consciousness

Meaning: Consciousness is awareness of oneself and the environment, and it exists on a continuum rather than as on or off. The MCAT groups states as alertness, drowsiness and sleep, and altered states such as hypnosis, meditation and drug-induced states. Alertness depends on attention: selective attention filters what you process, and divided attention splits it across tasks, usually at a cost to performance.

On the MCAT: Passages give EEG traces or sleep-stage graphs and ask you to identify the state. Alert waking shows beta waves (fast, low amplitude); relaxed, eyes-closed wakefulness shows alpha; stage 1 shows theta; stage 2 adds sleep spindles and K complexes; stage 3 shows slow, high-amplitude delta; REM shows fast, beta-like activity. Frequency falls and amplitude rises as non-REM sleep deepens.

Common confusion: Students treat sleep as the absence of consciousness. It is an altered state, and the brain is very active in REM sleep, which is why REM is called paradoxical sleep. Also keep this sense separate from Freud's unconscious, which is mental content outside awareness, not a loss of consciousness.