Extreme short-term environmental constraints do not update internal models of action as assessed from motor imagery in adults

According to the simulation theory, the internal simulation of a movement (imagined movement, IM) and its execution (actual movement, AM) are based on the same motor representations. The brain uses these representations for controlling action. The specific objective of this study was to investigate the updating process of internal models of action in adults, through massive environmental changes involved by microgravity (0G). 0G has multiple effects on motor control, including short-term adaptations with respect to the planification and performance of actions. However, the effects of 0G on internal representations of action are still largely unknown. To address this issue, thirteen subjects performed first sit-to-stand (STS) and back-to-sit (BTS) tasks, and subsequently had to imagine movement performance in these tasks. The tasks were performed under normogravity (1G) and 0G conditions. Based on durations of actual and IMs, two main results emerged from this study. In 1G, actual and IM's durations were similar. However, in 0G, AM durations were significantly longer than IM durations. Furthermore, IM durations in 0G were similar to the 1G value. These results show that although the planification and execution of action were immediately adapted to the 0G condition, the storage of afferent information was inadequate to recalibrate the predictive model. These results suggest that sudden change in gravity was not considered for updating internal models of action, and that forward model probably required more practice in order to integrate the modification of the sensorial feedback generated by the new environmental constraints.