“This realization could lead to novel approaches to treating a variety of anxiety disorders, including post-traumatic stress disorder (PTSD), because people who suffer from such conditions have trouble telling the difference between situations that merit fear and those that are innocuous,” say two Columbia University neurology professors, Mazen A. Kheirbek and Rene Hen, writing in the July 2014 issue of Scientific American magazine.
For years, scientists believed that adult humans stopped producing new neurons, but then about 20 years ago, evidence from the brains of adult rodents, monkeys, and even humans showed that new neurons are being produced continuously in two areas of the brain, one of which is involved with smell and the other involved in learning, memory, and emotion.
According to Kheirbek and Hen, one of the learning and memory functions that appear to involve new neurons involves pattern completion, which is laying a memory down so that it can be retrieved. The other involves pattern separation, which is recording details of an event so that it can be distinguished from other events.
The neurologists tested their theory in lab mice by shutting down neurogenesis, or the production of new neurons, in some mice and boosting it in others. Then they took the mice from their safe home cage and put them in another cage in which they got a mild electric shock.
“Animals lacking new neurons remained overly skittish, reacting in alarm in both environments, even after repeated trips to the harmless box proceeded without incident,” they reported. But that didn’t happen with mice with an increased number of new brain cells.
Several other studies have also shown that mice lacking in new neurons have been unable to distinguish between safety and danger, the neurologists said.
“If neurogenesis is, in fact, involved in pattern separation in humans, the finding could offer insights into the cause of anxiety disorders such as PTSD,” said the article, Add Neurons, Subtract Anxiety. “Psychologists have long suspected that the overgeneralization of memory contributes to anxiety disorders, which are marked by an exaggerated, sometimes crippling, fear response, even when the environment holds no immediate threat. Such inappropriate generalization could be the result of a diminished ability to distinguish between a past trauma and an innocuous event that shares some similarity with the traumatic event – for example, a picnic that is interrupted by a unexpected loud noise.
“Individuals with a normal capacity for pattern separation might flinch at the sudden boom but quickly realize that the park is not a war zone and continue with their lunch,” it said. “A veteran with impaired ability to carry out pattern separation, on the other hand, may be unable to separate the sound of a car backfiring from the memory of a battlefield – a mistake that could precipitate a full-blown panic attack.”
Researchers have found that most humans continue to add about 1,400 new brain cells per day to the hippocampus well into old age. While the authors speculated about a deficit in neuron production, they suggested no reason why so many vets would be experiencing this disorder.
They did, however, explain what cures the condition: exercise. Mice running on a wheel in their cages showed increased rates of neurogenesis.
And that may well explain why vets who are kayaking, whitewater rafting, hiking, and mountain climbing are able to alleviate the symptoms of PTSD. They’re producing more new brain cells and reducing their anxiety levels.