The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

As we saw earlier, one job of the right hemispheres is to take a detached, big-picture view of yourself and your situation. This job also extends to allowing you to “see” yourself from an outsider’s point of view. For example, when you are rehearsing a lecture, you may imagine watching yourself from the audience pacing up and down the podium.

This idea can also account for out-of-body experiences. Again, we only need to invoke disruption to the inhibitory circuits that ordinarily keep mirror-neuron activity in check. Damage to the right frontoparietal regions or anesthesia using the drug ketamine (which may influence the same circuits) removes this inhibition. As a result, you start leaving your body, even to the extent of not feeling your own pain; you see your pain “objectively” as if someone else were experiencing it. Sometimes you get the feeling that you have actually left your body and are hovering over it, watching yourself from outside. Note that if these “embodying” circuits are especially vulnerable to lack of oxygen to the brain, this could also explain why such out-of-body sensations are common in near-death experiences.

Odder still than most out-of-body sensations are the symptoms experienced by a patient named Patrick, a software engineer from Utah who had been diagnosed with a malignant brain tumor in his frontoparietal region. The tumor was on the right side of his brain, which was fortunate because he was less worried about it than he would have been had it been on the left. Patrick had been told he had less than two years to live even after the tumor had been removed, but he tended to play it down. What really intrigued him was much stranger than either he or anyone else could have imagined.

He noticed that he had an invisible but vividly felt “phantom twin” attached to the left side of his body. This was different from the more common sort of out-of-body experience in which a patient feels he is looking down on his own body from above. Patrick’s twin mimicked his every action in near-perfect synchrony. Patients like him have been studied extensively by Peter Brugger of the University Hospital Zürich. They remind us that even the congruence between different aspects of your mind such as subjective “ego” and body image can be deranged in brain disease. There must be a specific brain mechanism (or dovetailing suite of mechanisms) that ordinarily preserves such congruence; if there weren’t, it could not have been affected selectively in Patrick while leaving other aspects of his mind intact—for indeed, he was emotionally normal, introspective, intelligent, and amiable.¹⁰

Out of curiosity I irrigated his left ear canal with ice water. This procedure is known to activate the vestibular system and can provide a certain jolt to the body image; it can, for example, fleetingly restore awareness of the paralysis of the body to a patient with anosognosia due to a parietal stroke. When I did this for Patrick, he was astonished to notice the twin shrinking in size, moving, and changing posture. Ah, how little we know about the brain!

Out-of-body experiences are seen often in neurology, but they blend imperceptibly into what we call dissociative states, which are usually seen by psychiatrists. The phrase refers to a condition in which the person mentally detaches herself from whatever is going on in her body during a highly traumatic experience. (Defense lawyers often use the dissociative state diagnosis: that the accused was in a such a state, and that she was watching her body “acting out” the murder without personal involvement.)

The dissociative state involves the deployment of some of the same neural structures already discussed, but in addition two other structures: the hypothalamus and the anterior cingulate.¹¹ Ordinarily, when confronted with a threat, two outputs flow out from the hypothalamus: a behavioral output, such as running away or fighting; and an emotional output, such as fear or aggression. (We already mentioned the third output: autonomic arousal leading to sweating GSR, blood pressure, and heart-rate elevation.) The anterior cingulate is simultaneously active; it allows you to remain aroused and ever vigilant for new threats and new opportunities for fleeing. But the degree of threat determines the degree to which each of these three subsystems is engaged. When one is confronted with an extreme threat, it is sometimes best to lie still and do nothing at all. This could be regarded as a form of “playing possum,” shutting down both the behavioral and emotional output. The possum becomes completely still when a predator is so close that escape is no longer an option, and in fact any attempt would only activate the carnivore’s instinct to chase down fleeing prey. Nonetheless, the anterior cingulate remains powerfully engaged the whole time to preserve vigilance, just in case the predator isn’t fooled or a quick escape route becomes available.