To tackle this question, researchers needed to induce a fear response in subjects in the lab. Merel Kindt and his team gave a mild electric shock to participants, while they were shown pictures of spiders — this produced a kind of Pavlovian pairing between two stimuli. They determined the physiological fear response in each participant by measuring how much his or her eyes blinked when shown a picture of the spider by itself. This method is actually a measure of how much the amygdala, the hub of emotion, reacts to the stimulus. When the pictures by themselves elicited the blink response, the researchers considered the memory of the fear to be "consolidated", meaning that the reaction was remembered and would be evocable at a later time.
This method is actually a measure of how much the amygdala, the hub of emotion, reacts to the stimulus.
The researchers then gave half the participants propranolol and the other half placebo; three days later all the subjects were retested to see if they still had the same reaction. The control group, given placebo, still showed a strong fear reaction when shown pictures of the spiders; in the propranolol group, this response had disappeared.
The team points out that their results are in line with another study, which found that propranolol helped diminish the fear response in people with post-traumatic stress disorder. They hope that their findings will eventually lead to new clinical treatment for people suffering from phobias and anxiety disorders.
There are some drawbacks to the current study, however: for example, only tested over three days, the longevity of the propranolol effect is still unknown. Critics also point out that while the physiological reaction to the conditioned stimulus was diminished, the subjects still knew consciously that they "should" be afraid of the spider pictures; this knowledge may be part of the memory of fear itself and limit the effectiveness of propranolol clinically.