Well, I learned about the Biological model of I/E in my Theories of Personality class. I asked my teacher how well it related to Jung's version and she told me that there was some overlap but overall it is an unexplored area. So I thought I would do some casual exploration...
Basically, I's are more quickly and strongly aroused when exposed to external stimulation. Introverts are also more easily overwhelmed by the stimulation of parties, loud music, or social gatherings while extraverts find this kind of stimulation pleasant. On the other end of the coin, extraverts are quickly bored by slow-moving movie plots and soft music while introverts often find these subtle sources of stimulation engaging. When trying to study in a library, research has found that introverts typically will pick quiet study rooms away from all stimulation while extraverts prefer to sit out in the open amidst the stimulation.
Another difference, according to the biological model, is that extraverts are much more sensitive to the idea of rewards. This leads to impulsive behavior which makes them more likely to be found on a roller coaster or at a party. However, this also means that if an extravert thought going on a walk alone would be pleasing, they would be more likely than an introvert to actually do it.
Although I do not wish to address this piece of information as I do the others, research in the biological approach has found that extraverts are happier. This is, in short, due to their ability to appreciate and pursue social experience (commonly found to be a key factor in overall happiness) and their ability to appreciate rewards. As far as rewards go, research has found that extraverts are more pleased with their own accomplishments than introverts, and do not react as badly to failure as introverts. (Of course the problem I see with this is that social experience and rewards do not even begin to describe what constitutes happiness. These two things are items that are measurable by experimentation while I'm sure at least some of the things introverts find happiness in are not as easily measurable. I personally don't see happy as something experienced but rather something obtained.)
So, in summary: Introverts are much more sensitive to stimulation than extraverts.
So, I wish just wondering if whoever reads this feels like it is in some way descriptive of their MBTI I/E preference? I personally feel like the biological model of an introvert fits me well.