A theoretically based index of consciousness independent of sensory processing and behavior.
Level 3 - non-randomized controlled study
Non-randomized cross-sectional study evaluating a diagnostic index across multiple physiological and pathological conscious states.
PubMed 23946194 · doi:10.1126/scitranslmed.3006294
What was done
The authors evaluated the perturbational complexity index, a measure combining cortical integration and algorithmic information capacity, to assess consciousness independently of behavior. Cortex was perturbed with transcranial magnetic stimulation, and the spatiotemporal complexity of the resulting electrocortical responses was compressed and measured. The index was tested on electrophysiological datasets from healthy individuals during wakefulness, dreaming, non-rapid eye movement sleep, and anesthesia (midazolam, xenon, propofol), as well as in patients who had emerged from coma into vegetative state, minimally conscious state, or locked-in syndrome.
What was found
No specific numerical values, sensitivity, specificity, or effect estimates were reported in the abstract. The authors report that the index reliably discriminated the level of consciousness in single individuals across conscious, sleeping, anesthetized, and brain-injured states.
Why it matters
The method offers an objective, theoretically derived measure of consciousness that does not rely on sensory processing or motor responses. This could enable bedside detection of covert consciousness in severely brain-injured, unresponsive patients.
Limits
The abstract provides no sample size or numerical diagnostic performance metrics. TMS-EEG requires specialized equipment and technical expertise, and generalizability across diverse real-world clinical coma populations remains to be established.