However, the target choice signals Matsumoto and Takada observed occurred after the monkeys fixated the target but before delivery of the reward,

implying that these, too, encoded an expectation of reward. In fact, the same signals were present in trials where the monkeys made incorrect choices, consistent with the interpretation that they reflected monkeys’ subjective expectations rather than the reward outcome or a prediction error. The authors’ most intriguing finding resulted from an analysis of which neural responses were present in which cells. Although nearly all cells responded to the onset of the reward cue, cells responding to the sample stimulus were found almost entirely in dorsal and lateral regions of the midbrain, probably within the SNc. By contrast, find more cells responsive

to the size of the search array were more concentrated in medial and ventral ZD1839 in vivo regions, and there was a correlation between effect size and recording depth, most likely in the VTA. Such a gradient in function is broadly consistent with known anatomy: the SNc projects primarily to dorsolateral sensorimotor structures, whereas the VTA projects primarily to medial and limbic cortical areas associated with learning and motivation (Haber and Knutson, 2010). These observations endorse the authors’ conclusion that responses to the sample cue facilitate working memory by releasing dopamine in the dorsolateral prefrontal cortex. They are likewise consistent with the observation that factors influencing task difficulty are processed preferentially by systems responsible for calculating motivation and reward anticipation. old In addition to these tantalizing findings, the study also raises a number of important questions. Because the authors used spike waveforms to identify putative dopaminergic cells and recorded

only firing-rate responses, they could not verify the actual amount of dopamine released in response to task events; such verification could be provided by techniques such as voltammetry, which measures catecholamine release with millisecond precision. Furthermore, the difficulty of recording from small brainstem regions limited the number of cells recorded—enough so to suggest a gradient in function, perhaps, but the findings will benefit from replication. Finally, although both the location and timing of cell firing in response to the sample cue are consistent with the hypothesis that subsequent dopamine release facilitates working memory, future studies will need to verify this causally, perhaps by showing that selective activation or inactivation of lateral SNc neurons has an effect on the performance of working memory. What is most exciting about the work by Matsumoto and Takada is the finding that dopamine signaling in the brain is more heterogeneous and computationally specific than commonly thought.