Figure 9.
Top, Sustained activation of dopamine cells (percentage above baseline) is an inverted-U-shaped function of reward probability, qualitatively similar under moderate to large reward magnitudes, if and only if MSPN activation threshold for GABA and substance-P release from striatonigral terminals is a function of absolute reward magnitude. Plots are generated by assuming an arbitrary threshold at the time of predictive stimulus onset, proportional to the CS-induced DA signal. Bottom, Sustained activation of dopamine cells is an inverted-U-shaped function of expected reward value R̂ = p(R*|CS)|R*| (right) but not of conditional probability of reward p(R*|CS) (left), in contrast to Fiorillo et al. (2003), if MSPN activation threshold for GABA and substance-P release from striatonigral terminals is constant, ΓM = c in Equation 12.