Abstract

Selective attention, the ability to focus our cognitive resources on information relevant to our goals, influences working memory (WM) performance. Indeed, attention and working memory are increasingly viewed as overlapping constructs. Here, we review recent evidence from human neurophysiological studies demonstrating that top-down modulation serves as a common neural mechanism underlying these two cognitive operations. The core features include activity modulation in stimulus-selective sensory cortices with concurrent engagement of prefrontal and parietal control regions that function as sources of top-down signals. Notably, topdown modulation is engaged during both stimulus-present and stimulus-absent stages of WM tasks; that is, expectation of an ensuing stimulus to be remembered, selection and encoding of stimuli, maintenance of relevant information in mind and memory retrieval. Top-down modulation in perception and working memory Selective attention and working memory (WM) have traditionally been viewed as distinct cognitive domains (see Glossary). However, a growing number of psychological and neuroscientific studies have revealed extensive overlap between these two constructs, spurring a number of excellent reviews and theoretical discussions on this topic Top-down modulation underlies our ability to focus attention on task-relevant stimuli and ignore irrelevant distractions. Investigations using an array of methodologies (see Glossary) have offered complementary contributions showing that top-down signals generate neural contrast by enhancing activity in sensory regions for items that are relevant and suppressing activity for items irrelevant to task goals. For example, in visual areas, excitability changes are reflected in competition for representations in receptive fields of individual neurons, baseline firing rates of neurons and synchronization of neuronal ensembles In a typical visual WM task, participants are presented with an array of one or more items to be maintained in mind after the array is turned off over an interval of seconds (delay period) during which no stimulus information is present ('delayed-response' tasks). A single probe item or a probe array then appears, and the participant Glossary Selective attention: goal-directed focus on one aspect of the environment, while ignoring irrelevant aspects. Working memory: maintenance and/or manipulation of task-relevant information in mind for brief periods of time to guide subsequent behavior. Delay-response tasks: a commonly used cognitive paradigm to study working memory. It is particularly valuable for use in neural studies, because the stages of working memory can be dissociated in time: expectation, encoding, maintenance and retrieval. Types of delayed-response tasks include, delayed match to sample (DMTS), delayed non-match to sample (DNMTS), delayedrecognition tasks and the change-detection task. Top-down modulation: modulation of neural activity in neurons in lower-order sensory or motor areas based on an individual's goals. This may involve enhancement of task-relevant representations and/or suppression for taskirrelevant representations. Functional connectivity: an analytical approach used to reveal how brain regions interact as nodes within neural networks and how their interactions change according to experimental variables; most frequently assessed as correlations between distributed neurophysiological measures in the time or time-frequency domain (coherence). Functional MRI (fMRI): a whole-brain imaging technique that involves recording blood flow correlates of changes in neural activity during a task, often using the blood oxygen level-dependent (BOLD) signal. Spatial resolution is a strength (the order of millimeters), whereas temporal resolution is constrained by hemodynamic variables (the order of seconds) and is therefore a limitation. Electroencephalography (EEG) and magnetoencephalography (MEG): noninvasive physiological recordings that sample ongoing neural electrical activity through sensors on the scalp. The advantage is high temporal resolution (on the order of milliseconds), but spatial resolution is limited due to spatial and temporal summation of neural activity within the brain volume. Data are most frequently analyzed as event-related potentials or magnetic (ERPs), which rely on averaging signals time-locked to stimulus processing in the time-domain, and spectral measures in the time-frequency domain. Transcranial magnetic stimulation (TMS): non-invasive brain stimulation method that uses a rapidly changing magnetic field to induce an electrical current in underlying cortex. The unique strength is enabling functional assessment of brain regions and networks in a causal manner; a weakness is that accessibility to some cortical structures is limited.