Age-related changes of the cortical visual-vestibular interaction in healthy subjects

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.