Transmodal coding for reward prediction in the audiovisual thalamus

Unconsciously or consciously, we can extract biological significance from among the incoming environmental stimuli. However, it remains unresolved where and how, in the brain, the sensory physical information meets behaviorally relevant information, for example, motivational information. We recorded single neuron activity in the posterior thalamic region while rats performed an associative task with a delay imposed between auditory or visual stimuli and reward. We found that nonprimary thalamic neurons exhibited reward-related responses with two temporal response patterns, early component (EC) and late component (LC). EC (a phasic response) occurred shortly after the onset of the stimuli and depended on sensory modality. This magnitude resisted extinction and was correlated with the learning experience. LC gradually increased during the cue and delay periods, and peaked just prior to reward delivery. LC was independent of sensory modality and modulated by the value and timing of the reward. These observations suggest that even the sensory thalamus is involved in translating single-modality sensory information into transmodal rewarding information beyond a simple sensory relay function. Furthermore, in terms of contemporary reinforcement learning theories, EC probably represents the mnemonic process embedded in the "sensory state", whereas LC represents the predictive process corresponding to "value function". (C) 2003 Published by Elsevier B.V.