The Neural Basis of Reward Comparison

A great deal is known about the neural pathways involved in responding to the rewarding properties of food, water, or drugs of abuse, for example. However, these rewards are not typically experienced in a vacuum. At any given time, an animal can engage in a number of different activities, each of which may lead to a different rewarding outcome. The choice to select one behavior over another serves as evidence that very different rewards must be compared by some common neural substrate. However, there currently is little known about the neural pathways involved in such reward comparison processes. It is the focus of this laboratory to identify the substrate.

Rewards are compared in three ways over time. First, a reward can be compared with a different reward that is available closely in time. This form of reward comparison requires short-term memory processes. We have found that a brainstem relay, in particular the nucleus of the solitary tract, is involved in making this type of short-term memory dependent comparison process and that the effect is reflected in the activity of single taste cells in this nucleus (see figures). Second, a reward can be compared with the "memory" of another reward received 24h earlier. This form of reward comparison relies upon long-term memory and necessitates the involvement of the second gustatory relay, the parabrachial nucleus of the pons. Finally, a reward can be compared with another that is expected in the future. The anticipated reward may be a preferred gustatory stimulus or a drug of abuse, such as morphine or cocaine. This phenomenon seems to rely upon more complex associative processes occurring in the forebrain.

Given that drug addiction often is accompanied by an apparent devaluation of that which is naturally rewarding (e.g., relationships, employment, food...), we hope that these efforts will begin to illuminate the neural substrates by which natural rewards and drugs of abuse are compared.