We all take time to get used to (or habituate to) a new stimulus (a job interview, a party) before we begin to explore the unfamiliar. After all, a novel stimulus may serve as a signal for something dangerous or important. But shy individuals sense danger where it does not exist. Their nervous system does not accommodate easily to the new. Animal studies by Michael Bavis, Ph.D., of Yale University, indicate that the nerve pathways of shyness involve parts of the brain involved in the learning and expression of fear and anxiety.
Both fear and anxiety trigger similar physiologic reactions: muscle tension, increased heart rate, and blood pressure, all very handy in the event an animals has to fight or flee sudden danger. But there are important differences. Fear is an emotional reaction to a specific stimulus; it's quick to appear, and just as quick to dissipate when the stimulus passes. Anxiety is a more generalized response that takes much longer to dissipate.
Studies of cue conditioning implicate the amygdala as a central switchboard in both the association of a specific stimulus with the emotion of fear and the expression of that fear. Sitting atop the brain stem, the amygdala is crucial for relaying nerve signals related to emotions and stress. When faced with certain stimuli--notably strangers, authority figures, members of the opposite sex--the shy associate them with fearful reactions.
In contrast to such "explicit" conditioning is a process of "contextual" conditioning. It appears more slowly, lasts much longer. It is often set off by the context in which fear takes place. Exposure to that environment then produces anxiety-like feelings of general apprehension. Through contextual conditioning, shy people come to associate general environments--parties, group discussions where they will be expected to interact socially--with unpleasant feelings, even before the specific feared stimulus is present.
Contextual conditioning is a joint venture between the amygdala and the hippocampus, the cell cluster near the amygdala, which is essential to memory and spatial learning. Contextual conditioning can be seen as a kind of learning about unpleasant places.
But a crucial third party participates in contextual conditioning. It's the bed nucleus of the stria terminalis (BNST). The long arms of its cells reach to many other areas of the brain, notably the hypothalamus and the brain stem, both of which spread the word of fear and anxiety to other parts of the body. The BNST is principally involved in the generalized emotional-behavioral arousal characteristic of anxiety. The BNST may be set off by the neurotransmitter corticotropin releasing factor (CRF).
Once alerted, the hypothalamus triggers the sympathetic nervous system, culminating in the symptoms of inner turmoil experienced by the shy--from rapid heartbeat to sweaty paleness. Another pathway of information, from the amygdala to the brain stem, freezes movement of the mouth.
The shy brain is not different in structure from yours and mine; it's just that certain parts are more sensitive. Everyone has a "shyness thermostat," set by genes and other factors. The pin-pointing of brain structures and neuro-chemicals involved in shyness holds out the promise that specific treatment may eventually be developed to curb its most debilitating forms.