The footnote in that sentence leads you to this short paper linked from my bibliography, which reads:

The efficiency and accuracy with which neural activity can code the value of a stimulus (such as liquid volume) can be greatly increased if neurons make use of information about the probabilities of potential reward values. Neural activity can then be devoted to representing probable values at the expense of improbable values. Our evidence suggests that the transient dopamine response to conditioned stimuli may carry information on expected reward value, and previous work shows that the more sustained activity of dopamine neurons reflects a measure of reward uncertainty such as variance. If the system possesses prior information consisting of the expected value and variance of reward, then this information need not be represented redundantly at the time of reward. Discarding this old information may be achieved by subtracting the expected value from the absolute reward value and then dividing by the variance. Analogous normalization processes appear to occur in early visual neurons. It is not known to what extent the normalization processes observed in dopamine neurons are actually performed in dopamine neurons as opposed to their afferent input structures. Because the new information is by definition precisely the information that the system needs to learn, the activity of dopamine neurons would be an appropriate teaching signal.

I'm sorry I don't have more time to go into this, hopefully the paper link helps.