Electroencephalogram-Based Indices of Level of Consciousness

It is recognized that the EEG changes systematically in relation to the dose of anesthetic drug administered. As a consequence, the unprocessed EEG and various forms of processed EEG have been used to track the level of consciousness of patients receiving general anesthesia and sedation.

1. Basic Biophysics of EEG
   1. The EEG detects postsynaptic potentials in the cerebral cortex. As neurons synapse in a region of the cortex, they produce macroscopic extracellular currents that can be detected as potential differences by surface electrodes. The cortex is tied to subcortical structures like the thalamus.
   2. Therefore with the EEG, inferences can be made about both cortical and subcortical dynamics.
   3. The EEG signals are comprised of oscillations or waves with a variety of frequencies.
      1. The signal typically consists of many frequencies with different amplitudes (Figs. 11.1 and 11.2).