While many researchers use this strategy to study memory and learning in animals, LeDoux has focused all his attention on emotions. By using tracers, chemicals that stain neurons and thus show where they send their fibers, LeDoux found a direct pathway from the ear to a way station called the sensory thalamus that led directly to the amygdala, an almond-shaped structure in the forebrain. When this pathway was cut, rats could not be conditioned to fear a sound. Most of the time, the amygdala is quiet, LeDoux said. But when it receives a strong stimulus, it sets sirens in motion.
Hairs stand on end, the heart races and fight or flight hormones flood the body.
On closer examination, LeDoux found that the amygdala is designed to detect predators. When another rat hears this scream, a signal goes from the auditory cortex, where sounds are processed, directly to the amygdala. The human brain is similarly wired, LeDoux said. A visual stimulus, perhaps the sight of a snake on a dirt path, will travel to the amygdala in a few thousandths of a second. The human amygdala contains cells that fire in response to expressions on faces and may also react to objects of fear.