Can Fear Be Forgotten?

The Shock of Discovery

Down the corridor from LeDoux's office, near a paper sign reminding students of the lab's Wi-Fi password ("fearisgood"), heavy glass doors open to reveal the fear factory. Inside, 300 plump white rats live like rodent royalty. Each gets its own transparent acrylic cage and is fed a continuous supply of filtered water and top-notch rat chow. Their cages, neatly aligned on stainless-steel wire shelves, are scrubbed regularly and ventilated with oxygen-rich air. When we enter, we have to wear surgical masks to keep from sullying the rats with germs we might be tracking in from the outside world. According to Marie Monfils, a postdoc here, these rats are treated exceptionally well because happy, healthy, easygoing rats make ideal test subjects when it comes time to scare the holy crap out of them.

To understand why rats-and other animals, including humans-get scared, you have to start at the amygdala, the place where sensation and memory join forces to spawn the venerable beast we call fear. The amygdala is buried in the forebrain directly behind the eyes. LeDoux first started researching the amygdala in the late 1970s with early experiments that investigated how rats adapt to danger.

In one experiment, LeDoux played a tone to the rats and then dispensed a mild electric shock. After a few repetitions, the tone alone made the rats freeze-a classic Pavlovian response. He had expected this, but at the same time he wondered what was actually occurring inside their brains when they froze. He injected a dye that mapped out the connections in the rat brains and found that the auditory thalamus-the part of the brain that receives signals from the ears-connects directly to the amgdala. He then surgically cut the pathway that connects the auditory thalamus to the amygdala, repeated the tone, and found that the rats no longer feared the sound.

Somehow, the amygdala was forming and storing what LeDoux labeled a "fear memory" that preempted all other brain activity whenever it recognized the offending input. The rats were essentially oblivious to their freezing behavior, responding to the tone without the use of their higher brain functions, precisely the way I might squeal like a schoolgirl at the sight of a spider before I can reason that it's not going to eat my left arm.
The study revealed that when it comes to fear, the "thinking" part of your brain is instinctively subordinate to the amygdala. Your fears forestall your thoughts, and the amygdala is the reason why. It takes a new input, checks it against your fear memories and, if there's a match, initiates a response.

Without the fear memory, though, the chain falls apart: If my brain can't remember why I'm afraid of spiders, then I won't be afraid of spiders. Yet selectively eliminating a memory would seem to be impossible. LeDoux suspected it was not.