I came across this paper and find it interesting. It links enneagram with the left PFC, right PFC, left Amygdala and right Amygdala (so.... Social Reward, Social Punishment, and activation of and differentiation of fears). I wish the person had finished it.
http://petersavich.com/PersonalityAndTheBrain/
Edited to add: for those interested, the second paper refers to the optimism/pessimism (reward/punishment) as related specifically to OFCs.