In contrast, the brain of a deep sleeper acts like a stunted, badly tuned bell. Whereas the initial amplitude of the EEG is larger than when a subject is awake, its duration is much shorter, and it does not reverberate across the cortex to other connected regions. Although the neurons remain active in sleep, as evidenced by the strong, local response, integration has broken down. Little of the spatially differentiated and temporally variegated sequence of electrical activity that is typical for the awake brain is present. The same is also true of subjects who volunteered to undergo general anesthesia with propofol or xenon. The TMS pulse invariably produces a simple response that remains local, indicative of a breakdown of cortico-cortical interactions and a lessening of integration.
Probing the Mind of a Patient
This article was originally published with the title A Consciousness Meter.