High Memory Load Research Articles

The temporal locus of the interaction between working memory consolidation and the attentional blink

An increase in concurrent working memory load has been shown to amplify the attentional blink. The present study investigated the temporal locus of this phenomenon, by using a dual rapid serial visual presentation paradigm that enabled the measurement of lateralized event-related potentials. The P3 component was shown to be affected by both working memory load and the lag between the target stimuli, consistent with current models of temporal attention and a functional explanation of the P3 in terms of memory consolidation. P3 amplitude was reduced for short target lags and high memory loads. The P2 component was affected by lag only, and not memory load. Importantly, the N2pc component was modulated also by both lag and memory load. The results showed that early attentional processing (as marked by the N2pc) was suppressed by increased involvement of working memory, a phenomenon not well predicted by many current theories of temporal attention.

Age Differences in the Neural Representation of Working Memory Revealed by Multi-Voxel Pattern Analysis

Working memory function declines across the lifespan. Computational models of aging attribute such memory impairments to reduced distinctiveness between neural representations of different mental states in old age, a phenomenon termed dedifferentiation. These models predict that neural distinctiveness should be reduced uniformly across experimental conditions in older adults. In contrast, the Compensation-Related Utilization of Neural Circuits Hypothesis (CRUNCH) model predicts that the distinctiveness of neural representations should be increased in older adults (relative to young adults) at low levels of task demand but reduced at high levels of demand. The present study used multi-voxel pattern analysis to measure the effects of age and task demands on the distinctiveness of the neural representations of verbal and visuospatial working memory. Neural distinctiveness was estimated separately for memory encoding, maintenance, and retrieval, and for low, medium, and high memory loads. Results from sensory cortex during encoding and retrieval were consistent with the dedifferentiation hypothesis: distinctiveness of visual cortical representations during these phases was uniformly reduced in older adults, irrespective of memory load. However, maintenance-related responses in prefrontal and parietal regions yielded a strikingly different pattern of results. At low loads, older adults showed higher distinctiveness than younger adults; at high loads, this pattern reversed, such that distinctiveness was higher in young adults. This interaction between age group and memory load is at odds with the dedifferentiation hypothesis but consistent with CRUNCH. In sum, our results provide partial support for both dedifferentiation- and compensation-based models; we argue that comprehensive theories of cognitive aging must incorporate aspects of both models to fully explain complex patterns of age-related neuro-cognitive change.

Electrophysiological signature of working and long‐term memory interaction in the human hippocampus

Recent findings indicate that the hippocampus is not only crucial for long-term memory (LTM) encoding, but plays a role for working memory (WM) as well. In particular, it has been shown that the hippocampus is important for WM maintenance of multiple items or associations between item features. Previous studies using intracranial electroencephalography recordings from the hippocampus of patients with epilepsy revealed slow positive potentials during maintenance of a single item and during LTM encoding, but slow negative potentials during maintenance of multiple items. These findings predict that WM maintenance of multiple items interferes with LTM encoding, because these two processes are associated with slow potentials of opposing polarities in the hippocampus. Here, we tested this idea in a dual-task paradigm involving a LTM encoding task nested into a WM Sternberg task with either a low (one item) or a high (three items) memory load. In the high WM load condition, LTM encoding was significantly impoverished, and slow hippocampal potentials were more negative than in the low WM load condition. Time-frequency analysis revealed that a reduction of slow hippocampal activity in the delta frequency range supported LTM formation in the low load condition, but not during high WM load. Together, these findings indicate that multi-item WM and LTM encoding interfere within the hippocampus.

Age differences in prefontal recruitment during verbal working memory maintenance depend on memory load

Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) studies have revealed age-related under-activation, where older adults show less regional brain activation compared to younger adults, as well as age-related over-activation, where older adults show greater activation compared to younger adults. These differences have been found across multiple task domains, including verbal working memory (WM). Curiously, both under-activation and over-activation of dorsolateral prefrontal cortex (DLPFC) have been found for older adults in verbal WM tasks. Here, we use event-related fMRI to test the hypothesis that age-related differences in activation depend on memory load (the number of items that must be maintained). Our predictions about the recruitment of prefrontal executive processes are based on the Compensation-Related Utilization of Neural Circuits Hypothesis (CRUNCH; Reuter-Lorenz and Cappell, 2008). According to this hypothesis, more neural resources are engaged by older brains to accomplish computational goals completed with fewer resources by younger brains. Therefore, seniors are more likely than young adults to show over-activations at lower memory loads, and under-activations at higher memory loads. Consistent with these predictions, in right DLPFC, we observed age-related over-activation with lower memory loads despite equivalent performance accuracy across age groups. In contrast, with the highest memory load, older adults were significantly less accurate and showed less DLPFC activation compared to their younger counterparts. These results are considered in relation to previous reports of activation–performance relations using similar tasks, and are found to support the viability of CRUNCH as an account of age-related compensation and its potential costs.

Load effects in auditory selective attention: Evidence for distinct facilitation and inhibition mechanisms

It is unknown whether facilitation and inhibition of stimulus processing represent one or two mechanisms in auditory attention. We performed electrophysiological experiments in humans to address these two competing hypothesis. Participants performed an attention task under low or high memory load. Facilitation and inhibition were measured by recording electrophysiological responses to attended and ignored sounds and comparing them to responses to these same sounds when attention was considered to be equally distributed towards all sounds. We observed two late frontally distributed components: a negative one in response to attended sounds, and a positive one to ignored sounds. These two frontally distributed responses had distinct timing and scalp topographies and were differentially affected by memory load. Taken together these results provide evidence that attention-mediated top-down control reflects the activity of distinct facilitation and inhibition mechanisms.

High working memory load leads to more Ebbinghaus illusion

The evidence that distractor processing increases with greater load on working memory has come mainly from Stroop-type interference tasks, making it difficult to establish whether cognitive load affects distractor processing at the perceptual level or during response selection. We measured the Ebbinghaus illusion under varying levels of working memory load to test whether cognitive control is also relevant for preventing processing of distractors that do not produce any response conflict, and instead affect target processing at the perceptual level. The Ebbinghaus illusion was greater under high working memory load, suggesting that availability of cognitive control functions is critical to reduce distractor processing even for distractors that are not associated with a response. We conclude that the effect of loading working memory during selective attention leads to greater distractor perception.

Cognitive dissonance induced by writing a counterattitudinal essay facilitates performance on simple tasks but not on complex tasks that involve working memory

This article examines how performance is facilitated with simple tasks and is impaired with complex ones in individuals experiencing dissonance. Experiment 1 measured the performance of dissonance participants at a simple reaction time task. Reaction times were shortest in the dissonance condition. This facilitation effect was interpreted as resulting from increased arousal with dissonance. In Experiment 2, participants performed a more complex secondary memory task that required to memorize and to recall short and long series of numbers. Participants in the dissonance condition performed less well than participants in the no-dissonance condition only under a high memory load and did not differ with a low load. It is suggested that dissonance requires some working memory resources. Accordingly, it is assumed that the arousal properties of dissonance facilitate performance with simple task that do not involve working memory and require a dominant response, but that dissonance impairs performance with tasks that strongly involve working memory.

Binge Drinking Affects Attentional and Visual Working Memory Processing in Young University Students

Binge Drinking (BD) typically involves heavy drinking over a short time, followed by a period of abstinence, and is common among young people, especially university students. Animal studies have demonstrated that this type of alcohol consumption causes brain damage, especially in the nonmature brain. The aim of the present study was to determine how BD affects brain functioning in male and female university students, during the performance of a visual working memory task. Event-related potentials (ERPs) were recorded, with an extensive set of 32 scalp electrodes, in 95 first-year university students (age range 18 to 20 years), comprising 42 binge drinkers (BD) and 53 controls, in a visual "identical pairs" continuous performance task. Principal components analysis was used to identify and analyze the N2 (negative waveform with a latency around 200 to 300 ms related to attentional processes) and P3 (positive waveform with a latency around 300 to 600 ms related to working memory processes) components of the ERPs. In the matching condition of the task, the N2 component in central and parietal regions was significantly larger in the BD than in the control group. In the control group, the P3 component was larger in the matching than in the nonmatching condition in the frontal, central, and parietal regions, whereas the BD group did not show any significant differences between conditions in any region. The results of this study confirm the presence of electrophysiological differences between young university student binge drinkers and controls during the execution of a visual task with a high working memory load. The larger N2 in the BD group suggests higher levels of attentional effort required by this group to perform the task adequately. The absence of any differences in the P3 component in the different conditions (matching and nonmatching stimuli) in the BD group suggests a deficiency in the electrophysiological differentiation between relevant and irrelevant information, which may reflect some impairment of working memory processes.

Executive functioning and imitation: Increasing working memory load facilitates behavioural imitation

If perceptual and bodily states are closely linked and if perceiving action automatically leads to corresponding activations in one's own motor system, then why do not we imitate all the time? There is evidence suggesting that executive functioning (EF) may play a moderating role in inhibiting overt imitation [e.g. Luria, A. R. (1966). Higher cortical functions in man. New York: Basic Books]. In an experiment we tested this idea. 48 participants received either a high or low working memory (WM) load and were instructed to respond to either a finger cue or spatial cue with a finger movement. Results indicate that occupying WM facilitates reaction times to finger cues while responses to simple spatial cues are unchanged. The findings suggest that imitation is indeed a dominant response and EF is needed to inhibit the spontaneous tendency to imitate.

Increasing working memory load attenuates pain in healthy volunteers but not in chronic low back pain patients

Cognitive deficits and decreased ability to maintain high working memory loads have been well documented in chronic pain patients. The interplay between acute pain processing and working memory, and how that interaction may differ between chronic pain patients and healthy controls, are not as well defined. This study aims to characterize the modulatory effect of working memory- a system for temporary storage of information necessary for complex cognitive tasks-on pain processing and vice-versa. We hypothesize that in healthy controls, pain intensity will decrease the accuracy of performance on a working memory task and that increasing working memory loads will attenuate the pain perception of an acute, thermal stimulus.

Phenotypic and genetic correlations between evoked EEG/ERP measures during the response anticipation period of a delayed response task

We investigated the relationship between three electrophysiological indices of response anticipation in a spatial delayed response task with a low and high memory load manipulation: a slow cortical potential (SCP), theta desynchronization, and upper alpha synchronization. Individual differences in these three measures were examined in 531 adult twins and siblings. Heritability of the SCP at occipital-parietal leads varied from 30% to 43%. Heritability of upper alpha synchronization (35% to 65%) and theta desynchronization (31% to 50%) was significant at all leads. Theta desynchronization and upper alpha synchronization were significantly correlated (r approximately 43%), but SCP was not correlated with either. The effect of working memory load on all three measures was not heritable. Response anticipation reliably evokes an SCP, upper alpha synchronization and theta desynchronization, but variation in these measures reflects different (genetic) sources.

The Lateral and Ventromedial Prefrontal Cortex Work as a Dynamic Integrated System: Evidence from fMRI Connectivity Analysis

Recent functional magnetic resonance imaging (fMRI) investigations of the interaction between cognition and reward processing have found that the lateral prefrontal cortex (PFC) areas are preferentially activated to both increasing cognitive demand and reward level. Conversely, ventromedial PFC (VMPFC) areas show decreased activation to the same conditions, indicating a possible reciprocal relationship between cognitive and emotional processing regions. We report an fMRI study of a rewarded working memory task, in which we further explore how the relationship between reward and cognitive processing is mediated. We not only assess the integrity of reciprocal neural connections between the lateral PFC and VMPFC brain regions in different experimental contexts but also test whether additional cortical and subcortical regions influence this relationship. Psychophysiological interaction analyses were used as a measure of functional connectivity in order to characterize the influence of both cognitive and motivational variables on connectivity between the lateral PFC and the VMPFC. Psychophysiological interactions revealed negative functional connectivity between the lateral PFC and the VMPFC in the context of high memory load, and high memory load in tandem with a highly motivating context, but not in the context of reward alone. Physiophysiological interactions further indicated that the dorsal anterior cingulate and the caudate nucleus modulate this pathway. These findings provide evidence for a dynamic interplay between lateral PFC and VMPFC regions and are consistent with an emotional gating role for the VMPFC during cognitively demanding tasks. Our findings also support neuropsychological theories of mood disorders, which have long emphasized a dysfunctional relationship between emotion/motivational and cognitive processes in depression.

Postconcussion syndrome after minor head injury: Brain activation of working memory and attention

After minor head injury (MHI) postconcussive symptoms (PCS) such as memory and attention deficits frequently occur. It has been hypothesised that PCS are caused by microstructural damage to the brain due to shearing injury, which is not detectable with conventional imaging, and may be responsible for a functional deficit. The purpose of this study was to correlate functional magnetic resonance imaging brain activation of working memory and selective attention with PCS. 21 MHI patients and 12 healthy controls were scanned at 3T. Stimulation paradigms were the n-back and Counting Stroop tasks to engage working memory and selective attention, respectively. Functional data analysis consisted of random effects group analyses, correlating brain activation patterns with the severity of PCS as evaluated with the Rivermead postconcussion symptoms questionnaire. At minimal working memory load, activation was seen in patients with greater severity of PCS in the working memory network. With an increase of working memory load, increase of activation was more pronounced in patients with greater severity of PCS. At high and increased working memory load, activation associated with the severity of PCS was seen in the posterior parietal area, parahippocampal gyrus, and posterior cingulate gyrus. Activation related to selective attention processing was increased with greater severity of PCS. The increased activity in relation to working memory and attention, and the recruitment of brain areas outside the working memory network at high working memory load, may be considered a reflection of the brain's compensatory response to microstructural injury in patients with PCS.

Altered frontostriatal coupling in pre‐manifest Huntington’s disease: effects of increasing cognitive load

Functional neuroimaging studies have suggested a dysfunction of prefrontal regions in clinically pre-symptomatic individuals with the Huntington's disease (HD) gene mutation (pre-HD) during cognitive processing. The objective of this study was to test the impact of cognitive demand on prefrontal connectivity in pre-HD individuals. Sixteen healthy controls and sixteen pre-HD subjects were studied using functional MRI and a verbal working memory task with increasing cognitive load. Load-dependent functional connectivity of the left dorsolateral prefrontal cortex (DLPFC) was investigated by means of psychophysiological interactions. In pre-HD subjects, aberrant functional connectivity of the left DLPFC was found at high working memory load levels only. Compared with healthy controls, pre-HD individuals exhibited lower connectivity strength in the left putamen, the right anterior cingulate and the left medial prefrontal cortex. Pre-HD individuals close to the onset of motor symptoms additionally exhibited lower connectivity strength in the right putamen and the left superior frontal cortex. The connectivity strength in the left putamen was associated with several clinical measures including CAG repeat length, Unified Huntington's Disease Rating Scale motor score and predicted years to manifest symptom onset. These findings suggest that early prefrontal connectivity abnormalities in pre-HD individuals are modulated by cognitive demand.

Abstracts

To examine the effects of healthy ageing on the electrophysiological correlates of recognition memory, event-related potentials (ERPs) were recorded from 15 young (mean age = 21.73 years) and 15 healthy older adults (mean age = 66.67 years) while they performed a word recognition task with 4-or 8-word study list conditions. At test, participants were required to make old/new judgements to visually presented words. Although there was a trend for young participants to perform more accurately, especially on the long list, these differences only approached significance. Despite this, the N400 old/new effect was found to be significantly reduced (across both list lengths) in the old compared with the young participants. Only the young participants demonstrated an LPC old/new effect; and this was only in the long list condition at F4, suggesting that the LPC old/new effect may be sensitive to age-related cognitive changes specifically under higher memory load conditions. Overall the results indicate that these components may be more sensitive to the deleterious effects of age on recognition memory than behavioural measures of accuracy. The findings relating to the N400 are particularly interesting as the effects of ageing on the N400 from explicit recognition tasks have not been thoroughly explored.

Theta oscillations are affected by amnestic mild cognitive impairment and cognitive load

Amnestic mild cognitive impairment (aMCI) is classified primarily via substantial episodic memory deficits in the absence of a dementia diagnosis. To investigate the potential neurophysiological correlates of such deficits we compared QEEG power between 12 participants with aMCI and 12 healthy matched controls. EEG was acquired during performance of a modified Sternberg word recognition task with low and high memory load conditions. While recognition accuracy of aMCI participants was lower than that of controls, this difference was not significant. Nevertheless the aMCI group demonstrated significantly lower theta power at a number of electrode sites and significant correlations were observed between power at these sites and neuropsychological assessment scores. Furthermore in the aMCI sample only, theta power was significantly lower under high versus low memory load. Given current interpretations of the neural generator(s), as well as the role(s), of theta oscillations in cognitive processes, the present data indicate that aMCI may be associated with disruptions in the operation of neurocognitive networks (e.g., MTL-neocortical), particularly under high cognitive load.

Familiarity enhances visual working memory for faces.

Although it is intuitive that familiarity with complex visual objects should aid their preservation in visual working memory (WM), empirical evidence for this is lacking. This study used a conventional change-detection procedure to assess visual WM for unfamiliar and famous faces in healthy adults. Across experiments, faces were upright or inverted and a low- or high-load concurrent verbal WM task was administered to suppress contribution from verbal WM. Even with a high verbal memory load, visual WM performance was significantly better and capacity estimated as significantly greater for famous versus unfamiliar faces. Face inversion abolished this effect. Thus, neither strategic, explicit support from verbal WM nor low-level feature processing easily accounts for the observed benefit of high familiarity for visual WM. These results demonstrate that storage of items in visual WM can be enhanced if robust visual representations of them already exist in long-term memory.

Frontal Hypoactivation on Functional Magnetic Resonance Imaging in Working Memory after Severe Diffuse Traumatic Brain Injury

Working memory is frequently impaired after traumatic brain injury (TBI). The present study aimed to investigate working memory deficits in patients with diffuse axonal injury and to determine the contribution of cerebral activation dysfunctions to them. Eighteen patients with severe TBI and 14 healthy controls matched for age and gender were included in the study. TBI patients were selected according to signs of diffuse axonal injury on computed tomography (CT) and without any evidence of focal lesions on MRI clinical examination. Functional magnetic resonance (fMRI) was used to assess brain activation during n-back tasks (0-, 2-, and 3-back). Compared to controls, the TBI group showed significant working memory impairment on the Digits Backwards (p=0.022) and Letter-Number Sequencing subtests from the WAIS-III (p<0.001) under the 2-back (p=0.008) and 3-back (p=0.017) conditions. Both groups engaged bilateral fronto-parietal regions known to be involved in working memory, although patients showed less cerebral activation than did controls. Decreased activation in TBI patients compared to controls was observed mainly in the right superior and middle frontal cortex. The correlation patterns differed between patients and controls: while the control group showed a negative correlation between performance and activation in prefrontal cortex (PFC), TBI patients presented a positive correlation in right parietal and left parahippocampus for the low and high working memory load, respectively. In conclusion, severe TBI patients with diffuse brain damage show a pattern of cerebral hypoactivation in the right middle and superior frontal regions during working memory tasks, and also present an impaired pattern of performance correlations.

Reduced lateral prefrontal activation in adult patients with attention-deficit/hyperactivity disorder (ADHD) during a working memory task: A functional near-infrared spectroscopy (fNIRS) study

Near-infrared spectroscopy (NIRS) is an optical imaging method, which allows non-invasive in vivo measurements of changes in the concentration of oxygenated (O 2Hb) and deoxygenated (HHb) haemoglobin in cortical tissue. For the present study, we examined 13 adult ADHD patients and 13 age- and gender-matched healthy controls by means of multi-channel NIRS (Optical Topography; ETG-100, Hitachi Medical Co., Japan) during performance of a working memory ( n-back) paradigm. Compared to the healthy control group, ADHD patients showed reduced task-related increases in the concentration of O 2Hb in NIRS channels located over the ventro-lateral prefrontal cortex, indicating reduced activation during performance of the n-back task in this part of the brain. This finding was particularly apparent for the task condition with high working memory load (2-back), and was accompanied by a statistical trend towards an increased number of omission errors in the patient group. The data confirm previous findings of working memory deficits and prefrontal cortex dysfunction in patients suffering from ADHD, and are discussed in the light of imaging findings and theoretical models of working memory function.

Controlled Processing in Single-Prime Negative Priming

Negative priming refers to delayed responses to previously ignored distractors. Unlike conventional studies of negative priming in which the attentional selection of a target against its distractors is required in prime trials (prime-selection negative priming), in single-prime negative priming, a prime stimulus is presented briefly. To further investigate the nature of single-prime negative priming, its properties were examined. In Experiment 1, the proportion of repetition was varied. The effect of single-prime negative priming was reduced when the proportion of repetition was high. In addition, Experiment 2 showed that high memory load could hamper the single-prime negative priming effect. Overall, the current study indicates controlled processing in single-prime negative priming and similarities between single-prime negative priming and prime-selection negative priming.