Theta Transcranial Alternating Current Stimulation Over the Dorsomedial Prefrontal Cortex Does Not Enhance Long-Term Memory.

Long-term memory (LTM) has been associated with neural oscillation in the theta (3-8 Hz) range. Although previous studies have suggested that the dorsomedial prefrontal cortex (dmPFC) is a core region for LTM retrieval, causal evidence is sparse and mixed. Furthermore, the moderating effects of stimulus memorability have not yet been explored. In the present study, we used transcranial alternating current stimulation (tACS) to modulate theta oscillation in the dmPFC during the retrieval of visual images with varying levels of memorability. Specifically, we included n = 33 healthy volunteers who were exposed to 300 images of faces, scenes and items, which they had to memorize. Recognition accuracy was assessed 1 h later. During the retrieval phase, participants received either sham or verum (4 Hz, 2.5 mA) tACS and were asked whether they had seen the pictures before (150 new and 150 old). Contrary to our preregistered hypotheses, we found no significant effect of 4-Hz tACS applied during retrieval on LTM recognition. Furthermore, although the memorability effect was observed, it did not interact with tACS, indicating that stimulation neither improved nor worsened performance on low- and high-memorable images. Altogether, the present study does not support an active role of 4-Hz oscillations in the dmPFC for the recognition of images with varying levels of memorability, under the specific task and stimulation parameters used here. However, this null effect may be specific to the task and particular parameters used in this study.