Detecting EEG index of arousal in sleeping patients with unresponsive wakefulness syndrome.

Consciousness is assumed to be defined by two components: awareness and arousal. However, fewer electroencephalographic (EEG) indices have been identified for arousal than for awareness. Since patients clinically diagnosed with unresponsive wakefulness syndrome (UWS) show preserved sleep-wake cycles but markedly reduced reproducible behavioral evidence of awareness, their sleep-wake patterns may provide a useful model for examining arousal-related EEG fluctuations. This study recorded nighttime polysomnography in patients with UWS, patients in a minimally conscious state, and healthy controls. The EEG signal showed that spectral slope could index arousal, fluctuating among sleep stages in all three groups. Both spectral entropy and Lempel-Ziv complexity indexed awareness, showing a significant difference between conscious and unconscious states. These findings provide fundamental evidence for the two-component hypothesis of consciousness.