When a person is completely unconscious, she has no connection to the rest of the world. She may nonetheless be fully awake. This can be confusing because wakefulness often is identified with consciousness. However, these two concepts are distinct. The brain stem controls whether a person is awake, whereas areas in the cortex control whether a person is conscious. We can be conscious and asleep, as is the case when we are dreaming. We can also be awake and unconscious. This can be the case during a vegetative state. While unconscious, some brain areas can be more active than they are when we are awake and conscious.
Unconscious Sleep
Our brains are very active when we are asleep, even when we don't dream. When we are awake the brain stem secretes neurotransmitters such as noradrenaline and serotonin to keep the brain active. However, the brain stem contains different types of neurons that express chemicals that shut down the "awake" neurons during sleep, according to University of Colorado medical researcher Teofilo Lee-Chiong. These neurons remain active for as long as we are asleep, and they inactivate when we wake up. Not all five phases of the sleep cycle are unconscious phrases. In phrase one, we are in a state of light sleep and may have conscious visual and auditory perceptions of our surroundings. In phase two, eye movements stop but we often still have some conscious sensations. In phase three, our brainwaves start to slow down, and in phase four, we are completely unconscious. In the final phase, which is REM sleep, we become very conscious as we begin to dream. Each cycle takes 90 minutes.
Coma and Anesthesia
People in a coma or under anesthesia cannot be awakened, and they are deeply unconscious. Their brain waves are so slow that hardly any signal is detectable on EEG scans. However, electrical activity is just one kind of brain activity. Blood flow and metabolism, measured on fMRI and PET scans, respectively, are kinds of brain activity that do not cease completely during a coma or anesthesia. In fact, some comatose patients have higher levels of metabolites from the brain's major neurotransmitters noradrenaline, serotonin and dopamine, perhaps suggesting that remaining healthy neurons overcompensate for the neurological damage, reports a Greek research team in the January 1996 issue of Acta Neurochirurgica.
Seizures
Seizures begin when activity in a few neurons increase to a point that exceeds what the brain can manage. The hyper-excitability may spread to the rest of the brain, causing a grand mal seizure. People who undergo a grand mal seizure are not conscious. But their neurons fire in uncontrollable ways. So, as pointed out by Normal Delanty M.D., editor of Seizures: Medical Causes and Management, a person who lacks consciousness due to a grand mal seizure in fact shows more activity throughout the affected brain areas than a conscious person who is not having a grand mal seizure.
Vegetative State
People in a vegetative state can be fully awake yet they fail to be consciously aware of their surroundings. So, despite being awake, they are unconscious. They sometimes show quite remarkable brain stem activity and activity in the brain's limbic system. They can cry, laugh, fall asleep, wake up and move their gaze around, yet they have no connection to the rest of the world. People in a vegetative state may have more activity in the limbic system than healthy people, reports Delanty. One reason for this is that the cortex can down-modulate activity in the limbic system. In the absence of cortical activity, the limbic system may fire in an uncontrollable and irregular pattern. Some patients in a vegetative state have shown activity in the anterior cingulate and limbic areas of the temporal lobe when told stories about their mothers, report Belgian neurologist Steven Laureys. There is also evidence that people in a vegetative state show activity in lower visual areas. Activity in these areas, however, does not suffice for consciousness, which requires activity in the prefrontal lobe.
References
- "Sleep: a comprehensive handbook"; Teofilo L. Lee-Chiong; 2006
- "Acta Neurochirurgica"; CSF Neurotransmitter metabolites in comatose head injury patients during changes in their clinical state; Markianos, et al.; January 1996
- "Seizures: medical causes and management"; Norman Delanty; 2002
- "The Lancet Neurology"; Brain function in coma, vegetative state,, and related disorders; laureys, et al.; September 2004
- "The Conscious Mind"; David J. Chalmers; 1996
