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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, top-down modulation is engaged during both stimulus-present and stimulus-absent stages of WM tasks, i.e., expectation of an ensuing stimulus to be remembered, selection and encoding of stimuli, maintenance of relevant information in mind and memory retrieval.

In this study, we assess the impact of normal aging on top-down modulation, a cognitive control mechanism that supports both attention and memory by the suppression and enhancement of sensory processing in accordance with task goals. Using fMRI (functional magnetic resonance imaging), we show that healthy older adults demonstrated a prominent deficit in the suppression of cortical activity associated with task-irrelevant representations, whereas enhancement of task-relevant activity was preserved. Moreover, this suppression-specific attention deficit correlated with impaired working memory performance.

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Aging is associated with performance decrements across multiple cognitive domains. The neural noise hypothesis, a dominant view of the basis of this decline, posits that aging is accompanied by an increase in spontaneous, noisy baseline neural activity. Here we analyze data from two different groups of human subjects: intracranial electrocorticography from 15 participants over a 38 year age range (15-53 years) and scalp EEG data from healthy younger (20 -30 years) and older (60 -70 years) adults to test the neural noise hypothesis from a 1/f noise perspective. Many natural phenomena, including electrophysiology, are characterized by 1/f noise. The defining characteristic of 1/f is that the power of the signal frequency content decreases rapidly as a function of the frequency ( f) itself. The slope of this decay, the noise exponent (), is often ϽϪ1 for electrophysiological data and has been shown to approach white noise (defined as ϭ 0) with increasing task difficulty. We observed, in both electrophysiological datasets, that aging is associated with a flatter (more noisy) 1/f power spectral density, even at rest, and that visual cortical 1/f noise statistically mediates age-related impairments in visual working memory. These results provide electrophysiological support for the neural noise hypothesis of aging.

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In this study, electroencephalography (EEG) was used to examine the relationship between two leading hypotheses of cognitive aging, the inhibitory deficit and the processing speed hypothesis. We show that older adults exhibit a selective deficit in suppressing task-irrelevant information during visual working memory encoding, but only in the early stages of visual processing. Thus, the employment of suppressive mechanisms are not abolished with aging but rather delayed in time, revealing a decline in processing speed that is selective for the inhibition of irrelevant information. EEG spectral analysis of signals from frontal regions suggests that this results from excessive attention to distracting information early in the time course of viewing irrelevant stimuli. Subdividing the older population based on working memory performance revealed that impaired suppression of distracting information early in the visual processing stream is associated with poorer memory of task-relevant information. Thus, these data reconcile two cognitive aging hypotheses by revealing that an interaction of deficits in inhibition and processing speed contributes to agerelated cognitive impairment.aging ͉ working memory ͉ inhibitory deficit ͉ distraction ͉ attention C ognitive impairment associated with normal aging impacts multiple domains [e.g., attention, working memory (WM) and episodic memory (1)], prompting a search for underlying neural mechanisms that might account for such widespread deficits. Two of the leading cognitive aging hypotheses are the ''processing speed hypothesis,'' in which performance deficits are attributed to a decline in processing speed (2), and the ''inhibitory deficit hypothesis,'' which proposes that impairment in diverse cognitive abilities are the result of an inability to reduce interference from task-irrelevant information (3). Despite widespread behavioral evidence, physiological data characterizing the neural underpinnings of these age-related deficits, and notably the interactions between them, are limited.A recent functional magnetic resonance imaging (fMRI) study supports the presence of an age-related top-down modulation deficit in inhibitory control (4). Top-down modulation is the neural process that underlies our ability to focus on relevant information and ignore irrelevant distractions via both the enhancement and suppression of sensory cortical activity (5, 6). The fMRI data revealed that, although older adults were able to enhance visual cortical activity for relevant information to the same extent as younger individuals, they were unable to adequately suppress activity associated with irrelevant information, and this suppression deficit correlated with their impaired WM performance (4).The current study is directed at exploring the relationship between the inhibitory deficit and processing speed hypothesis, a goal that necessitates obtaining high temporal resolution neural data to dissect the time-course of age-related processing changes. Because of the vascular nature of the fMRI blood o...

Selective attention filters information to limit what is encoded and maintained in working memory. Although the prefrontal cortex (PFC) is central to both selective attention and working memory, the underlying neural processes that link these cognitive abilities remain elusive. Using functional magnetic resonance imaging to guide repetitive transcranial magnetic stimulation with electroencephalographic recordings in humans, we perturbed PFC function at the inferior frontal junction prior to participants performing a selective-attention, delayed-recognition task. This resulted in diminished top-down modulation of activity in posterior cortex during early encoding stages, which predicted a subsequent decrement in working memory accuracy. Participants with stronger fronto-posterior functional connectivity displayed greater disruptive effects. Data further suggested that broad alpha band (7–14 Hz) phase coherence subserved this long distance top-down modulation. The results establish top-down modulation mediated by the prefrontal cortex as a causal link between early attentional processes and subsequent memory performance.