A new index called the perturbational complexity index (PCI) can objectively measure a person's level of consciousness without requiring them to respond or interact. PCI works by using transcranial magnetic stimulation to briefly perturb the cortex and then measuring the algorithmic complexity of the resulting brain activity patterns. The index was tested on healthy individuals during wakefulness, dreaming, nonrapid eye movement sleep, and under sedation with midazolam, xenon, and propofol, as well as on patients who had emerged from coma. PCI reliably distinguished conscious from unconscious states in single individuals across all conditions, including vegetative state, minimally conscious state, and locked-in syndrome.
Ketamine alters consciousness by disrupting connectivity within and between specific resting-state brain networks, particularly the default mode network (DMn) and salience network (SALn), while leaving sensory and motor networks largely intact. In healthy volunteers given stepwise ketamine infusions until they lost responsiveness, DMn connectivity between the medial prefrontal cortex and other network regions decreased (from 0.20 to 0.07), and the normal anticorrelated activity between the DMn and sensory regions reversed (e.g., right sensory cortex shifted from -0.07 to 0.04). SALn connectivity was also suppressed but nonuniformly. These specific changes, including preserved sensory network connectivity, are shared with propofol-induced unconsciousness.