How much would you pay me for a deck of 100 cards, half blue and half red, if I told you that if you drew a card from the deck without looking and correctly specified its color you would get $100? Most people would off about $45 for such a deck, indicating an aversion to risk. If offered a second deck and not told how many of the cards are blue and how many are white, the offer drops to $42, indicating that much larger an aversion to risk plus ambiguity. The odds of getting the $100 are actually 50-50 in both cases, but in the first people think 'they know the probability."
Hsu et al use functional brain imaging to show that the brain treats the two decks in different ways. Ambiguity in choice (the second deck) correlates positively with activation of the amygdala and orbitofrontal cortex, regions involving emotional values of options, and negatively with a striatal system. Striatal activation correlates more strongly with risk and and expected reward (the first deck).
By the way, if you want to learn a bit more about the brain, and brain structures mentioned above, try Neuroscience for Kids. Or better, the more advanced Digital Anatomist or Braininfo sites.