Functional roles of rhythmic neuronal activity in the human visual and somatosensory system

Abstract

The main aim of this thesis was to investigate the functional role of synchronised oscillations in sensory systems of the human brain. In the first study we found high-frequency gamma-oscillations in the somatosensory system in response to mechanical tactile stimulation. These stimulus-related high-frequency oscillations have been described before in other sensory systems, but not in the somatosensory system in response to tactile stimulation. Its functional relevance was assessed by means of a spatial selective attention task. We found that performance of this task caused a variety of changes in neural activity with different temporal dynamics: The main affect of attention on stimulus-related activity was an enhancement of somatosensory gamma-band-activity, while low-frequency activity in the alpha- and beta-band were altered largely independently from stimulation. Tactile stimulation also altered neural activity in occipital cortex. We therefore investigated how simultaneous visual and tactile stimulation influence activity in visual cortex compared to unimodal stimulation, and as a function of the spatial relation between the two stimuli. We found that in occipital cortex, primarily gamma-band activity was enhanced by tactile stimulation (irrespective of location) and that this effect corresponded with the shortening of response latencies to visual stimuli by tactile stimulation. We further investigated the role of oscillatory activity in sensory information transmission using a visual detection task, where stimuli were presented at threshold level. The main result from this study was that (presumably non-stimulus-related) beta-band-activity in a parieto-frontal network discriminated best between successful and unsuccesful performance. Together the results show that rhythmic activity in sensory cortex is not only altered by stimulation but also correlates with various cognitive and behavioural parameters, suggesting it has a function role. A differential pattern of high- and low-frequency activity in response to bottom-up and top-down-activation was observed and is of interest for further research.