Trends in Cognitive Sciences
ReviewOscillatory Control over Representational States in Working Memory
Section snippets
Selective Control over Working Memory-Guided Attention
Everyday life overloads us with sensory information, forcing us to ignore distractions and selectively focus on what is relevant to our current goal. Attention is the collection of mechanisms that serves this challenging task. Attention can be regarded as an input filter [1], which, through the preactivation of sensory representations relevant to our behavioral goal, prioritizes the sensory processing of matching information [2]. This preactivated sensory filter is often referred to as the
Different Representational States in Working Memory
The recent use of multitask sequences has brought important new insights for our understanding of working memory. Specifically, studies have investigated how, besides serving our current perceptual goals, working memory also allows us to plan ahead and maintain representations for future goals across a series of tasks. To prevent interference, such prospective representations should be shielded from interacting with the current sensory input. Indeed, evidence indicates that while stimuli
Control over Perception through Posterior Alpha (∼8–14 Hz) Oscillations
A large body of work points to a crucial role for alpha-band oscillations in modulating visual processing in the posterior brain. Perhaps counterintuitive, a reduction in the amplitude of alpha oscillations, also termed alpha suppression or event-related desynchronization, goes hand in hand with an increase in neural activity in that region [16,17], suggesting that alpha-band oscillations functionally reflect inhibition. Thus, alpha suppression reflects an increase in the excitability (or
Control over Mnemonic Representations through Posterior Alpha Oscillations
Importantly, modulations of alpha-band activity not only support the prioritization of incoming sensory information, but also of mnemonic representations [31]. In a number of studies, observers were instructed to memorize items presented on each side of fixation and only during maintenance were informed, by means of a post hoc cue, which of the items would be eligible for report. This resulted in alpha being more suppressed contralateral to the cued items [32, 33, 34, 35], more enhanced
Spatiotemporal Specificity of Alpha Oscillations in Working Memory
One important outstanding question is whether there is any functional significance to the spatial source of alpha-band activity and whether alpha can also be used to track the representational state of nonspatial information in working memory. Most research on posterior alpha oscillations in working memory has so far presented items at distinct locations, such that each modulation in alpha power at a specific location can be assigned to a specific item, usually by separating items by hemifield
Executive Control through Frontal Low-Frequency (∼2–8 Hz) Oscillations
While the above sections illustrate the importance of alpha oscillations in sensory regions for the prioritization of task-relevant memories during multitask sequences, it is unlikely that they carry the task goals themselves, and the planning thereof (i.e., ‘I should find a red circle first and a blue circle second’). These types of higher-order cognitive processes are typically associated with the frontal cortex. Indeed, the frontal cortex plays a crucial role in the planning of goal-directed
Involvement of Other Frequency Bands
The theta/delta–alpha interactions are probably not the only oscillatory basis of cognitive control over representational states. One study has shown how beta power in prefrontal cortex is modulated by delta phase when observers need to remember multiple items in working memory [82]. More recent work in monkeys implicates alpha and beta (10–30 Hz) oscillations in executive control over prefrontal working memory representations, which are in turn encoded in gamma-band activity ([83]; Box 4).
Concluding Remarks
In this article we have reviewed the evidence on how neural oscillations may control the representational state of items maintained in working memory depending on their moment-by-moment task-relevance. Based on the reviewed literature, we postulate that modulations in alpha activity above sensory cortices are the direct result of the top-down control mechanism by which internal attention selects which working memory representation should currently be prioritized (i.e., activated in sensory
Acknowledgments
This work was supported by the European Research Council (ERC) Consolidator grant (ERC-2013-CoG-615423) to C.N.L.O. Furthermore, we would like to thank Joram van Driel for useful discussions.
Glossary
- Attentional template
- an active (also commonly termed prioritized or attended) working memory representation that serves to bias sensory processing towards task-relevant input. It is also commonly referred to as target template, search template, or attentional set. The attentional template in working memory undergoes a flexible functional transformation from abstract goal towards task-specific feature filter that optimizes target detection by enhancing matching sensory input [12,86,87], utilizing
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