The influence of memory load upon delay-interval activity in a working-memory task: an event-related functional MRI study

Working Memory: Imaging the Magic Number Four

Many brain regions have been implicated in memory performance, but the relationship between memory capacity and neural activity has not been clear. Recent studies show that activity in the posterior parietal cortex increases with working memory load, implicating this region in the storage of representations in visual memory.

Dynamic and strategic aspects of executive processing

Executive cognitive functions have been postulated to include both dynamic behavioral selection and strategic goal-setting or response preparation. To investigate the relation between these aspects of executive processing, we embedded an event-related oddball paradigm within a blocked design. Subjects responded to infrequent targets presented within a series of standard stimuli that required no response; this task alternated with a visually similar nontask condition. Using functional magnetic resonance imaging (fMRI), we found that a set of brain regions including dorsolateral prefrontal cortex (dlPFC), insular cortex, cingular cortex, and the basal ganglia demonstrated transient activation both to target stimuli and to the onset of task blocks. Within the parietal cortex, there was a dissociation such that the supramarginal gyrus exhibited greater activity to the target stimuli than to block onsets, while the converse pattern was observed in the intraparietal sulcus. Sustained positive activity during task blocks was present in the caudate and supplementary motor area, while sustained negative activity was present in the precuneus and medial parietal cortex. We conclude that dlPFC and related brain regions mediate both dynamic and strategic processing, through the preparation and selection of rules for behavior.

Interaction between the Amygdala and the Medial Temporal Lobe Memory System Predicts Better Memory for Emotional Events

Emotional events are remembered better than neutral events possibly because the amygdala enhances the function of medial temporal lobe (MTL) memory system (modulation hypothesis). Although this hypothesis has been supported by much animal research, evidence from humans has been scarce and indirect. We investigated this issue using event-related fMRI during encoding of emotional and neutral pictures. Memory performance after scanning showed a retention advantage for emotional pictures. Successful encoding activity in the amygdala and MTL memory structures was greater and more strongly correlated for emotional than for neutral pictures. Moreover, a double dissociation was found along the longitudinal axis of the MTL memory system: activity in anterior regions predicted memory for emotional items, whereas activity in posterior regions predicted memory for neutral items. These results provide direct evidence for the modulation hypothesis in humans and reveal a functional specialization within the MTL regarding the effects of emotion on memory formation.

Emotion–attention network interactions during a visual oddball task

Emotional and attentional functions are known to be distributed along ventral and dorsal networks in the brain, respectively. However, the interactions between these systems remain to be specified. The present study used event-related functional magnetic resonance imaging (fMRI) to investigate how attentional focus can modulate the neural activity elicited by scenes that vary in emotional content. In a visual oddball task, aversive and neutral scenes were presented intermittently among circles and squares. The squares were frequent standard events, whereas the other novel stimulus categories occurred rarely. One experimental group [N=10] was instructed to count the circles, whereas another group [N=12] counted the emotional scenes. A main effect of emotion was found in the amygdala (AMG) and ventral frontotemporal cortices. In these regions, activation was significantly greater for emotional than neutral stimuli but was invariant to attentional focus. A main effect of attentional focus was found in dorsal frontoparietal cortices, whose activity signaled task-relevant target events irrespective of emotional content. The only brain region that was sensitive to both emotion and attentional focus was the anterior cingulate gyrus (ACG). When circles were task-relevant, the ACG responded equally to circle targets and distracting emotional scenes. The ACG response to emotional scenes increased when they were task-relevant, and the response to circles concomitantly decreased. These findings support and extend prominent network theories of emotion-attention interactions that highlight the integrative role played by the anterior cingulate.

Functional parcellation of attentional control regions of the brain

Recently, a number of investigators have examined the neural loci of psychological processes enabling the control of visual spatial attention using cued-attention paradigms in combination with event-related functional magnetic resonance imaging. Findings from these studies have provided strong evidence for the involvement of a fronto-parietal network in attentional control. In the present study, we build upon this previous work to further investigate these attentional control systems. In particular, we employed additional controls for nonattentional sensory and interpretative aspects of cue processing to determine whether distinct regions in the fronto-parietal network are involved in different aspects of cue processing, such as cue-symbol interpretation and attentional orienting. In addition, we used shorter cue-target intervals that were closer to those used in the behavioral and event-related potential cueing literatures. Twenty participants performed a cued spatial attention task while brain activity was recorded with functional magnetic resonance imaging. We found functional specialization for different aspects of cue processing in the lateral and medial subregions of the frontal and parietal cortex. In particular, the medial subregions were more specific to the orienting of visual spatial attention, while the lateral subregions were associated with more general aspects of cue processing, such as cue-symbol interpretation. Additional cue-related effects included differential activations in midline frontal regions and pretarget enhancements in the thalamus and early visual cortical areas.

The prefrontal cortex and working memory: physiology and brain imaging

Sustained activity has been recorded in the prefrontal cortex during working memory tasks. First, we compare the anatomical distribution of this activity in humans and monkeys. Then, we show that it reflects many factors, maintenance of the items presented, preparation for the response, transformation of the items during the delay, task rules and task goals. Finally, we point out that sustained activity has also been recorded in other areas, such as the parietal cortex. We suggest that the key to prefrontal cortex lies not in the maintenance of sensory information but in the prospective use of that information for behaviour.

Orienting Attention to Locations in Perceptual Versus Mental Representations

Extensive clinical and imaging research has characterized the neural networks mediating the adaptive distribution of spatial attention. In everyday behavior, the distribution of attention is guided not only by extrapersonal targets but also by mental representations of their spatial layout. We used eventrelated functional magnetic resonance imaging to identify the neural system involved in directing attention to locations in arrays held as mental representations, and to compare it with the system for directing spatial attention to locations in the external world. We found that these two crucial aspects of spatial cognition are subserved by extensively overlapping networks. However, we also found that a region of right parietal cortex selectively participated in orienting attention to the extrapersonal space, whereas several frontal lobe regions selectively participated in orienting attention within on-line mental representations.

Identifying regional activity associated with temporally separated components of working memory using event-related functional MRI

This study describes the neural circuitry underlying temporally separated components of working memory (WM) performance-stimulus encoding, maintenance of information during a delay, and the response to a probe. While other studies have applied event-related fMRI to separate epochs of WM tasks, this study differs in that it employs a methodology that does not make any a priori assumptions about the shape of the hemodynamic response (HDR). This is important because no one model of the HDR is valid across the range of activated brain regions and stimulus types. Systematic modeling inaccuracies may lead to the misattribution of activity to adjacent events. Twelve healthy subjects performed a numerical version of the Sternberg Item Recognition Paradigm adapted for rapid presentation event-related fMRI. This paradigm emphasized maintenance rather than manipulative WM processes and used a subcapacity WM load. WM trials with different delay lengths were compared to fixation. The HDR of the entire WM trial for each trial type was estimated using a finite impulse response (FIR). Regional activity associated with the Encode, Delay, and Probe epochs was identified using contrasts that were based on the FIR estimates and by examining the HDRs. Each epoch was associated with a distinct but overlapping pattern of regional activity. Activation of the dorsolateral prefrontal cortex, thalamus, and basal ganglia was exclusively associated with the probe. This suggests that frontostriatal neural circuitry participates in selecting an appropriate response based on the contents of WM.

Frontal activations associated with accessing and evaluating information in working memory: an fMRI study

To investigate the involvement of frontal cortex in accessing and evaluating information in working memory, we used a variant of a Sternberg paradigm and compared brain activations between positive and negative responses (known to differentially tax access/evaluation processes). Participants remembered two trigrams in each trial and were then cued to discard one of them and maintain the other one as the target set. After a delay, a probe letter was presented and participants made decisions about whether or not it was in the target set. Several frontal areas--anterior cingulate (BA32), middle frontal gyrus (bilateral BA9, right BA10, and right BA46), and left inferior frontal gyrus (BA44/45)--showed increased activity when participants made correct negative responses relative to when they made correct positive responses. No areas activated significantly more for the positive responses than for the negative responses. It is suggested that the multiple frontal areas involved in the test phase of this task may reflect several component processes that underlie more general frontal functions.

Cortical capacity constraints for visual working memory: dissociation of fMRI load effects in a fronto-parietal network

Working memory (WM) capacity limitations and their neurophysiological correlates are of special relevance for the understanding of higher cognitive functions. Evidence from behavioral studies suggests that restricted attentional resources contribute to these capacity limitations. In an event-related functional magnetic resonance imaging (fMRI) study, we probed the capacity of the human visual WM system for up to four complex nonnatural objects using a delayed discrimination task. A number of prefrontal and parietal areas bilaterally showed increased blood oxygen level-dependent activity, relative to baseline, throughout the task when more than one object had to be held in memory. Monotonic increases in response to memory load were observed bilaterally in the dorsolateral prefrontal cortex (DLPFC) and the presupplementary motor area (pre-SMA). Conversely, activity in the frontal eye fields (FEFs) and in areas along the intraparietal sulcus (IPS) peaked when subjects had to maintain only two or three objects and decreased in the highest load condition. This dissociation of memory load effects on cortical activity suggests that the cognitive operations subserved by the IPS and FEF, which are most likely related to attention, fail to support visual WM when the capacity limit is approached. The correlation of brain activity with performance implies that only the operations performed by the DLPFC and pre-SMA, which support an integrated representation of visual information, helped subjects to maintain a reasonable level of performance in the highest load condition. These results indicate that at least two distinct cortical subsystems are recruited for visual WM, and that their interplay changes when the capacity limit is reached.

Dissecting contributions of prefrontal cortex and fusiform face area to face working memory

Interactions between prefrontal cortex (PFC) and stimulus-specific visual cortical association areas are hypothesized to mediate visual working memory in behaving monkeys. To clarify the roles for homologous regions in humans, event-related fMRI was used to assess neural activity in PFC and fusiform face area (FFA) of subjects performing a delay-recognition task for faces. In both PFC and FFA, activity increased parametrically with memory load during encoding and maintenance of face stimuli, despite quantitative differences in the magnitude of activation. Moreover, timing differences in PFC and FFA activation during memory encoding and retrieval implied a context dependence in the flow of neural information. These results support existing neurophysiological models of visual working memory developed in the nonhuman primate.

Brain activity evoked by the perception of human walking: controlling for meaningful coherent motion

Many functional neuroimaging studies of biological motion have used as stimuli point-light displays of walking figures and compared the resulting activations with those evoked by the same display elements moving in a random or noncoherent manner. Although these studies have established that biological motion activates the superior temporal sulcus (STS), the use of random motion controls has left open the possibility that coordinated and meaningful nonbiological motion might activate these same brain regions and thus call into question their specificity for processing biological motion. Here we used functional magnetic resonance imaging and an anatomical region-of-interest approach to test a hierarchy of three questions regarding activity within the STS. First, by comparing responses in the STS with animations of human and robot walking figures, we determined (1) that the STS is sensitive to biological motion itself, not merely to the superficial characteristics of the stimulus. Then we determined that the STS responds more strongly to biological motion (as conveyed by the walking robot) than to (2) a nonmeaningful but complex nonbiological motion (a disjointed mechanical figure) and (3) a complex and meaningful nonbiological motion (the movements of a grandfather clock). In subsequent whole-brain voxel-based analyses, we confirmed robust STS activity that was strongly right lateralized. In addition, we observed significant deactivations in the STS that differentiated biological and nonbiological motion. These voxel-based analyses also revealed regions of motion-related positive activity in other brain regions, including MT or V5, fusiform gyri, right premotor cortex, and the intraparietal sulci.

Persistent activity in the prefrontal cortex during working memory

The dorsolateral prefrontal cortex (DLPFC) plays a crucial role in working memory. Notably, persistent activity in the DLPFC is often observed during the retention interval of delayed response tasks. The code carried by the persistent activity remains unclear, however. We critically evaluate how well recent findings from functional magnetic resonance imaging studies are compatible with current models of the role of the DLFPC in working memory. These new findings suggest that the DLPFC aids in the maintenance of information by directing attention to internal representations of sensory stimuli and motor plans that are stored in more posterior regions.

Brain activation evoked by perception of gaze shifts: the influence of context

Prior studies from our laboratory [Journal of Neuroscience 18 (1998) 2188; Cognitive Neuropsychology 17 (2000) 221] have demonstrated that discrete regions of the superior temporal sulcus (STS) are activated when a subject views a face in which the eyes shift their gaze. Here we investigated the degree to which activity in the STS and other brain regions is modulated by the context of the perceived gaze shift; that is, when the shift correctly or incorrectly acquires a visual target. Fifteen subjects participated in an event-related functional magnetic resonance imaging experiment in which they viewed an animated face that remained present throughout each run. On each of 21 trials within each run, a small checkerboard appeared and flickered at one of six locations within the character's visual field. On "correct" trials, the character shifted its gaze towards the checkerboard after a delay of 1 or 3s. On "incorrect" trials, the character shifted its gaze towards empty space after the same delays. On "no shift" trials, the character's eyes did not move. Significantly larger hemodynamic responses (HDR) were evoked by gaze shifts compared to no gaze shifts in primarily right hemisphere STS. The gaze-evoked HDR was significantly delayed in peak amplitude for 3s compared to 1s shifts. For 1s shifts, a strong effect of context was observed in which errors evoked a HDR with extended duration. Although this study focused upon STS, similar effects were also observed in the intraparietal sulcus and fusiform gyrus.

Prefrontal activity associated with working memory and episodic long-term memory

Many recent neuroimaging studies have highlighted the role of prefrontal regions in the sustained maintenance and manipulation of information over short delays, or working memory (WM). In addition, neuroimaging findings have highlighted the role of prefrontal regions in the formation and retrieval of memories for events, or episodic long-term memory (LTM), but it remains unclear whether these regions are distinct from those that support WM. We used event-related functional magnetic resonance imaging (fMRI) to identify patterns of prefrontal activity associated with encoding and recognition during WM and LTM tasks performed by the same subjects. Results showed that the same bilateral ventrolateral prefrontal regions (at or near Brodmann's Areas [BA] 6, 44, 45, and 47) and dorsolateral prefrontal regions (BA 9/46) were engaged during encoding and recognition within the context of WM and LTM tasks. In addition, a region situated in the left anterior middle frontal gyrus (BA 10/46) was engaged during the recognition phases of the WM and LTM tasks. These results support the view that the same prefrontal regions implement reflective processes that support both WM and LTM.

Tracking the time-course of attentional involvement in spatial working memory: an event-related potential investigation

Spatial working memory is a cognitive brain mechanism that enables the temporary maintenance and manipulation of spatial information. Recent neuroimaging and behavioral studies have led to the proposal that directed spatial attention is the mechanism by which location information is maintained in spatial working memory. Yet it is unclear whether attentional involvement is required throughout the period of active maintenance or is only invoked during discrete task-phases such as mnemonic encoding. In the current study, we aimed to track the time-course of attentional involvement during spatial working memory by recording event-related brain potentials (ERPs) from healthy volunteers. In Experiment 1, subjects performed a delayed-recognition task. Each trial began with the presentation of a brief stimulus (S1) that indicated the relevant location that subjects were to maintain in working memory. A 4.8-5.3 sec delay interval followed during which a single task-irrelevant probe was presented. The delay interval concluded with a test item (S2) to which subjects made a response indicating whether the S2-location was the same as the S1-memory location. To determine if attention was differentially engaged during discrete phases of the trial, task-irrelevant probes were presented early (400-800 msec following S1-offset) or late (2600-3000 msec following S1-offset) during the delay interval. Sensory-evoked ERPs (P1 and N1) elicited by these irrelevant probes showed attention-like modulations with greater amplitude responses for probes occurring at the S1-memory locations in comparison to probes presented at other locations. This pattern was obtained for both early- and late-delay probes. Probe-evoked activity during delayed-recognition trials was similar to activity observed when spatial attention was explicitly focused on a location in visual space (Experiment 2). These results are consistent with a model of spatial working memory in which perceptual level selective attention is utilized throughout the entire period of active maintenance to keep relevant spatial information in mind.

The role of prefrontal cortex in working-memory capacity, executive attention, and general fluid intelligence: an individual-differences perspective

We provide an "executive-attention" framework for organizing the cognitive neuroscience research on the constructs of working-memory capacity (WMC), general fluid intelligence, and prefrontal cortex (PFC) function. Rather than provide a novel theory of PFC function, we synthesize a wealth of single-cell, brain-imaging, and neuropsychological research through the lens of our theory of normal individual differences in WMC and attention control (Engle, Kane, & Tuholski, 1999; Engle, Tuholski, Laughlin, & Conway, 1999). Our critical review confirms the prevalent view that dorsolateral PFC circuitry is critical to executive-attention functions. Moreover, although the dorsolateral PFC is but one critical structure in a network of anterior and posterior "attention control" areas, it does have a unique executive-attention role in actively maintaining access to stimulus representations and goals in interference-rich contexts. Our review suggests the utility of an executive-attention framework for guiding future research on both PFC function and cognitive control.

Dissociable prefrontal brain systems for attention and emotion

The prefrontal cortex has been implicated in a variety of attentional, executive, and mnemonic mental operations, yet its functional organization is still highly debated. The present study used functional MRI to determine whether attentional and emotional functions are segregated into dissociable prefrontal networks in the human brain. Subjects discriminated infrequent and irregularly presented attentional targets (circles) from frequent standards (squares) while novel distracting scenes, parametrically varied for emotional arousal, were intermittently presented. Targets differentially activated middle frontal gyrus, posterior parietal cortex, and posterior cingulate gyrus. Novel distracters activated inferior frontal gyrus, amygdala, and fusiform gyrus, with significantly stronger activation evoked by the emotional scenes. The anterior cingulate gyrus was the only brain region with equivalent responses to attentional and emotional stimuli. These results show that attentional and emotional functions are segregated into parallel dorsal and ventral streams that extend into prefrontal cortex and are integrated in the anterior cingulate. These findings may have implications for understanding the neural dynamics underlying emotional distractibility on attentional tasks in affective disorders.

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