Figure 2. Optogenetic stimulation of DA neurons mimics RPE and drives reward learning.

A. Behavioral protocol. Rats were trained to associate an auditory stimulus (cue A) with sucrose. Once this association was learned, as attested by stable levels of conditioned approach to the sucrose delivery port, a visual stimulus (cue X) was added to the auditory stimulus and this compound stimulus (cue AX) was paired with the sucrose reward. During compound stimulus conditioning sessions, DA neurons were photoactivated during reward consumption (Paired Stim.) to artificially create a normally-absent RPE, the sucrose reward being perfectly predicted at this stage by cue A. Control (Unpaired Stim.) rats received DA neuron activation of DA neurons during the intertrial interval. At test, conditioned responding to stimulus X alone was tested in absence of sucrose or optical stimulation. B. Optical stimulation of DA neurons during a compound cue trial. Optical stimulation of DA neurons was synchronized with the delivery of the anticipated reward in Paired Stimulation rats; Unpaired Stimulation rats received optical stimulation of DA neurons at a variable time after cue and reward delivery. C. In rats that received previous stimulation of DA neurons during reward, the presentation of cue X elicited approach to the location of previous sucrose delivery. In contrast rats that received DA neuron manipulation unpaired with reward showed low, or blocked, responding to cue X (Steinberg et al., 2013).