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From specific to generalized fear: neuronal encoding of the switch

“I can’t get the memories out of my mind!… I am right back in Vietnam, in the middle of the monsoon season at my guard post. My hands are freezing, yet sweat pours from my entire body…I smell a damp sulfur smell. Suddenly I see what’s left of my buddy Troy, his head on a bamboo platter, sent back to our camp by the Viet Cong.”

This veteran of the US army, who served in Vietnam, has intense flashbacks of his decapitated friend whenever he hears a clap of thunder, touches a bamboo mat, or sees an Oriental woman. Although the traumatic incident happened decades ago in a battlefield half way around the world, his vivid memories continue to produce a state of hyperarousal and intense fear similar to what he experienced that fateful day.

How did that specific trauma morph into a state of generalized fear and anxiety, leading to the soldier’s post-traumatic stress disorder (PTSD)?

Now, a study published in Nature Neuroscience, by Professor Sumantra Chattarji, a neuroscientist at the National Centre for Biological Sciences (NCBS) in Bangalore and his student Supriya Ghosh, gives new insight into how the brain’s ability to distinguish safe from dangerous stimuli can go badly wrong, potentially leading to a state of generalized fear.

The fear conditioning experiments, done with live rats, showed that individual neurons in the amygdala, the emotional hub of the brain, that were initially capable of telling apart safe from dangerous stimuli can start firing indiscriminately–causing the rat to become fearful of non-threatening stimuli. Faced with the potential for greater danger, the neurons reflect the animal’s tendency to err on the side of caution.

Fear conditioning is learning by association. A widely known example of this type of learning came from the work of the Russian physiologist Ivan Pavlov. In his experiments, a dog did not respond to the ring of a bell on its own. However, when the same bell ring was followed by food, a later exposure to the bell alone caused the dog to salivate. It remembered that the bell predicts a reward.

Interestingly, Pavlov’s dogs salivated not just to the original bell, but to other sounds as well; especially to sounds similar to the original bell. In other words, animals generalize from one stimulus to another not only because they cannot discriminate them, but because they expect that the stimuli are likely to have the same consequence. If the consequence is not a reward but a painful punishment or a dangerous situation, then the stakes are obviously higher. If an animal under-generalizes it may overlook future signs of danger, whereas if it over-generalizes it may be too afraid to explore and thereby miss opportunities for feeding, mating, etc. So, striking the right balance is essential for survival.