Its dark out, and youre home alone. The house is quiet other than the sound of the show youre watching on TV. You see it and hear it at the same time: The front door is suddenly thrown against the door frame.

Your breathing speeds up. Your heart races. Your muscles tighten. A split second later, you know its the wind. No one is trying to get into your home. For a split second, you were so afraid that you reacted as if your life were in danger, your body initiating the fight-or-flight response that is critical to any animals survival. But really, there was no danger at all. What happened to cause such an intense reaction? What exactly is fear? In this article, well examine the psychological and physical properties of fear, find out what causes a fear response and look at some ways you can defeat it.

Fear is a chain reaction in the brain that starts with a stressful stimulus and ends with the release of chemicals that cause a racing heart, fast breathing and energized muscles, among other things, also known as the fight-or-flight response. The stimulus could be a spider, a knife at your throat, an auditorium full of people waiting for you to speak or the sudden thud of your front door against the door frame.

The brain is a profoundly complex organ. More than 100 billion nerve cells comprise an intricate network of communications that is the starting point of everything we sense, think and do. Some of these communications lead to conscious thought and action, while others produce autonomic responses. The fear response is almost entirely autonomic: We dont consciously trigger it or even know whats going on until it has run its course.

Because cells in the brain are constantly transferring information and triggering responses, there are dozens of areas of the brain at least peripherally involved in fear. But research has discovered that certain parts of the brain play central roles in the process:

Thalamus - decides where to send incoming sensory data (fromeyes, ears, mouth, skin)

Sensory cortex - interprets sensory data

Hippocampus - stores and retrieves conscious memories; processes sets of stimuli to establish context

Amygdala - decodes emotions; determines possible threat; stores fear memories

Hypothalamus - activates "fight or flight" response

T­he process of creating fear begins with a scary stimulus and ends with the fight-or-flight response. But there are at least two paths between the start and the end of the process. In the next section, well take a closer look at how fear is created.

The process of creating fear takes place in the brain and is entirely unconscious. There are two paths involved in the fear response: The low road is quick and messy, while the high road takes more time and delivers a more precise interpretation of events. Both processes are happening simultaneously.

The idea behind the low road is "take no chances." If the front door to your home is suddenly knocking against the frame, it could be the wind. It could also be a burglar trying to get in. Its far less dangerous to assume its a burglar and have it turn out to be the wind than to assume its the wind and have it turn out to be a burglar. The low road shoots first and asks questions later. 

The process looks like this:

The door knocking against the door frame is the stimulus. As soon as you hear the sound and see the motion, your brain sends this sensory data to the thalamus. At this point, the thalamus doesnt know if the signals its receiving are signs of danger or not, but since they might be, it forwards the information to the amygdala. The amygdala receives the neural impulses and takes action to protect you: It tells the hypothalamus to initiate the fight-or-flight response that could save your life if what youre seeing and hearing turns out to be an intruder.

The high road is much more thoughtful. While the low road is initiating the fear response just in case, the high road is considering all of the options. Is it a burglar, or is it the wind? 

The long process looks like this:

When your eyes and ears sense the sound and motion of the door, they relay this information to the thalamus. The thalamus sends this information to the sensory cortex, where it is interpreted for meaning. The sensory cortex determines that there is more than one possible interpretation of the data and passes it along to the hippocampus to establish context. The hippocampus asks questions like, "Have I seen this particular stimulus before? If so, what did it mean that time? What other things are going on that might give me clues as to whether this is a burglar or a wind storm?" The hippocampus might pick up on other data being relayed through the high road, like the tapping of branches against a window, a muffled howling sound outside and the clatter of patio furniture flying about. Taking into account this other information, the hippocampus determines that the door action is most likely the result of wind. It sends a message to the amygdala that there is no danger, and the amygdala in turn tells the hypothalamus to shut off the fight-or-flight response.

The sensory data regarding the door the stimulus is following both paths at the same time. But the high road takes longer than the low road. Thats why you have a moment or two of terror before you calm down.

Regardless of which path were talking about, all roads lead to the hypothalamus. This portion of the brain controls the ancient survival reaction called the fight-or-flight response. In the next section, well take a closer look at the fight-or-flight response.

If we couldnt be afraid, we wouldnt survive for long. Wed be walking into oncoming traffic, stepping off of rooftops and carelessly handling poisonous snakes. Wed be hanging out with people who have tuberculosis. In humans and in all animals, the purpose of fear is to promote survival. In the course of human evolution, the people who feared the right things survived to pass on their genes. In passing on their genes, the trait of fear and the response to it were selected as beneficial to the race.

During the 19th-century debate surrounding evolution, the "face of fear" that wide-eyed, gaping grimace that often accompanies sheer terror became a talking point. Why do people make that face when theyre terrified? Some said God had given people a way to let others know they were afraid even if they didnt speak the same language. Charles Darwin said it was a result of the instinctive tightening of muscles triggered by an evolved response to fear. To prove his point, he went to the reptile house at the London Zoological Gardens. Trying to remain perfectly calm, he stood as close to the glass as possible while a puff adder lunged toward him on the other side. Every time it happened, he grimaced and jumped back. In his diary, he writes, "My will and reason were powerless against the imagination of a danger which had never been experienced." He concluded that the entire fear response is an ancient instinct that has been untouched by the nuances of modern civilization [ref].

Most of us are no longer fighting (or running) for our lives in the wild, but fear is far from an outdated instinct. It serves the same purpose today as it did when we might run into a lion while carrying water back from the river. Only now, were carrying a wallet and walking down city streets. The decision not to take that shortcut through the deserted alley at midnight is based on a rational fear that promotes survival. Only the stimuli have changed were in as much danger today as we were hundreds of years ago, and our fear serves to protect us now as it did then.

Darwin had never experienced the bite of a poisonous snake, and yet he reacted to it as if his life were in danger. Most of us have never been anywhere near The Plague, but our heart will skip a beat at the sight of a rat. For humans, there are other factors involved in fear beyond instinct. Human beings have the sometimes unfortunate gift of anticipation, and we anticipate terrible things that might happen things we have heard about, read about or seen on TV. Most of us have never experienced a plane crash, but that doesnt stop us from sitting on a plane with white-knuckle grips on the armrests. Anticipating a fearful stimulus can provoke the same response as actually experiencing it. This also is an evolutionary benefit: Those humans who felt rain, anticipated lightning and remained in the cave until the storm passed had a better chance of not getting struck with thousands of volts of electricity.

Studies have shown that rats with damaged amygdalas will walk right up to cats [ref]. Most of us arent too keen on the prospect of going at our amygdala with an ice pick, though. So scientists are exploring other ways to overcome fear.

Fear extinction

Whereas Little Albert learned to fear white rats in the 1920s, rats learned to fear a simple noise more than 80 years later. Scientist Mark Barad of UCLA performed an experiment in which he and his team combined a noise with an electric shock. They would play the tone and then immediately apply a shock to the metal floor of the rats cage. It was classical conditioning, and it didnt take long for the rats to brace themselves for the shock as soon as they heard the sound. At that point, their amygdalas paired the sound with the shock, and the sound created a fear response. The researchers then began the process of fear-extinction training, in which they made the sound but did not apply the shock. After hearing the sound very often without the shock, the rats stopped fearing the noise.

Fear extinction involves creating a conditioned response that counters the conditioned fear response. While 

studies situate the amygdala as the location of fear memories formed by conditioning, scientists theorize that fear-extinction memories form in the amygdala but then are transferred to the medial prefrontal cortex (mPFC) for storage. The new memory created by fear extinction resides in the mPFC and attempts to override the fear memory triggered in the amygdala.

Most behavioral therapies for fear extinction focus on exposure. For instance, therapy for a person with a fear of snakes might involve visiting a snake farm repeatedly and taking small steps toward touching one. First, the person might get within 10 feet of the snake and see that nothing terrible happens. Then he might get within 5 feet of the snake. When nothing terrible happens within 5 feet of the snake, he might get close enough to touch it. This process continues until new, fear-extinction memories are formed memories that say "snakes are not going to harm you" and serve to contradict the fear of snakes that lives in the amygdala. The fear still exists, but the idea is to override it with the new memory.

FEAR DISORDERS

According to the National Institute of Mental Health, 19 million people in the United States alone suffer from mental illnesses that involve irrational fear responses. These disorders include generalized anxiety disorder, panic disorder and post-traumatic stress disorder.