Perceptual compasses: spatial navigation in multisensory environments

Moving through space is a crucial activity in daily human life. The main objective of my Ph.D. project consisted of investigating how people exploit the multisensory sources of information available (vestibular, visual, auditory) to efficiently navigate. Specifically, my Ph.D. aimed at i) examining the multisensory integration mechanisms underlying spatial navigation; ii) establishing the crucial role of vestibular signals in spatial encoding and processing, and its interaction with environmental landmarks; iii) providing the neuroscientific basis to develop tailored assessment protocols and rehabilitation procedures to enhance orientation and mobility based on the integration of different sensory modalities, especially addressed to improve the compromised navigational performance of visually impaired (VI) people. To achieve these aims, we conducted behavioral experiments on adult participants, including psychophysics procedures, galvanic stimulation, and modeling. In particular, the experiments involved active spatial navigation tasks with audio-visual landmarks and selfmotion discrimination tasks with and without acoustic landmarks using a motion platform (Rotational-Translational Chair) and an acoustic virtual reality tool. Besides, we applied Galvanic Vestibular Stimulation to directly modulate signals coming from the vestibular system during behavioral tasks that involved interaction with audio-visual landmarks. In addition, when appropriate, we compared the obtained results with predictions coming from the Maximum Likelihood Estimation model, to verify the potential optimal integration between the available multisensory cues. i) Results on multisensory navigation showed a sub-group of integrators and another of non-integrators, revealing inter-individual differences in audio-visual processing while moving through the environment. Finding these idiosyncrasies in a homogeneous sample of adults emphasizes the role of individual perceptual characteristics in multisensory perception, highlighting how important it is to plan tailored rehabilitation protocols considering each individual’s perceptual preferences and experiences. ii) We also found a robust inherent overestimation bias when estimating passive self-motion stimuli. This finding shed new light on how our brain processes and elaborates the available cues building a more functional representation of the world. We also demonstrated a novel impact of the vestibular signals on the encoding of visual environmental cues without actual self-motion information. The role that vestibular inputs play in visual cues perception, and space encoding has multiple consequences on humans’ ability to functionally navigate in space and interact with environmental objects, especially when vestibular signals are impaired due to intrinsic (vestibular disorders) or environmental conditions (altered gravity, e.g. spaceflight missions). Finally, iii) the combination of the Rotational-Translational Chair and the acoustic virtual reality tool revealed a slight improvement in self-motion perception for VI people when exploiting acoustic cues. This approach shows to be a successful technique for evaluating audio-vestibular perception and improving spatial representation abilities of VI people, providing the basis to develop new rehabilitation procedures focused on multisensory perception. Overall, the findings resulting from my Ph.D. project broaden the scientific knowledge about spatial navigation in multisensory environments, yielding new insights into the exploration of the brain mechanisms associated with mobility, orientation, and locomotion abilities.