Psychophysic and cortical source analysis of movement illusions induced by vibratory stimulation

It is known that vibratory stimuli applied to muscle tendons induce illusory perception of movement. Tendon muscle vibration, indeed, activates spindle receptors and almost selectively elicits trains of action potentials in the primary endings connected to Ia afferent fibers, which are then interpreted as muscle lengthening and limb displacement. However, while the anatomical substrate that may mediate such illusions is quite defined, their functional and psychophysical correlates are understudied. The present research aims at qualifying and investigating vibration-induced movement illusory perceptions in terms of their electrophysiological and psychophysical features (frequency, strength, and duration of illusion). 17 right-handed volunteers took part to the study, two of them who never reported movement illusions was then excluded from the analyses. Participants were asked to wear a 28-channel EEG system and a wrist splint on the right forearm. A vibratory stimulator was then placed perpendicularly to the flexor tendons and the arm was hidden behind a screen. The vibratory stimulation was at a frequency of about 90 Hz. The experimental design included 15 trials and participants were asked to report whether they felt their right hand moving during stimulations and to qualify those perceptions based on psychophysics features. EEG frequency data were analysed and used as input for signal source localization (sLORETA). The analysis of psychophysical data revealed significant correlations each other. In particular, the more frequently participants reported movement illusions the stronger their perception, and participants who perceived stronger illusions over-estimated their duration. The analysis of EEG data contrasting baselines and stimulation trials inducing movement illusions highlighted a significant decrease of upper-alpha power – mirroring greater activation – especially in insular and prefrontal areas. Finally, correlation analyses between psychophysical and cortical localization data yielded significant relationship between the activation of parietal areas and the frequency of illusory perception. Present findings point out a clear consonance between psychophysical and electrophysiological features of motor illusory phenomena and suggest that pre- and post-central structures may play different roles in their definition. The advanced analyses of EEG components, in particular, helped in shedding light on the potential contribution of superior parietal areas to conscious feeling of movement – even if illusory.