Consciousness and complexity: from theory to practice

Simone Sarasso
Department of Biomedical and Clinical Sciences "L. Sacco",
Università degli Studi di Milano.

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 (a clinical condition known as the vegetative state). 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. Theoretical principles suggest 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). Inspired by these theoretical principles, we employed different experimental models of loss and recovery of consciousness (sleep, anesthesia and disorders of consciousness following severe brain injuries) and recently validated an empirical measure of brain complexity that is independent of behavior (the Perturbational Complexity Index, PCI). Practically, this index captures the algorithmic complexity (information) generated by causal distributed interactions in the brain (integration) and is based on the combination of Transcranial Magnetic Stimulation and high-density Electroencephalography (TMS/EEG). Results showed that PCI reliably assesses the presence/absence of consciousness with unprecedented sensitivity and specificity and offers a reliable stratification of unresponsive patients that has important physiopathological as well as therapeutic implications.