Image credit: Rama (CC BY-SA 2.0 FR)
To survive, animals must identify and react to stimuli in their environment that signal danger. But they must also adapt their behavior when those stimuli no longer signal danger – hiding whenever you hear a loud noise might keep you safe, but it also prevents you from searching for food. In the laboratory, we can study this form of learning using procedures called fear conditioning and extinction.
During fear conditioning, animals learn that a stimulus, such as a tone, signals that an unpleasant event is about to occur. That event might involve receiving a mild shock to the foot, for example. After experiencing the tone and shock paired together multiple times, animals will initially show signs of fear – such as freezing – when they hear the tone. But if later the tone occurs without being followed by the shock, these fear responses fade. This fading process is called extinction. Extinction does not involve erasing the old memory about the tone-shock relationship. That is, it is not a form of forgetting. Instead, the animals learn that the tone no longer signals an impending shock.
By monitoring brain activity in mice trained to associate a shock with a tone, Salinas-Hernández et al. reveal how the brain begins to learn that the shock no longer follows the tone. When the mice do not receive the anticipated shock to the foot, a group of brain cells that produce the chemical dopamine increase their activity. These neurons also fire whenever animals receive a reward, particularly one that exceeds their expectations. The more the dopamine neurons fire, the faster the mice reduce their fear responses to the tone. Preventing the neurons from increasing their activity prevents the mice from extinguishing their fear memory. By contrast, activating the neurons speeds up the extinction process.
Understanding how the brain extinguishes learned fear responses has therapeutic implications. Many anxiety disorders, such as post-traumatic stress disorder, involve impaired fear extinction learning. Indeed, exposure therapy – used to treat anxiety disorders such as phobias – is a form of fear extinction. Manipulating the activity of dopamine neurons during extinction could therefore help to treat anxiety disorders.
