Neural Mechanisms of Motor Imagery and Training-Induced Plasticity: Hypnotizability Relevance to Microgravity Adaptation

Long exposure to extreme environments, such as microgravity during spaceflights, induces weightlessness-related physiological alteration regarding sensory information, sensorimotor integration, and interoceptive and vestibular feedback disruption. Such alterations impair postural control, spatial orientation, and fine motor skills. Motor imagery (MI), defined as the mental simulation of movement without overt execution, can counteract sensorimotor alterations by activating the sensorimotor system independently from actual movement execution. On Earth, both physical and MI training (MIT) are reported to improve motor performance. However, the effects of MI training are extremely variable among the general population, which prevents the prediction of its outcome. Among the factors modulating the physiological correlates of MI, this thesis examines the role of hypnotizability, which is a psychophysiological trait measured by standardized scales, in the efficacy of MI and MIT. The individuals with high hypnotizability scores (highs) display peculiar structural and functional brain characteristics compared to low hypnotizables (lows) - i.e., reduced grey matter volume of the insula and of the cerebellar left lobules IV-VI -, and behavioral/physiological differences, i.e., greater functional equivalence (FE) between actual and imagined perception/action, greater excitability of the motor cortex, lower interoceptive accuracy and more adaptive interoceptive sensibility. Since highs exhibit greater FE compared to lows, and mediums have not been studied yet, although they represent 70% of general population, this research work aimed a) to support the EEG findings regarding stronger FE and different mode of information processing in highs and lows by employing functional Near Infrared Spectroscopy (fNIRS) (Study 1); b) to investigate whether MIT induces neural plasticity and improvement in the MI cortical correlates after a 15-day MIT in mediums and lows (Study 2); c) to study the role of different cognitive conditions in the duration of an extreme interoceptive condition like voluntary apnea (Study 3). Other hypnotizability-related traits were also considered to possibly interact with hypnotizability, i.e., interoceptive sensibility, absorption, and the ability to imagine movement through visual and kinesthetic imagery (measured by questionnaires). fNIRS confirmed a stronger FE in highs than in lows and a different mode of information processing in the two groups (Study 1); Study 2 revealed better effects of MIT in highs, although MIT induced neural plasticity also in mediums and lows; Study 3 disclosed a different relevance of hypnotizability to apnea duration associated with attention focused on apnea sensation or with imagery of normal breathing. In conclusion, hypnotic assessment may predict the adaptation to space environment, mental training may improve the performance of medium-to-low hypnotizables, and motor imagery may buffer the experience of the unpleasant condition of apnea. The reported findings also indicate that, on Earth, hypnotizability may also be relevant to neurorehabilitation treatments.