Retrospective and prospective coding for predicted reward in the sensory thalamus

Reward is important for shaping goal-directed behaviour(1-4). After stimulu s-reward associative learning, an organism can assess the motivational valu e of the incoming stimuli on the basis of past experience (retrospective pr ocessing), and predict forthcoming rewarding events (prospective processing )(1-5). The traditional role of the sensory thalamus is to relay current se nsory information to cortex. Here we rnd that non-primary thalamic neurons respond to reward-related events in two ways. The early, phasic responses o ccurred shortly after the onset of the stimuli and depended on the sensory modality. Their magnitudes resisted extinction and correlated with the lear ning experience. The late responses gradually increased during the cue and delay periods, and peaked just before delivery of the reward. These respons es were independent of sensory modality and were modulated by the value and timing of the reward. These observations provide new evidence that single thalamic neurons can code for the acquired significance of sensory stimuli in the early responses (retrospective coding) and predict upcoming reward v alue in the late responses (prospective coding).