the amygdala is one of the most-studied areas of the brain in relation to habituation. a common approach is to observe the visual processing of facial expressions. a study by breiter and colleagues used fmri scans to identify which areas of the brain habituate and at what rate. their results showed that the human amygdala responds and rapidly habituates preferentially to fearful facial expressions over neutral ones. they also observed significant amygdala signal changes in response to happy faces over neutral faces.