"Associating an emotional expression of the face with a “motion of the mind” was an astonishing insight by Da Vinci and a surprisingly modern metaphor. Today we correlate specific patterns of electrochemical dynamics (i.e. “motions”) of the central nervous system, with emotional feelings. Consciousness, the substrate for any emotional feeling, is itself a “motion of the mind,” an ephemeral state characterized by certain dynamical patterns of electrical activity. Even if all the neurons, their constituent parts and neuronal circuitry remained structurally the same, a change in the dynamics can mean the difference between consciousness and unconsciousness.
But what kind of motion is it? What are the patterns of electrical activity that correspond to our subjective state of being conscious, and why? Can they be measured and quantified? This is not only a theoretical or philosophical question but also one that is of vital interest to the anesthesiologist trying to regulate the level of consciousness during surgery, or for the neurologist trying to differentiate between different states of consciousness following brain trauma.
Recently, Casali et al have presented a quantitative metric. It provides, according to the authors, a numerical measure of consciousness, separating vegetative states from minimally conscious states. The study provides hints of being able to identify the enigmatic locked-in state, in which the subject is conscious but is unable to communicate with the external world due to motor deficits. What is most interesting is the claim that the measures provide scientific insight into consciousness, by providing an objective measure.
Their metric, like other existing clinical measures of consciousness, is based on Electroencephalography (EEG), where voltages recorded from electrodes placed on the scalp provide a coarse picture of neural activity in the brain. EEG can be used to measure either ongoing brain activity, or that evoked by an external stimulus. In Casali’s case, the activity in question is evoked directly in the brain using a transient magnetic field (Transcranial Magnetic Stimulation). This involves applying a transient magnetic field, which generates an electric field in a particular region of the brain due to Faraday’s law, a bit like attaching a battery to the neural circuitry. This causes currents to flow in the brain, not just in the stimulated region, but in other regions connected to it as well. The spatial and temporal patterns of these currents in the brain are then inferred from the EEG measurements and quantified to produce the metric.
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