Introverts have wide-open information channels, causing them to be flooded with stimulation and over-aroused, while extroverts have tighter channels, making them prone to under-arousal. Over-arousal doesn't produce anxiety so much as the sense that you can't think straight--that you've had enough and would like to go home now. Under-arousal is something like cabin fever. Not enough is happening: you feel itchy, restless, and sluggish, like you need to get out of the house already.