Computational models

have demonstrated the feasibility of this corticostriatal output-gating find more architecture for solving hierarchical tasks 18, 22•• and 42••, and at least one such model has been supported by data from fMRI [42••]. Moreover, human diffusion tractography confirms a prediction motivated by this model — namely, that any given area of striatum is more likely to also receive projections from frontal areas more rostral, rather than caudal, to its primary input source [47]. Though a variety of computational modeling thus indicates that corticostriatal circuits can support output gating, empirical studies have only begun to test the function of this hypothesized system. We PD0325901 recently confirmed the differential importance of output gating in hierarchical control [48••]. Our task used three sequentially presented and completely reorderable stimuli: two ‘item’ stimuli and a ‘context’ stimulus that specified which of the two items would be relevant for responses.

The core logic was straightforward: when the context appears first, it can be used to drive selective input gating of only the relevant subsequent item into working memory; however, when context appeared last, it could only be used for selectively output gating the relevant item out of all those seen. All trials showed sustained recruitment of a relatively caudal sector of frontal Dichloromethane dehalogenase cortex (the dorsal premotor cortex, or PMd), but a somewhat more rostral area (the pre-PMd) transiently increased its recruitment specifically when context was provided last, and was therefore implicated output gating ( Figure 3a). An overlapping region of the pre-PMd also increased its coupling with the BG in the same conditions ( Figure 3b). These two dynamics in pre-PMd

each predicted a distinct kind of individual difference during selective output gating alone: whereas bilateral prePMd recruitment predicted the mean efficiency of responses during selective output gating, its bilateral coupling with BG predicted response variability, as expected of a stochastic BG-mediated output gate. The rapidly developing literature on working memory input and output control has been strongly guided by the numerous models to posit that BG-mediated gating processes may address these problems. Unfortunately, computational models differ widely in how they treat a third kind of control problem. How is working memory reallocated when already-stored information is later revealed to be irrelevant? By some accounts, an active removal process is necessary; by others, passive decay could be sufficient [49]. Finally, a third class of models posit that irrelevant representations will tend to linger until (or unless) they are overwritten with new information, such as by input gating mechanisms 6, 10, 15 and 23••.