Studying sleep and consciousness with TMS EEG

Why does consciousness fade during non-REM sleep?
This is a puzzling question, explains neurophysiologist Marcello Massimini, because the brain does not “shut down” when we fall asleep. Rather, neurons can show comparable firing rates, albeit with different patterns of synchrony from what is seen in wakefulness. Here, Massimini explains how he and his colleagues applied TMS to subjects while they were awake, and after they fell into unconscious sleep, while simultaneously recording the EEG. With this "perturb-and-measure" approach they tested the hypothesis that the two states differ in terms of levels and patterns of connectivity – predictions made by the Integrated Information Theory of consciousness. It’s a technical tour-de-force, with strikingly clear results.

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Relevant papers:
Massimini, M., Ferrarelli, F., Huber, R., Esser, S. K., Singh, H., & Tononi, G. (2005). Breakdown of cortical effective connectivity during sleep. Science, 309(5744), 2228-2232.

Ferrarelli, F., Massimini, M., Sarasso, S., Casali, A., Riedner, B. A., Angelini, G., ... & Pearce, R. A. (2010). Breakdown in cortical effective connectivity during midazolam-induced loss of consciousness. Proceedings of the National Academy of Sciences, 107(6), 2681-2686.