Using Granger Causality to assess the interaction between brain areas during different consciousness states

To analyze cortico-cortical interactions in different consciousness states, namely during NREM sleep and wakefulness, we compared evoked potentials from 5 mA intra-cerebral stimulations in an epileptic subject undergoing clinical evaluation. We collected recordings from 16 different cortical areas and analyzed the perturbation effects in a 200ms time range after the stimulus using both cross-Coherence and Granger causality and comparing the two procedures. Results show that the overall interaction intensity involves a wider frequency range during wakefulness than during NREM sleep. Moreover, comparing similar Coherence intensity thresholds, the number of interacting areas is sharply higher during wakefulness. However, during the NREM phase, interactions show a highly directional behavior that is not present during wakefulness. The study displays which areas are mainly involved in reciprocal G-causal interactions, paving the way to a following research on their functional meaning.