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Free keywords:
perceptual awareness, consciousness, somatosensory perception, fMRI, functional connectivity, neural network, graph theory
Abstract:
Introduction: Are functional network topologies of the brain candidates for a neural account of sensory awareness? Pre-stimulus network topologies have been reported to predict access to consciousness for tactile and auditory stimuli. Post-stimulus network topologies have been shown to vary with visual awareness. Here we test to what extent functional brain networks vary with somatosensory awareness.
Methods: We acquired fMRI while participants (N=38) had to report the perception of near-threshold electrical pulses and their decision confidence. Functional networks were modelled using the generalized psychophysiological interaction (gPPI) for two networks: (a) data-driven network of 19 nodes in frontal, parietal and somatosensory cortices, and (b) whole-brain network of 264 nodes. The 19-nodes network was derived from the BOLD signal contrasts between aware, unaware and catch trials. gPPI has the advantage of controlling its context-dependent functional connectivity estimates for the stimulus-evoked BOLD response and the baseline functional connectivity. Graph theoretical analyses of the functional networks were performed using the Brain Connectivity Toolbox.
Results: The 19-nodes network showed an increased modularity with somatosensory awareness. Modularity is a measure of global segregation into distinct networks. Measures of local segregation (clustering), integration (path length) and centrality (participation) showed no consistent significant differences. The same analysis for the 264-nodes whole-brain network resulted in no significant differences. When the connectivity strengths between the 19 nodes was tested directly, aware compared to unaware trials showed a higher intra-parietal and parietal-frontal connectivity, along with a two-fold pattern for somatosensory cortex: (1) primary somatosensory cortex had an increased and (2) secondary somatosensory cortex a decreased functional connectivity to parietal cortex.
Conclusions: Our results indicate that somatosensory awareness is accompanied by functional brain network topology alterations. These network alterations can be observed between functionally relevant nodes that form more modules during somatosensory awareness, indicating functional network segregation and specialization.
