Soutenance de thèse de Malika Azaroual-Sentucq: "Somatosensation and plasticity: perceptual, cognitive and physiological effects"

Abstract

Somatosensation is an essential function for human perception, action and cognition, being crucial for fine motor skills and self bodily awareness. My PhD work is interested in somatosensation and its plasticity at cognitive, perceptual and physiological levels.
While it is widely accepted that somatosensation contributes to building multiple mental body representations (MBRs), its contribution to each MBR remains unclear. A first aim of my work was to answer this question by leveraging repetitive somatosensory stimulation (RSS), known to temporarily improve tactile acuity (TA) by inducing plastic changes in the primary (SI) and secondary (SII) somatosensory cortex. This randomized sham-controlled double-blind study conducted on 33 adults investigated the effects of RSS on three MBRs of the stimulated right index finger (rD2): the body image (BI), the body model, and the superficial schema. The results revealed that the BI is selectively affected by RSS, as the stimulated rD2 was perceived significantly smaller after RSS, while the other MBRs were left unaffected. This suggests that somatosensory processes contribute differently to the BI than to the other two MBRs.
Somatosensation can be assessed by measuring TA. Accurately measuring this feature of touch is essential as it is used in clinical practice and research attempting to restore tactile perception. A widely used -but criticized- task is the two-point discrimination task (2PDT), while the grating orientation (GOT) and two-point orientation (2POT) tasks are suggested to be more reliable alternatives. Critically, whether these tasks measure similar aspects of TA has yet to be determined. The second aim of my thesis was to answer this question by comparing the performance in these tasks and linking them to anatomical measures at the fingertips, and by leveraging RSS. In this study, RSS was applied on the rD2 of 29 adults and its impact on the tasks was assessed at the rD2 as well as at the lD2 (control) and lD3 which has been recently found to display TA improvement following RSS. At baseline, 2POT and GOT correlated to the fingertip area. Following RSS, 2PDT and GOT were both improved at the rD2, 2PDT and 2POT also at lD3. Overall, the results suggest that the three tasks capture both similar and different aspects of TA.
Because RSS is used to induce plasticity in the somatosensory system, understanding its mechanisms of action is important. While cortical changes in the SI and SII representations of the stimulated finger have been associated to the local effect of RSS, the physiological mechanisms responsible for local and remote effects (on the unstimulated hand) have not been explored yet. My third aim was to investigate them through EEG, testing the hypothesis of a modulation of cortical inhibition between the fingers’ representations of both hands. This study is made of two randomized sham-controlled double-blind experiments, each conducted on 41 adults, undergoing EEG and 2PDT. Because we identified a methodological bias in our first design, we conducted a second experiment aimed at obviating it. We found that after both sham and RSS, the intra- and inter-hemispheric inhibition significantly increased, potentially driven by the inhibition increase between lD2 and lD3 and between lD3 and rD3 which appear (non-significantly) larger than in other pairs, as well as larger after RSS than sham. Because of potential issues also in the second experiment, these results are preliminary, and another experiment is planned to solve them. The final experiment would hopefully clarify if RSS affects inhibition.
Overall, studying somatosensation at multiple levels, my work shows that somatosensation contributes differently to the BI than to the other MBRs, which allows to refine current MBR models, and multiple tasks should be used to comprehensively assess TA, while it does not allow to conclude on the neural mechanisms underlying the effects of RSS.