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Abstract

Neuroimaging and brain damage studies suggest that dorsolateral prefrontal cortex (dlPFC) is associated with cognitive control of episodic recollection. Brain stimulation studies can provide more direct evidence. If dlPFC is causally involved in retrieval, then transcranial direct current stimulation (tDCS) of this brain region should increase recollection accuracy, especially when recollection is difficult and requires high levels of cognitive control. Here, we report the first series of brain stimulation experiments to directly test this hypothesis. In four experiments, we administered tDCS to dlPFC immediately after studying to-be-learned material but just prior to recollection testing, thereby targeting retrieval processes. In Experiment 1, we found that stimulation of dlPFC significantly increased recollection accuracy, relative to a no-stimulation sham condition and to a control, parietal stimulation condition. These dlPFC stimulation effects were behaviorally selective, increasing accuracy only when participants needed to recollect difficult information. In Experiments 2 and 3, we manipulated the difficulty of the stimuli to be remembered, but failed to replicate the effects of stimulation in our whole sample analyses. However, we did find post hoc evidence of tDCS effects in morning participants. In Experiment 4 we attempted to replicate this morning effect, but we were again unable to find effects of dlPFC stimulation on recollection accuracy, although post hoc analyses revealed that early morning participants were in the predicted direction. We also failed to find a stimulation effect in an eyewitness identification paradigm. Taken together, these results indicate that the effect of tDCS over dlPFC on recollection accuracy is not very robust. These results argue against the dominant theory that dlPFC is causally involved in retrieval, or they at least indicate that tDCS is ineffective at increasing dlPFC’s contribution to the retrieval process.

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