcranial magnetic stimulation (rTMS) given as intermittent (excitatory) theta burst stimulation (iTBS) applied over the hand area of the primary sensory region (S1) can enhance tactile sensation in a group of 21 healthy subjects and one patient with cervical spinal cord injury. Four sessions covered all combinations of real and sham stimulations of the RHI and the TBS: real TBS and real RHI, real TBS and sham RHI, sham TBS and real RHI, and both conditions sham. The condition sham TBS and real RHI shows the greatest effect on the proprioceptive drift (median 2.3 cm, IQR 2) and on the score of RHI questionnaires (median 3, IQR 2) in the control group as well as in the real-real condition (median 2, IQR 2). The sham TBS and real RHI condition also shows the best results in the electrical perception test of the patient (median 1.9 mA). Conversely, the upregulation of the cortical excitability of S1 via TBS seems to impair the effect of the RHI. This might be due to a strengthening of the top-down connection between the central nervous system and the periphery, diminishing the RHI. This finding helps in understanding the mechanisms of top-down and bottom-up mechanisms in healthy subjects and patients with spinal cord injury. The RHI paradigm could represent an interesting therapeutic approach in improving tactile sensation and rTMS techniques could modulate these effects. Yet, further studies are needed, to examine the direction of the interaction effect of TMS and RH.

Connectivity Analysis during Rubber Hand Illusion—A Pilot TMS-EEG Study in a Patient with SCI

“…Hence, the connectivity observed might reflect the mechanism of processing a prediction error, without influencing the bodily self-perception yet. This matches with the behavioral data of our earlier work [ 17 ] as the sham RHI conditions revealed a stronger effect than expected. It seems that the rubber hand is integrated well and quickly into the own body perception, while it might actually be the asynchronous sham stimulation that really causes struggle in the feeling of embodiment.…”

“…In this pilot study, we investigated the effect of RHI and repetitive TMS in healthy participants and one patient with cervical SCI on HD-EEG. In our earlier work [ 17 ], we demonstrated a behavioral suppression of the RHI after facilitating TBS stimulation over S1. Additionally, we found an increase in tactile sensation in the patient after applying the RHI.…”

“…On the one hand, there might be an overlapping effect of the RHI over the TMS regarding communication of the brain areas. On the other hand, TMS does have an influence on the behavioral data of the study [ 17 ]. Hence, processing the multisensory information during the RHI shows a greater effect regarding connectivity than a change of cortical excitability.…”

“…In patients with SCI, this effect retained, yet to a lower extent than in healthy participants, in contrast to our data obtained during RHI. Lateralization can be observed mainly after sTBS, increasing the effectivity of the RHI according to behavioral data [ 17 ]. The increased laterality in the patient points to a higher effectivity of the RHI compared to the healthy control group.…”

“…It is important to gain a better understanding of the connectivity during RHI and its possible manipulation of the bottom-up and top-down processes. This could trigger new therapeutic approaches, which influence neuroplastic changes in patients with SCI and improve their sensory perception, as demonstrated in our earlier behavioral study [ 17 ]. In order to test these pathways, modulation of processing capacities would allow for establishing causal relationships.…”