Figure 2.

Risk signal in dopamine neurons. (a) Phasic reward value signal reflecting reward prediction (left) and more sustained risk signal during the stimulus–reward interval in a single dopamine neuron. Visual stimuli predicting reward probabilities (i) 0.0, (ii) 0.25, (iii) 0.5, (iv) 0.75 and (v) 1.0 alternated semi-randomly between trials. Both rewarded and unrewarded trials are shown at intermediate probabilities; the longer vertical marks in the rasters indicate delivery of the juice reward. (b) Population histograms of responses shown in (a). Histograms were constructed from every trial in 35–44 neurons per stimulus type (638 total trials at p=0 and 1200–1700 trials for all other probabilities). Both rewarded and unrewarded trials are included at intermediate probabilities. (i) 0.0, (ii) 0.25, (iii) 0.5, (iv) 0.75 and (v) 1.0. (c) Median sustained risk-related activation of dopamine neurons as a function of reward probability. Plots show the sustained activation as inverted U function of reward probability, indicating relationship to risk as opposed to value. Data from different stimulus sets and animals are shown separately. Reprinted with permission from Fiorillo et al. (2003). Copyright © American Association for the Advancement of Science.