also known as specific phobia) are triggered by a stressful or fearful experience or object. This fearful sensory input is then sent to the amygdala (the fear processing center of the brain), where it ‘remembers’ the fearful stimulus. The activated amygdala also starts the cascade known as the fight or flight response, where the sympathetic nervous system is activated and stress hormones are released. This culminates in the various physical changes from this fight or flight response (fear response), which includes increased heart rate, increased rate of breathing, sweating, muscle tension, trembling, tunnel vision, butterflies in the stomach, lump in the throat, and nausea, just to name a few symptoms.
In addition, another part of the brain called the hippocampus remembers the fearful stimuli, and makes sure the fear is triggered (via its connection to the amygdala) whenever encountering the fearful stimuli in the future.
However, the brain has a way to prevent the amygdala from remembering the feared stimuli, and hence prevent the development of a phobia. The last resort is the prefrontal cortex (the thinking part of the brain), which can suppress the fear response via its connection with the amygdala. If the prefrontal cortex can’t suppress the fear response, then fear conditioning proceeds, and a phobia has developed.
So phobias are triggered by the environment, and then hard-wired into the anxiety memory banks of the brain, the amygdala and hippocampus. But phobias only develop if the prefrontal cortex is not able to suppress the fear response via its connection with the amygdala.
Therefore, phobias are not really caused by biological factors alone. The development of phobias is both a biological and psychological response to an environmental stimuli, mediated by the amygdala, hippocampus, and prefrontal cortex.