The diagnosis was diffuse axonal injury. “The textbook definition is essentially a blow that shuts down the bundle of wires responsible for keeping us conscious,” said Dr. Jonathan Fellus, a neurologist at Kessler Institute for Rehabilitation in West Orange, N.J., who has overseen Adam’s gradual recovery. “It’s as if the major highways have taken a hit, and now the brain has to use back roads to function. But every brain responds differently. I have given up making predictions.”

Researchers who have taken images of the brain as it processes information related to personal identity have noticed that several areas are particularly active. Called cortical midline structures, they run like an apple core from the frontal lobes near the forehead through the center of the brain.

These frontal and midline areas communicate with regions of the brain that process memory and emotion, in the medial temporal lobe, buried deep beneath each ear. And studies strongly suggest that in delusions of identity, these emotion centers are either not well connected to frontal midline areas or not providing good information. Mom looks and sounds exactly like Mom, but the sensation of her presence is lost. She seems somehow unreal.

The classic delusion of identification is called Capgras syndrome, after the French psychiatrist Dr. Jean Marie Joseph Capgras, who with Dr. Jean Reboul-Lachaux described in 1923 the case of a 53-year-old patient “who transformed everyone in her entourage, even those closest to her, such as her husband and daughter, into various and numerous doubles.”

In an analysis of such cases published in January in the journal Neurology, Dr. Orrin Devinsky, a neurologist at New York University, documented that people with the delusion typically have more damage to their right hemisphere than their left. Linear reasoning and language tend to be predominantly left-hemisphere functions, while holistic judgments  of intonation, of emphasis  tend to be processed more in the right. Dr. Devinsky argues that when people lack a familiar emotional glow in the company of a parent or loved one, the left hemisphere, unchecked by a damaged right, resolves the conflict with its categorical logic. The person must be an impostor.

“And if you have other damage in the cortical areas that check reality, that make judgments about what’s right and wrong, then you have no way to correct that error,” Dr. Devinsky said.

Image SURROUNDED BY FRIENDS AND FAMILY Adam engaging with those close to him. The brain needs to be active, scientists say, to have a chance to heal. Credit... Jon Reis for The New York Times

On good days, like that morning in physical therapy, Adam’s emotional centers appeared to be joining the working circuits of his brain. As he stared into the mirror, his smile changed from uncertain to mischievous, and he answered the question.