Figure 1.
Figure 1A. Corticostriatal inputs (SA, SB) coding for sensory events synapse upon striatal output cells coding for behavioral responses (RA, RB). A behavioral response (RA) leading to delivery of a reward or reward-predicting stimulus causes a phasic increase in midbrain DA activity and a consequent increase in striatal DA release. DA-mediated LTP strengthens the currently-active synapses (SB to RA; bottom diagram, bold box in striatum) and increases the likelihood that the reward-procuring behavioral response (RA) will occur in response to the same sensory stimulus (SB) in the future. If a striatal output cell is activated by convergent inputs from cortical cells representing two sensory modalities, DA activation will increase the synaptic strength of both inputs to the striatal cell. Such a cell will come to show preferential activity in response to the conjunction of the two sensory conditions, e.g., to a visual stimulus only when presented in proximity to a particular tactile field [55].
Figure 1B. Corticostriatal inputs (OA, OB) coding for expected reward outcomes synapse upon striatal output cells coding for behavioral responses (RA, RB). Selective strengthening of O-R synapses occurs in a manner identical to that depicted for S-R synapses in figure 1A.