Effects of TMS on the decoding, electrophysiology, and representational geometry of priority in working memory

The flexible control of working memory (WM) requires prioritizing immediately task-relevant information while maintaining information with potential future relevance in a deprioritized state. Using double-serial retrocuing (DSR) with simultaneous EEG recording, we investigated how single pulses of transcranial magnetic stimulation (spTMS) to right intraparietal sulcus impacts neural representations of unprioritized memory items (UMI), relative to irrelevant memory items (IMI) that are no longer needed for the trial. Twelve human participants (8 female) performed DSR plus a single-retrocue task, while spTMS was delivered during delay periods. Multivariate pattern analysis revealed that spTMS restored decodability of the UMI concurrent with stimulation, and that of the IMI several timesteps later, after the evoked effects of spTMS were no longer present in the EEG signal. This effect was carried by the alpha and low-beta frequency bands. Analyses of the EEG showed two effects selective to the UMI: both the retrocue designating the UMI and spTMS produced phase shifts in the low-beta band. These results were mirrored by representational geometry analyses, which revealed larger cue-related and spTMS-related rotations of the UMI-representing subspace, compared to the prioritized subspace spTMS. These findings demonstrate that deprioritization involves active neural mechanisms distinct from the processing of the IMI, and that these are supported by low-beta oscillatory dynamics in parietal cortex. We hypothesize that the mechanism underlying spTMS-triggered involuntary retrieval of the UMI is the disruption of the encoding of priority status, which may depend on oscillatory dynamics in the low-beta band.