Processing fear is a chain reaction in the brain triggered by stressful or threatening stimuli and resulting in physical symptoms of the fight-or-flight response. They include rapid heart beat, increased adrenaline and quickened breathing. Specific areas in the brain play major roles in the automatic fight-or-flight response, which follows two simultaneous paths once prompted: the low road, which progresses from thalamus to amygdala then to hypothalamus; and the high road, which takes longer and follows the path of thalamus through hippocampus, sensory cortex and amygdala to hypothalamus.
The amygdala determines possible threats, based on stored memories of frightening situations and prior knowledge of received data, and analyzes any emotional significance attached to the received information. Defensive and aggressive behavior is initially mediated by the amygdala, as its role is to receive information about stimuli, then alert the hypothalamus via neural impulses to initiate the fight-or-flight response, which may save your life in a dangerous situation.
The hippocampus stores and retrieves conscious memories; it uses this information to process sets of stimuli to determine if there is a threat and whether the fight-or-flight response will be initiated. Once the hippocampus establishes a context to process the stimuli, which includes determining if you've encountered the same stimulus before and what it meant that time, it signals the amygdala if there is immediate danger.
The hypothalamus activates the age-old survival reaction to fear, known today as the fight-or-flight response. To achieve this response, the hypothalamus triggers both the sympathetic nervous system (SNS), which initiates body reactions via nerve pathways, and the adrenal-cortical system, which sends information through the bloodstream.
During the fight-or-flight response, your body tenses and becomes alert. As adrenaline flows into the bloodstream, you become capable of quicker actions. These physical responses, triggered by the hypothalamus, also include increased blood pressure, dilated pupils and skin covered in goosebumps. They are the tools given by nature to help you survive threatening situations and decide whether to run or fight for your life.
The sensory cortex receives and analyzes information about potentially threatening visual stimuli and dangerous situations from the thalamus. This brain area recognizes there are several possibilities associated with any set of stimuli; it passes the sensory data along to the hippocampus to examine further and decide whether to signal the amygdala there is danger and initiate the fight-or-flight response.
The thalamus area of the human brain decides where to send incoming sensory data from the skin, mouth, eyes, nose and ears. Once it receives this sensory information, the thalamus determines if any of it meets the criteria for danger. If so, it alerts the amygdala and sensory cortex simultaneously to a potential threat. Therefore, the thalamus is the first brain area involved in the fight-or-flight response, as it brings the possibility of danger to the attention of your brain.