Abstract
Persistently active neurons during mnemonic periods have been regarded as the mechanism underlying working memory maintenance. Alternatively, neuronal networks could instead store memories in fast synaptic changes, thus avoiding the biological cost of maintaining an active code through persistent neuronal firing. Such "activity-silent" codes have been proposed for specific conditions in which memories are maintained in a nonprioritized state, as for unattended but still relevant short-term memories. A hallmark of this "activity-silent" code is that these memories can be reactivated from silent, synaptic traces. Evidence for "activity-silent" working memory storage has come from human electroencephalography (EEG), in particular from the emergence of decodability (EEG reactivations) induced by visual impulses (termed pinging) during otherwise "silent" periods. Here, we reanalyze EEG data from such pinging studies. We find that the originally reported absence of memory decoding reflects weak statistical power, as decoding is possible based on more powered analyses or reanalysis using alpha power instead of raw voltage. This reveals that visual pinging EEG "reactivations" occur in the presence of an electrically active, not silent, code for unattended memories in these data. This crucial change in the evidence provided by this dataset prompts a reinterpretation of the mechanisms of EEG reactivations. We provide 2 possible explanations backed by computational models, and we discuss the relationship with TMS-induced EEG reactivations.
Highlights
A hallmark of the activity-silent working memory framework [1] is that memories stored silently in synaptic traces through short-term synaptic plasticity can be reactivated through nonspecific stimuli [1,2,3,4,5]
Evidence supporting activity-silent working memory has recently emerged from human electroencephalography (EEG) [6,7], in particular from EEG reactivations of unattended memories induced by visual impulses [7]—the so-called visual pinging
If there is an active EEG code for both attended and unattended stimuli prior to the visual impulse, as our analyses suggest, what is the interpretation of the observed increase in EEG decodability [7,18]? We reasoned that EEG reactivation events may emerge from either an increase in the signal about the stimulus or through a reduction in the across-trial variability (S3C Fig)
Summary
A hallmark of the activity-silent working memory framework [1] is that memories stored silently in synaptic traces through short-term synaptic plasticity can be reactivated through nonspecific stimuli [1,2,3,4,5]. Evidence supporting activity-silent working memory has recently emerged from human electroencephalography (EEG) [6,7], in particular from EEG reactivations of unattended memories induced by visual impulses [7]—the so-called visual pinging. Not silent, working memories publications (Wolff et al, 2015; Wolff et al, 2017; Rose et al 2016)
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