USC Brain Project: Hippocampus and Navigation Group

The TAM-WG Model: Rewarding Stimuli

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Theoretical background:

Midbrain dopamine systems are crucially involved in motivational processes underlying the learning and execution of goal-directed behavior. Schultz et al., (1995); Mirenowicz and Schultz, (1996); Apicella et al., (1991) found that dopamine neurons in monkeys are uniformly activated by unpredicted appetitive stimuli such as food and liquid rewards and conditioned reward-predicting stimuli (see Lavoie and Mizumori, 1994; and Wiener, 1993 for reward neurons in rats). Schultz et al. (1995) hypothesized that dopamine neurons may mediate the role of unexpected rewards to bring about learning. For them, the information conveyed by the dopamine neurons can be viewed as an error signal, which sugests that the dopamine signal could be an important substrate for reinforcement learning. In constrast to the responses of dopamine neurons to unexpected rewards, neurons in the striatum and in structures projecting to the striatum, such as the amygdala and orbitofrontal and anterior cingulate cortex, respond to primary rewards in well-established behavioral tasks and are not affected by the uncertainty of reward. This suggests that different aspects of the reward signal are distributed over different neuronal systems in the brain and that the function of reward signals accordingly may vary among these systems: "The neuronal response to reward in striatal, amygdalar, and cortical neurons during well-established behaviors suggests a function in maintaining rather than bringing about learning."

In the present work, reward information is used to learn expectations of future reinforcements.

For more information on reward neurons, please refer to the papers listed in the Hippocampus and Navigation Group homepage.

Representing rewarding stimuli ...

When food is sensed, for example, it is represented by excitation coarsely coded accross a population of neurons. Its presence is also coded as an incentive which will raise the hunger level. The window below depicts the situation in which food is sensed straigh ahead.

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Author: Alex Guazzelli <aguazzel@rana.usc.edu>