In other words, was the presence of cued trials necessary for the deactivations observed
during uncued reward trials? To achieve this, two different monkeys (M22 and M23), who were naive with respect to the stimuli used, performed a variant of experiment 1 that consisted solely of fixation and uncued reward trials—hence, BKM120 clinical trial without cued trials. Within this paradigm, uncued reward activity, as monitored by a ROI analysis within the cue-representation (measured during an independent localizer scan), showed no significant reduction in activity (Figures 4 and S3). These results suggest that the deactivations observed during uncued reward trials in experiment 1 require the presence of randomly intermixed cue-reward trials. We hypothesized that by manipulating PE during uncued reward through changes in reward size, we could alter the strength of the reward modulations in visual cortex. Importantly, the use of different reward
sizes allowed us to examine the dependence of reward modulation on PE in the absence of visual stimulation without the need to compare rewarded trials to unrewarded ones (e.g., uncued reward versus fixation). Hence, we could also rule out the possibility that the perception of “reward omission” during unrewarded trial types (fixation and cued trials) see more accounted for the activity modulations observed. To test the effect of reward size on reward modulations in experiment 3, we replaced the single reward level (0.2 ml) used in experiment 1 with large (0.3 ml) and small (0.1 ml) reward. Consistent with electrophysiological studies (Tobler et al., 2005), reward-responsive regions in the ventral midbrain, presumably corresponding to the VTA, displayed either stronger responses for larger unpredicted reward (Figures 5A and 5D). The fMRI responses within
the cue representation also showed stronger deactivations associated with larger uncued reward (Figures 5A and 5D). These differences cannot be explained by visual stimulation, as no visual cues were presented during either trial type. Furthermore, a reward omission signal cannot account for this effect as both trial types were rewarded. In addition, we observed substantial colocalization between voxels more strongly deactivated by larger uncued reward and voxels representing the cue (Figures 5B and 5E). We quantified the dependency of the effect of reward size (large versus small uncued reward) upon cue localizer activity by calculating the voxel-by-voxel correlation between the beta values of these two signals. We found a significant correlation between the two (Figure S4), confirming that the strongest deactivations evoked by administering the larger uncued reward were most prevalent within those voxels best driven by the visual cue.
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