Subjective neuronal coding of reward: temporal value discounting and risk

A key question in the neurobiology of reward relates to the nature of coding. Rewards are objects that are advantageous or necessary for the survival of individuals in a variety of environmental situations. Thus reward appears to depend on the individual and its environment. The question arises whether neuronal systems in humans and monkeys code reward in subjective terms, objective terms or both. The present review addresses this issue by dealing with two important reward processes, namely the individual discounting of reward value across temporal delays, and the processing of information about risky rewards that depends on individual risk attitudes. The subjective value of rewards decreases with the temporal distance to the reward. In experiments using neurophysiology and brain imaging, dopamine neurons and striatal systems discount reward value across temporal delays of a few seconds, despite unchanged objective reward value, suggesting subjective value coding. The subjective values of risky outcomes depend on the risk attitude of individual decision makers; these values decrease for risk-avoiders and increase for risk-seekers. The signal for risk and the signal for the value of risky reward covary with individual risk attitudes in regions of the human prefrontal cortex, suggesting subjective rather than objective coding of risk and risky value. These data demonstrate that important parameters of reward are coded in a subjective manner in key reward structures of the brain. However, these data do not rule out that other neurons or brain structures may code reward according to its objective value and risk.