What we might see as “delusions” are rational from the perspective of a patient’s altered reality.

When Arthur was a student, he received a severe blow to the head in a near-fatal car accident. For weeks, he lay in a coma. After he awoke, he embarked on the usual grueling path toward rehabilitation – gradually relearning to walk, talk, and recall the past. Miraculously, he was able to make a full recovery, although there was one small problem: he declared his parents had been replaced by imposters, and no amount of evidence or persuasion could convince him otherwise.

Arthur had acquired Capgras syndrome – a delusion wherein the sufferer believes their loved ones have been replaced by duplicates. Arthur admitted that these duplicates looked just like his parents, but he insisted they really weren’t.

Capgras syndrome is usually directed at close relatives, but other people, animals, or even things can be perceived as “replaced.” For example, one patient believed his pet poodle was an imposter. A much more tragic and grisly case involved a patient who, believing his stepfather to be a robot, decapitated him and opened his skull in search of microchips.

Most would be tempted to conclude that these patients are acting irrationally. Certainly, from our perspective, they are. But for a neurologist, this dismissive attitude doesn’t help explain why these patients manifest such a specific delusion. For a neurologist, the salient issue is to find out what has changed in a patient’s brain that would make such a delusion seem rational from the patient’s perspective.

Our biggest clue to understanding what’s going on comes from studies that measure changes in the galvanic skin response, or sweat, on the palms of patients’ hands when they look at familiar faces. Capgras patients exhibit no change whatsoever when they see an image of their mother. This indicates a total lack of response from the limbic system, which isn’t typical behavior.

It’s possible that the emotional response elicited from the limbic system by a familiar face plays a part in helping the brain identify who we’re looking at. So when Arthur looks at his mother and feels absolutely nothing toward her, perhaps the way the brain makes sense of this contradiction is to conclude that this person isn’t really his mother.

If this interpretation is correct, then it explains how the “duplicate” delusion might seem rational to a brain not receiving the emotional feedback that a loved one is present; in this case, the familiar face must be a copy.