Anatomical and functional impact of critical brain areas to network activity and basic visual function
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A set of widely distributed brain areas, collectively known as the fronto-parietal network, serve to modulate aspects of visual perception. However, the unique influence exerted by these regions on low-level visual processing remains unclear. The goals of this thesis were (1) to examine how right frontal, parietal and occipital brain areas interact to process and modulate visual function and (2) to investigate the ability to improve foveal visual performance by means of noninvasive neurostimulation. In a first set of experiments, visual percepts known as 'phosphenes' were measured following low-frequency neurostimulation of the right occipital pole, Intraparietal Sulcus (IPS) or Frontal Eye Fields (FEF). Stimulation of the occipital pole and IPS were capable of evoking phosphenes with similar appearances. Furthermore, occipital or IPS stimulation decreased the excitability of the locally stimulated region but had no effect on the non-stimulated brain area. These results indicate a lack of sufficient inter-regional interactions capable of supporting long-range changes in brain activity. In a second set of experiments, contrast sensitivity and reaction times were assessed as the capacity to detect centrally located, high or low spatial frequency stimuli. Low-frequency rTMS to the FEF, but not the occipital pole or IPS improved contrast sensitivity for high spatial frequency stimuli. Stimulation of the occipital pole decreased reaction times for low spatial frequency stimuli and was shown to depend on transcollicular projections. Finally, stimulation of the IPS decreased reaction times for both types of stimuli. These effects however did not appear to depend on transcollicular pathways, indicating that performance was enhanced through cortico-cortical connections. In a final set of experiments, we investigated whether patterns of individual white matter connectivity linking stimulated brain regions could predict the effects of neurostimulation on visual processing and performance. None of the probability measures however correlated with changes in visual performance. Overall, these data suggest that occipital, parietal, frontal and tectal areas uniquely contribute to the modulation of visual perception. Moreover, results show that targeted stimulation to these brain regions serves to generate lasting improvements in visual performance, which could be used to enhance aspects of vision in healthy and clinical populations.
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