We normally assess another individual’s level of consciousness based on her/his ability to interact with the surrounding environment and communicate. Usually, if we observe purposeful behavior, appropriate responses to sensory inputs, and above all appropriate answers to questions, we can be reasonably sure that the person is conscious. However, we know that consciousness can be entirely generated within the brain, even in the absence of any interaction with the external world; this happens almost every night, while we dream. The dissociation between consciousness and responsiveness is particularly relevant in the case of brain-injured patients who may emerge from coma with their eyes open, but immobile and not reacting to sensory stimulation. Yet, to this day, we still lack an objective, dependable measure of the level of consciousness that is independent of processing sensory inputs and producing appropriate motor outputs.
To overcome this problem, one should directly assess brain properties that are necessary and sufficient for consciousness to emerge. Empirically, however, this task is fraught with challenges. Once again, one of the most striking paradoxes is offered by sleep. During early dreamless sleep, in fact, several brain parameters such as mean firing rates and synchronization levels are comparable to quiet wakefulness, still consciousness is lost.
My work is guided by theoretical principles suggesting that consciousness depends on the ability of neural elements to engage in complex activity patterns that are, at once, distributed within a system of interacting cortical areas (integrated) and differentiated in space and time (information-rich) (i.e. brain complexity). In practice, my research focuses on the development of a theory-driven empirical method to assess brain complexity based on a combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG).
Over the last ten years, we tested this method in several conditions where consciousness was unambiguously present (wakefulness, dreaming, locked-in syndrome) or lost (NREM sleep, anesthesia, and vegetative state patients). Invariably, when consciousness was present a direct cortical perturbation induced a widespread and reproducible response characterized by complex spatiotemporal dynamics. On the contrary, whenever consciousness was lost, the same perturbation induced simpler cortical activations that were either circumscribed to the stimulation site (loss of integration) or involved the entire cortical mantle at once (loss of information).
Overall, this assessment of brain complexity seem to provide a reliable measuring scale along the unconsciousness/consciousness spectrum. This represents a first step towards a robust and objective assessment of unresponsive individuals whose level of consciousness is unknown. Most important, a theoretical and empirical link between consciousness and complexity may shed new light on the cortical mechanisms that underlie loss and recovery of consciousness in pathological condition. TMS/EEG measures suggest that also in vegetative patients brain complexity may collapse through network bistability, as in sleep and anesthesia. Since bistability is, in principle, reversible and its mechanisms are well-understood at the cellular and network level, it may represent a suitable target for novel therapeutic approaches in patients in whom consciousness is impaired, in spite of preserved cortical activity.