Abstract

The visual and vestibular systems play one of the central roles in the perception of verticality, spatial orientation, maintenance of balance and distinguishing self-motion from motion of the environment. As the brain continuously and simultaneously receives an enormous quantity of information through their receptor organs, collaboration between these systems at different levels of information processing is crucial for the proper execution of the above mentioned functions. Psychophysical and neuroimaging research in humans has provided support for the concept of a reciprocal inhibitory visual-vestibular interaction, the functional significance of which lies in suppression of potential mismatch between incongruent sensory inputs delivered from the two systems. Functional magnetic resonance imaging (fMRI) enabled visualization of this interaction through detection of blood-oxygen-level-dependant (BOLD) signal increases or signal decreases in the visual and vestibular networks during unisensory stimulation. Specifically, visual stimulation related to the percept of self-motion, such as optokinetic stimulation, was shown to elicit BOLD signal increases in areas involved in visual processing along with BOLD signal decreases in areas involved in vestibular processing. Increasing age was shown to alter the morphological and functional properties of the sensory, motor and cognitive systems. Previous research has revealed that senescence associates with deterioration of both, visual and vestibular functions, as well as a change in the psychophysical measurements related to their interaction. However, the effects of age on the BOLD signal pattern reflecting the visual-vestibular interaction have not yet been investigated. Exploring these effects in healthy subjects could offer the possibility to detect early age-related changes in the cortical function occurring before a decline in behavioural measurements can be detected. Aside broadening the scientific knowledge on the physiological changes with age in the sensory systems and their interactions, such research would also help to better understand the pathophysiological processes underlying various visual and vestibular disorders investigated in neuroimaging studies. Therefore, the aim of this doctoral thesis was to explore how the BOLD signal related to the visualvestibular interaction during optokinetic nystagmus (OKN) changes with age in healthy subjects. It specifically aimed to investigate the age-related changes in the spatial and temporal patterns of the signal during unaltered oculomotor performance. In order to obtain information on the diverse effects of age, the changes in the mean of the BOLD signal, as well as the changes in its temporal variability were analyzed. For the purpose of differentiating between global and task-related changes with age, the alterations of the BOLD signal during OKN were compared to the alterations of the BOLD signal elicited by a pure visual and a pure motor task. In the frame of this work, we were able to show that significant age-related changes in the mean of the BOLD signal and in its temporal fluctuations occur prior to any measurable decline in OKN performance. The changes in the mean of the BOLD signal were taskspecific and possibly reflected age-related alterations in neurovascular coupling and neural processing related to OKN. They were found only in cortical and subcortical areas of the visual system. The changes in the temporal fluctuations of the BOLD signal were not specific for the OKN task, but rather region-specific, affecting mostly areas know to be part of the multimodal vestibular processing network.