In humans, facial expressions of negative affect are examples of fearful stimuli. In fact, facial expressions convey such strong emotional information that merely observing anger or fearful faces elicits visceral responses, including increased heart rate and sweating (Ohman and Soares 1998). Neuroimaging studies (Davis and Whalen 2001) have illustrated that these fearful responses to facial expressions are processed and largely mediated by the amygdala (having connections to both early sensory processing areas and autonomic reflex centers). Furthermore, amygdala responses to fearful faces have been observed with neuroimaging scans even in the absence of conscious awareness of their presentation to subjects (Whalen et al 1998).
