"Joseph LeDoux at New York University has shown how the BLA learns fear.fn14,20 Expose a rat to an innate trigger of fear--a shock. When this "unconditioned stimulus" occurs, the central amygdala activates, stress hormones are secreted, the sympathetic nervous system mobilizes, and, as a clear end point, the rat freezes in place--"What was that? What do I do?" Now do some conditioning. Before each shock, expose the rat to a stimulus that normally does not evoke fear, such as a tone. And with repeated coupling of the tone (the conditioned stimulus) with the shock (the unconditioned one), fear conditioning occurs--the sound of the tone alone elicits freezing, stress hormone release, and so on.fn15"