Fear-potentiated startle

Fear-potentiated startle (FPS) is a reflexive physiological reaction to a presented stimulus, and is an indicator of the fear reaction in an organism. The FPS response can be elicited in the face of any threatening stimulus (e.g., any object, person or situation that would cause someone to experience feelings of fear), but it can also be elicited by a neutral stimulus as a result of fear conditioning, a process that occurs when a benign stimulus comes to evoke fear and anxiety upon being paired with a traumatic or fear-provoking event. The stimulus in question is usually of auditory (e.g., loud noise) or visual (e.g., bright light) nature, and startle response measures include eyeblink rates and pulse/heart rate. The negative impact of heightened FPS in the face of neutral stimuli can be treated pharmacologically, using psychotropic medications that are typically used to reduce anxiety in humans. Recent literature, moreover, has implicated increased FPS responses as a correlate in posttraumatic stress disorder (PTSD) and other anxiety disorders.

Neurobiology of FPS

The central brain structure through which fear-associated responses are mediated has been determined to be the amygdala, which is located in the brain's temporal lobe. When the central nucleus of the amygdala is stimulated - what is popularly referred to as the "fight-or-flight" response is activated - the organism in question reacts passively (is rendered frozen in its tracks, becomes hyper-vigilantly attentive, etc.), or displays a physiological reaction geared toward facilitating an aggressive reaction (e.g., increased heart-rate/pulse, rapid breathing). These fear-induced reactions result from communication between the amygdala and a variety of other brain regions (such as the brain stem and hypothalamus), resulting in a variety of physiological responses in the organism. For instance, communication between an activated amygdala and the lateral hypothalamus results in increased blood pressure and dilation of the pupils; the initiation of the central grey via communication from the amygdala results in the organism's becoming frozen in its tracks; communication between the activated amygdala and the paraventricular nucleus of the hypothalamus releases hormones associated with stress.

Literature has linked the FPS response to interplay between the central nucleus of the amygdala and both the central grey and nucleus reticular pontis caudalis.