Balancing the Self

The vestibular system is composed of otolith organs and semi-circular canals that encode linear and angular accelerations, as well as the position of the head with respect to gravity. Thus, the detection of self-motion, the distinction between self- and object-motion, as well as gaze stabilisation and maintenance of postural stability are the core vestibular functions. Recent research shows that vestibular information interacts with higher-level cognitive processes, such as space perception, attention orienting, body schema and bodily self-consciousness. In order to contribute to these faculties, vestibular information is dynamically combined with visual, somatosensory and proprioceptive signals. In the present thesis we explore such multimodal interactions using a human centrifuge (rotating chair). In Part 1 we show that visual and vestibular cues are integrated in accordance with statistical optimality even when large directional conflicts are introduced between these modalities. Participants were significantly better in discriminating rotation magnitude when simultaneously presented with visual and vestibular cues, as compared to each modality independently, despite the fact that the axes of rotation implied by the two cues were different (Study 1). We also demonstrate that visuo-vestibular integration is present and optimal in patients with unilateral vestibular loss (Study 2). Part 2 of the present thesis examined vestibularsomatosensory interactions. We show that vestibular stimulation in the form of passive whole-body rotations increases tactile sensitivity at the fingertips, as compared to a no-rotation baseline (Study 3). We also demonstrate that the effect of vestibular stimulation on touch is not direct, but mediated by visual information about self-motion: visual and vestibular cues first combine, and only subsequently influence tactile sensitivity (Study 4). In Part 3 of this thesis, we explored how vestibular stimulation affects visual attention and awareness. We show that when acting as an exogenous cue, vestibular stimulation orients attention at short cue-to-target delays. When acting as an endogenous cue, vestibular stimulation strongly orients attention at all cue-to-target delays (Study 5). Vestibular stimulation also affects visual awareness. Using a continuous flash suppression paradigm to suppress an optic flow stimulus during passive whole-body rotations, we show that optic flow that is congruent (i.e. counterdirectional) with the direction of the vestibular rotation breaks suppression faster than incongruent optic flow (Study 6). In sum, our findings refine the existing knowledge on multisensory processing in general and vestibular interactions with other senses in particular. Our results are of relevance for the understanding of how visual, vestibular, proprioceptive and somatosensory information are combined by the brain in order to form a coherent representation of the self in space.