"I try to act extraverted, but I just am not. I guess I'm not wired that way."
Research shows that reward-related areas of the brain show more activation in extraverts than in introverts (2), but researchers are now looking into differences in sensitivity in the brain. A recent study by Fishman and colleagues (2011) tested to see if brain waves (using ERP, which is a form of EEG that measures the brain's electrical response to a specific stimulus) showed differences in response to pictures of faces depending upon whether the subject was more introverted or extraverted. They found higher P300 amplitude (1) in subjects with higher extraversion scores, indicating higher interest/greater salience of human faces than for introverts.
While it is a very cool conclusion that introverts are simply less sensitive to social stimuli, receive less reward from it, find less significance in faces, and respond differently in their brains (ignoring arguments of plasticity for the sake of argument), it is not entirely clear that this is what these data actually mean.
This would also hardly be my blog unless I asked, "Exactly what do you mean by introversion?" This set of researchers define introversion with the following descriptors: "reserved, withdrawn, or shy." Naturally, this also doesn't mean "socially uninterested." If you're shy, you have anxiety, but you aren't necessarily of mere lower sociability. They go on to explain that "...social engagement and preference for other people's company is one of the fundamental features of extraversion," citing Ashton and colleagues from 2002. They also cite Eysenck and Allport...
Allport & Allport (1921) described introversion as a difference in self-expressive tendency. Eysenck liked a 2-factor model with sociability and impulsivity at the core. You will note that neither made extraversion synonymous with high sociability and introversion with low sociability.
Why am I complaining about this? There is some really cool research out there probing introvert-extravert differences, and they have certainly found some neat neural differences. But let's be cautious in drawing conclusions: if you are a social introvert, your P300 might be attenuated (you might not find such compelling motivational salience with the presentation of faces), but if you are a thinking, anxious, or inhibited introvert -- all bets are off! Unless your P300 is peaked when you look at a picture of a book...
(1) The P300 is a signal in your brain that occurs in response to stimuli that are of more interest or personal salience. It is "proportional to the amount of attentional resources engaged in processing a given stimulus" (Fishman et al., 2011). In other words, more attention to something means a bigger P300.
(2) Fishman et al. reference Cohen et al., 2005; Depue & Collins, 1999; Johnson et al., 1999.