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

Abstract

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 (8-13 Hz) and low-beta (13-20 Hz) frequency bands. Analyses of the raw EEG signal showed two effects selective to the epoch containing the UMI: the retrocue and spTMS each produced phase shifts in the low-beta band. 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. Significance StatementThis study provides key insights into how the brain maintains information at different levels of priority in working memory. Importantly, it shows how deprioritizing information in working memory is different from simply "dropping" it. By combining transcranial magnetic stimulation with high temporal resolution measurement of neural signals (with EEG), we replicate previous findings that spTMS triggers the involuntary retrieval of previously unprioritized information and provide new insight into how it does this. Specifically, at the level of EEG dynamics, spTMS is shown to uniquely alter oscillatory dynamics in the low-beta band.