The Washington Post has a heartbreaking story on something most of us have heard about in the news. Parents, busy and stressed, somehow manage to forget their children in the car (or on top of the car in a carseat, or any number of scenarios), and the child ends up dying. A majority of the article is focused on the legal and moral implications of this tragedy, but near the end of the article, a psychologist is interviewed talking about the nature of memory. Memory often works well, but under stress, it oftentimes makes errors.

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David Diamond is picking at his breakfast at a Washington hotel, trying to explain.

“Memory is a machine,” he says, “and it is not flawless. Our conscious mind prioritizes things by importance, but on a cellular level, our memory does not. If you’re capable of forgetting your cellphone, you are potentially capable of forgetting your child.”

Diamond is a professor of molecular physiology at the University of South Florida and a consultant to the veterans hospital in Tampa. He’s here for a national science conference to give a speech about his research, which involves the intersection of emotion, stress and memory. What he’s found is that under some circumstances, the most sophisticated part of our thought-processing center can be held hostage to a competing memory system, a primitive portion of the brain that is — by a design as old as the dinosaur’s — inattentive, pigheaded, nonanalytical, stupid.

Diamond is the memory expert with a lousy memory, the one who recently realized, while driving to the mall, that his infant granddaughter was asleep in the back of the car. He remembered only because his wife, sitting beside him, mentioned the baby. He understands what could have happened had he been alone with the child. Almost worse, he understands exactly why.

The human brain, he says, is a magnificent but jury-rigged device in which newer and more sophisticated structures sit atop a junk heap of prototype brains still used by lower species. At the top of the device are the smartest and most nimble parts: the prefrontal cortex, which thinks and analyzes, and the hippocampus, which makes and holds on to our immediate memories. At the bottom is the basal ganglia, nearly identical to the brains of lizards, controlling voluntary but barely conscious actions.

Diamond says that in situations involving familiar, routine motor skills, the human animal presses the basal ganglia into service as a sort of auxiliary autopilot. When our prefrontal cortex and hippocampus are planning our day on the way to work, the ignorant but efficient basal ganglia is operating the car; that’s why you’ll sometimes find yourself having driven from point A to point B without a clear recollection of the route you took, the turns you made or the scenery you saw.

Ordinarily, says Diamond, this delegation of duty “works beautifully, like a symphony. But sometimes, it turns into the ‘1812 Overture.’ The cannons take over and overwhelm.”

By experimentally exposing rats to the presence of cats, and then recording electrochemical changes in the rodents’ brains, Diamond has found that stress — either sudden or chronic — can weaken the brain’s higher-functioning centers, making them more susceptible to bullying from the basal ganglia. He’s seen the same sort of thing play out in cases he’s followed involving infant deaths in cars.

“The quality of prior parental care seems to be irrelevant,” he said. “The important factors that keep showing up involve a combination of stress, emotion, lack of sleep and change in routine, where the basal ganglia is trying to do what it’s supposed to do, and the conscious mind is too weakened to resist. What happens is that the memory circuits in a vulnerable hippocampus literally get overwritten, like with a computer program. Unless the memory circuit is rebooted — such as if the child cries, or, you know, if the wife mentions the child in the back — it can entirely disappear.”