Shared neural mechanisms between imagined and illusory egocentric motion

Mental imagery is an important strategy to infer the future state of one's body. Neuroimaging studies have shown that mental simulations activate sensory or motor structures in the brain that resemble an overt action or a perception generated by external stimuli. Similarly, it is suggested that the mental rotation of one’s own body relies on brain areas involved in processing self-motion, most prominently the vestibular system. Evidence for the involvement of the vestibular system in simulated changes of self-location stems from behavioral experiments with healthy participants showed an altered capacity for egocentric mental rotation (EMR) during vestibular stimulation. However, those findings are challenged by the conflicting nature of the stimulation and the cognitive tasks that imply different individual strategies. To address these open questions, we used fMRI to investigate the brain structures that underlie simulated changes of the self in mental space and vestibular processing within the same individuals. Participants performed an EMR task during simultaneous Galvanic Vestibular Stimulation (GVS). In line with previous literature, we hypothesized that GVS negatively affects the ability to perform EMR. At the neural level, we expected an overlap between brain structures activated for vestibular processing and EMR within the parieto-insular vestibular cortex (PIVC). In line with our predictions, the data showed an overlap of brain activity within the PIVC for both egocentric mental rotation and vestibular processing. In contrast to our expectations, GVS did not influence the ability to perform egocentric mental rotation. Our results constitute the first neural evidence for shared neural mechanisms underlying illusory and simulated self-motion. They also suggest that the vestibular stimulation was not effective enough to influence EMR. We conclude that mental rotations of one's body rely on the PIVC, but also suggest that those mental simulations of one's body might be protected from external interference.