Figure 5. (A) Top: testing risky rewards: an animal chooses between a safe reward whose amount is adjusted by the experimenter (left) and a fixed, binary, equiprobable gamble (right). Height of each bar indicates juice volume; two bars indicate occurrence of each indicated amount with P=0.5 (risky reward). Bottom: Two psychophysical assessments of risk attitude in a monkey. CE indicates amount of safe reward at choice indifference against gamble (certainty equivalent). EV = expected value of gamble. A CE > EV suggests risk seeking (subjective gamble value CE exceeds objective gamble value EV, left), CE < EV indicates risk avoidance (right). (B) Positive utility prediction error responses to unpredicted juice rewards. Red: utility function derived from binary, equiprobable gambles. Black: corresponding, nonlinear increase of population response (n=14 dopamine neurons) in the same animal. A and B reproduced from ref 25: Stauffer WR, Lak A, Schultz W. Dopamine reward prediction error responses reflect marginal utility. Curr Biol. 2014;24:2491-2500. Copyright © Cell Press 2014.