J Neurophysiol. 2023 Sep 14. doi: 10.1152/jn.00069.2023. Online ahead of print.
INTRODUCTION: Despite being studied for >50 years, the neurophysiological mechanisms underlying vibration(VIB)-induced kinesthetic illusions are still unclear. The aim of this study was to investigate how corticospinal excitability tested by transcranial magnetic stimulation (TMS) is modulated during VIB-induced illusions.
METHODS: Twenty healthy adults received vibration over wrist flexor muscles (80 Hz, 1 mm, 10 seconds). TMS was applied over the primary motor cortex representation of wrist extensors at 120% of resting motor threshold in four random conditions (10 trials/condition) : baseline (without VIB), 1s, 5s and 10s after VIB onset. Means of motor evoked potentiels (MEP) amplitudes and latencies were calculated.
RESULTS: Statistical analysis found a significant effect of conditions (stimulation timings) on MEP amplitudes (p=0.035). Paired-comparisons demonstrated lower corticospinal excitability during VIB at 1s compared to 5s (p=0.025) and 10s (p=0.003), although none of them differed to baseline values.
DISCUSSION: Results suggest a time-specific modulation of corticospinal excitability in muscles antagonistic to those vibrated, i.e. muscles involved in the perceived movement. An early decrease of excitability was observed at 1s followed by a stabilization of values near baseline at subsequent time-points. At 1s, the illusion is not yet perceived or not strong enough to up-regulate corticospinal networks coherent with the proprioceptive input. Spinal mechanisms, as reciprocal inhibition, could also contribute to lower the corticospinal drive of non-vibrated muscles in short period before the illusion emerges.
CONCLUSION: Our results suggest that neuromodulatory effects of VIB are likely time-dependent, and that future work is needed to further investigate underlying mechanisms.