Only small amounts of visual information, as determined by the capacity of working memory, can be held in an active and accessible state. Thus, it is important to select and maintain information that is relevant while ignoring irrelevant information. However, the underlying neural mechanism of these processes has yet to be identified. One potential candidate are alpha oscillations (8-14 Hz), which have been shown to inhibit stimulus processing in perceptual tasks. During memory maintenance, alpha power increases with set size suggesting that alpha oscillations are involved either in memory maintenance or in the inhibition of task-irrelevant information to protect relevant information from interference. The need for such a protection should increase with the amount of distracting information, but most previous studies did not show any distractors. Therefore, we directly tested whether alpha oscillations are involved in inhibition of distractors during memory maintenance. Participants memorized the orientation of one or two target lines embedded among irrelevant distractors. Distractors were either strong or weak and were present during the retention interval after which participants reported the orientation of probed targets. Computational modeling showed that performance decreased with increasing set size and stronger distraction. Alpha power in the retention interval generally increased with set size, replicating previous studies. However, here stronger distractors reduced alpha power. This finding is in clear contradistinction to previous suggestions, as alpha power decrease indicates higher neuronal excitability. Thus, our data do not support the suggested role of alpha oscillations in inhibition of distraction in working memory.

Although statistical regularities in the environment often go explicitly unnoticed, traces of implicit learning are evident in our neural activity. Recent perspectives have offered evidence that both pre-stimulus oscillations and peri-stimulus event-related potentials are reliable biomarkers of implicit expectations arising from statistical learning. What remains ambiguous, however, is the origination and development of these implicit expectations. To address this lack of knowledge and determine the temporal constraints of expectation formation, pre-stimulus increases in alpha/beta power were investigated alongside a reduction in the N170 and a suppression in peri-/post-stimulus gamma power. Electroencephalography was acquired from naive participants who engaged in a gender classification task. Participants were uninformed, that eight face images were sorted into four reoccurring pairs which were pseudorandomly hidden amongst randomly occurring face images. We found a reduced N170 for statistically expected images at left parietal and temporo-parietal electrodes. Furthermore, enhanced gamma power following the presentation of random images emphasized the bottom-up processing of these arbitrary occurrences. In contrast, enhanced alpha/beta power was evident pre-stimulus for expected relative to random faces. A particularly interesting finding was the early onset of alpha/beta power enhancement which peaked immediately after the depiction of the predictive face. Hence, our findings propose an approximate timeframe throughout which consistent traces of enhanced alpha/beta power illustrate the early prioritisation of top-down processes to facilitate the development of implicitly cued face-related expectations.

Working memory is inherently limited, which makes it important to select and maintain only task-relevant information and to protect it from distraction. Previous research has suggested the contralateral delay activity (CDA) and lateralized alpha oscillations as neural candidates for such a prioritization process. While most of this work focused on distraction during encoding, we examined the effect of external distraction presented during memory maintenance. Participants memorized the orientations of three lateralized objects. After an initial distraction-free maintenance interval, distractors appeared in the same location as the targets or in the opposite hemifield. This distraction was followed by another distraction-free interval. Our results show that CDA amplitudes were stronger in the interval before compared with the interval after the distraction (i.e., CDA amplitudes were stronger in response to targets compared with distractors). This amplitude reduction in response to distractors was more pronounced in participants with higher memory accuracy, indicating prioritization and maintenance of relevant over irrelevant information. In contrast, alpha lateralization did not change from the interval before distraction compared with the interval after distraction, and we found no correlation between alpha lateralization and memory accuracy. These results suggest that alpha lateralization plays no direct role in either selective maintenance of task-relevant information or inhibition of distractors. Instead, alpha lateralization reflects the current allocation of spatial attention to the most salient information regardless of task-relevance. In contrast, CDA indicates flexible allocation of working memory resources depending on task-relevance.

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