Context-dependent processing of vestibular signals for balance and orientation

The control of balance and orientation comprises various forms of sensory input, reflexive action and anticipatory mechanisms. An important sensory input is the vestibular system. When a destabilising or disorientating perturbation is sensed by the vestibular apparatus a corrective response is generated. This thesis investigated how the processing of vestibular signals is affected by postural and sensory context. Orientation reflexes evoked by a vestibular signal of head roll were shown to be continuously modulated and even reversed direction during self-generated head pitch movements (Chapter 2). Results also raised the possibility that the direction of a vestibular-evoked balance reflex is automatically rotated following adaptation of motor output (Chapter 3). In addition to modulating the response direction, the context was also found to affect the response amplitude. Passive cutaneous sensory input was shown to attenuate a vestibular-evoked balance reflex (Chapter 4). If, however, such changes in sensory context were anticipated, then response amplitude was unchanged (Chapter 5). Furthermore, the initial balance reflex was not affected by a fear of falling (Chapter 6). The present findings demonstrate that the processing of vestibular signals is indeed context-dependent. However, the modulation of vestibular-evoked reflexes is seemingly automatic, and is not affected by cognition or emotion