The neural bases of distracter-resistant working memory

A tale of two n-backs: Diverging associations of dorsolateral prefrontal cortex activation with n-back task performance

Background

In studying the neural correlates of working memory (WM) ability via functional magnetic resonance imaging (fMRI) in health and disease, it is relatively uncommon for investigators to report associations between brain activation and measures of task performance. Additionally, how the choice of WM task impacts observed activation-performance relationships is poorly understood. We sought to illustrate the impact of WM task on brain-behavior correlations using two large, publicly available datasets.

Methods

We conducted between-participants analyses of task-based fMRI data from two publicly available datasets: the Human Connectome Project (HCP; n = 866) and the Queensland Twin Imaging (QTIM) Study (n = 459). Participants performed two distinct variations of the n-back WM task with different stimuli, timings, and response paradigms. Associations between brain activation ([2-back - 0-back] contrast) and task performance (2-back % correct) were investigated separately in each dataset, as well as across datasets, within the dorsolateral prefrontal cortex (dlPFC), medial prefrontal cortex, and whole cortex.

Results

Global patterns of activation to task were similar in both datasets. However, opposite associations between activation and task performance were observed in bilateral pre-supplementary motor area and left middle frontal gyrus. Within the dlPFC, HCP participants exhibited a significantly greater activation-performance relationship in bilateral middle frontal gyrus relative to QTIM Study participants.

Conclusions

The observation of diverging activation-performance relationships between two large datasets performing variations of the n-back task serves as a critical reminder for investigators to exercise caution when selecting WM tasks and interpreting neural activation in response to a WM task.

Lateral Prefrontal Stimulation of Active Cortex With Theta Burst Transcranial Magnetic Stimulation Affects Subsequent Engagement of the Frontoparietal Network

BACKGROUND

A critical unanswered question about therapeutic transcranial magnetic stimulation is what patients should do during treatment to optimize its effectiveness. Here, we address this lack of knowledge in healthy participants, testing the hypotheses that stimulating the left dorsolateral prefrontal cortex (dlPFC) while participants perform a working memory task will provide stronger effects on subsequent activation, perfusion, connectivity, and performance than stimulating resting dlPFC.

METHODS

After a baseline functional magnetic resonance imaging session to localize dlPFC activation and the associated frontoparietal network (FPN) engaged by an n-back task, healthy participants (N = 40, 67.5% female) underwent 3 counterbalanced sessions, separated by several weeks, during which they received intermittent theta burst stimulation (iTBS) followed by magnetic resonance imaging scans as follows: 1) iTBS to the dlPFC while resting passively (passive), 2) iTBS to the dlPFC while performing the n-back task (active), and 3) iTBS to a vertex site, while not engaged in the n-back task and resting passively (control).

RESULTS

We found no difference in n-back performance between the 3 conditions. However, FPN activation was reduced while performing the n-back task in the active condition relative to the passive and control conditions. There was no differential activity in the FPN on comparing passive with control conditions, i.e., there was no effect of the site of stimulation. We found no effects of state or site of stimulation on perfusion or connectivity with the dlPFC.

CONCLUSIONS

In this study, the state of the brain while receiving iTBS affected FPN activation, possibly reflecting greater efficiency of FPN network activation when participants were stimulated while engaging the FPN.

Neuromonitoring Correlates of Expertise Level in Surgical Performers: A Systematic Review

Surgical expertise does not have a clear definition and is often culturally associated with power, authority, prestige, and case number rather than more objective proxies of excellence. Multiple models of expertise progression have been proposed including the Dreyfus model, however, they all currently require subjective evaluation of skill. Recently, efforts have been made to improve the ways in which surgical excellence is measured and expertise is defined using artificial intelligence, video recordings, and accelerometers. However, these aforementioned methods of assessment are still subjective or indirect proxies of expertise, thus uncovering the neural mechanisms that differentiate expert surgeons from trainees may enhance the objectivity of surgical expertise validation. In fact, some researchers have already suggested that their neural imaging-based expertise classification methods outperform currently used methods of surgical skill certification such as the Fundamentals of Laparoscopic Surgery (FLS) scores. Such imaging biomarkers would not only help better identify the highest performing surgeons, but could also improve residency programs by providing more objective, evidence-based feedback and developmental milestones for those in training and perhaps act as a marker of surgical potential in medical students. Despite the potential advantages of using neural imaging in the assessment of surgical expertise, this field of research remains in its infancy. This systematic review identifies studies that have applied neuromonitoring in assessing surgical skill across levels of expertise. The goals of this review are to identify (1) the strongest neural indicators of surgical expertise, (2) the limitations of the current literature on this subject, (3) the most sensible future directions for further study. We found substantial evidence that surgical expertise can be delineated by differential activation and connectivity in the prefrontal cortex (PFC) across multiple task and neuroimaging modalities. Specifically, novices tend to have greater PFC activation than experts under standard conditions in bimanual and decision-making tasks. However, under high temporal demand tasks, experts had increased PFC activation whereas novices had decreased PFC activation. Common limitations uncovered in this review were that task difficulty was often insufficient to delineate between residents and attending. Moreover, attending level involvement was also low in multiple studies which may also have contributed to this issue. Most studies did not analyze the ability of their neuromonitoring findings to accurately classify subjects by level of expertise. Finally, the predominance of fNIRS as the neuromonitoring modality limits our ability to uncover the neural correlates of surgical expertise in non-cortical brain regions. Future studies should first strive to address these limitations. In the longer term, longitudinal within-subjects design over the course of a residency or even a career will also advance the field. Although logistically arduous, such studies would likely be most beneficial in demonstrating effects of increasing surgical expertise on regional brain activation and inter-region connectivity.

Shared increased entropy of brain signals across patients with different mental illnesses: A coordinate-based activation likelihood estimation meta-analysis

Entropy is a measurement of brain signal complexity. Studies have found increased/decreased entropy of brain signals in psychiatric patients. There is no consistent conclusion regarding the relationship between the entropy of brain signals and mental illness. Therefore, this meta-analysis aimed to identify consistent abnormalities in the brain signal entropy in patients with different mental illnesses. We conducted a systematic search to collect resting-state functional magnetic resonance imaging (rs-fMRI) studies in patients with psychiatric disorders. This work identified 9 eligible rs-fMRI studies, which included a total of 14 experiments, 67 activation foci, and 1383 subjects. We tested the convergence across their findings by using the activation likelihood estimation method. P-value maps were corrected by using cluster-level family-wise error p < 0.05 and permuting 2000 times. Results showed that patients with different psychiatric disorders shared commonly increased entropy of brain signals in the left inferior and middle frontal gyri, and the right fusiform gyrus, cuneus, precuneus. No shared alterations were found in the subcortical regions and cerebellum in the patient group. Our findings suggested that the increased entropy of brain signals in the cortex, not subcortical regions and cerebellum, might have associations with the pathophysiology across mental illnesses. This meta-analysis study provided the first comprehensive understanding of the abnormality in brain signal complexity across patients with different psychiatric disorders.

Exploring brain-behavior relationships in the N-back task

Working memory (WM) function has traditionally been investigated in terms of two dimensions: within-individual effects of WM load, and between-individual differences in task performance. In human neuroimaging studies, the N-back task has frequently been used to study both. A reliable finding is that activation in frontoparietal regions exhibits an inverted-U pattern, such that activity tends to decrease at high load levels. Yet it is not known whether such U-shaped patterns are a key individual differences factor that can predict load-related changes in task performance. The current study investigated this question by manipulating load levels across a much wider range than explored previously (N ​= ​1-6), and providing a more comprehensive examination of brain-behavior relationships. In a sample of healthy young adults (n ​= ​57), the analysis focused on a distinct region of left lateral prefrontal cortex (LPFC) identified in prior work to show a unique relationship with task performance and WM function. In this region it was the linear slope of load-related activity, rather than the U-shaped pattern, that was positively associated with individual differences in target accuracy. Comprehensive supplemental analyses revealed the brain-wide selectivity of this pattern. Target accuracy was also independently predicted by the global resting-state connectivity of this LPFC region. These effects were robust, as demonstrated by cross-validation analyses and out-of-sample prediction, and also critically, were primarily driven by the high-load conditions. Together, the results highlight the utility of high-load conditions for investigating individual differences in WM function.

Turning down the heat: Neural mechanisms of cognitive control for inhibiting task-irrelevant emotional information during adolescence

One major question in the cognitive neuroscience of cognitive control is whether prefrontal regions contribute to control by upregulating the processing of task-relevant material or by downregulating the processing of task-irrelevant material. Here we take a unique approach to addressing this question by using multi-voxel pattern analysis, which allowed us to determine the degree to which each of the task-relevant and task-irrelevant dimensions of a stimulus are being processed in posterior cortex on a trial-by-trial basis. In our study, adolescent participants performed an emotion word - emotional face Stroop task requiring them to determine the emotional valence (positive, negative) of a task-relevant word in the context of a task-irrelevant emotional face. Using mediation models, we determined whether activation of a major cognitive control region, the dorsolateral prefrontal cortex (DLPFC), influences reaction time on a trial-by-trial basis directly or if it does so indirectly by modulating processing of the task-relevant and/or task-irrelevant information in posterior brain regions. To examine the specificity of the effects observed for the DLPFC, similar analyses were performed for the amygdala, a brain region involved in processing of the salient task-irrelevant emotional information. For both congruent and incongruent trials, increased DLPFC activity on a given trial was associated with reduced perceptual processing of the task-irrelevant face, consistent with the idea that top-down cognitive control can modulate processing of task-irrelevant information. No effect of DLPFC activity was observed on processing of the task-relevant word. However, increased processing of the task-relevant word was associated with longer RT on congruent trials but not incongruent trials, which may reflect a need for greater processing of the task-relevant word to overcome any influence of the pre-potent task-irrelevant face. In a more exploratory aspect of our investigation, multi-level moderated mediation models were used to examine the influence of individual differences on the observed brain-behavior relationships. For congruent trials, the influence of task-irrelevant face processing on RT was decreased in individuals with higher self-reported Executive Control and increased in those with higher levels of self-reported Negative Affect. These results suggest that cognitive control regions in prefrontal cortex during adolescence can suppress the processing of task-irrelevant information in sensory cortex to influence performance (RT). The processing of task-relevant information may also influence performance, but such processing did not reveal evidence of being modulated by cognitive control regions. Moreover, these effects are sensitive to individual differences in the self-reported ability to exert cognitive and affective control. As such, we provide insights into the more precise mechanisms by which cognitive control influences task performance on a trial-by-trial basis during adolescence.

Enhancement of pain inhibition by working memory with anodal transcranial direct current stimulation of the left dorsolateral prefrontal cortex

The aim of this study was to examine whether transcranial direct current stimulation (tDCS) of the dorsolateral prefrontal cortex (DLPFC) enhances pain inhibition by improving working memory (WM). Forty healthy volunteers participated in two tDCS sessions. Pain was evoked by electrical stimulation at the ankle. Participants performed an n-back task (0-back and 2-back). The experimental protocol comprised five counterbalanced conditions (0-back, 2-back, pain, 0-back with pain and 2-back with pain) that were performed twice (pre-tDCS baseline and during tDCS). Compared with the pre-tDCS baseline values, anodal tDCS decreased response times for the 2-back condition (p < 0.01) but not for the 0-back condition (p > 0.5). Anodal tDCS also decreased pain ratings marginally in the 2-back with pain condition, but not the 0-back with pain condition (p = 0.052 and p > 0.2, respectively). No effect was produced by sham tDCS for any condition (p > 0.2). These results indicate that tDCS of the left DLPFC may enhance pain inhibition by improving WM.

Nicotinic acetylcholine receptor availability in cigarette smokers: effect of heavy caffeine or marijuana use

RATIONALE

Upregulation of α4β2* nicotinic acetylcholine receptors (nAChRs) is one of the most well-established effects of chronic cigarette smoking on the brain. Prior research by our group gave a preliminary indication that cigarette smokers with concomitant use of caffeine or marijuana have altered nAChR availability.

OBJECTIVE

We sought to determine if smokers with heavy caffeine or marijuana use have different levels of α4β2* nAChRs than smokers without these drug usages.

METHODS

One hundred and one positron emission tomography (PET) scans, using the radiotracer 2-FA (a ligand for β2*-containing nAChRs), were obtained from four groups of males: non-smokers without heavy caffeine or marijuana use, smokers without heavy caffeine or marijuana use, smokers with heavy caffeine use (mean four coffee cups per day), and smokers with heavy marijuana use (mean 22 days of use per month). Total distribution volume (Vt/fp) was determined for the brainstem, prefrontal cortex, and thalamus, as a measure of nAChR availability.

RESULTS

A significant between-group effect was found, resulting from the heavy caffeine and marijuana groups having the highest Vt/fp values (especially for the brainstem and prefrontal cortex), followed by smokers without such use, followed by non-smokers. Direct between-group comparisons revealed significant differences for Vt/fp values between the smoker groups with and without heavy caffeine or marijuana use.

CONCLUSIONS

Smokers with heavy caffeine or marijuana use have higher α4β2* nAChR availability than smokers without these drug usages. These findings are likely due to increased nicotine exposure but could also be due to an interaction on a cellular/molecular level.

Intergenerational transmission of self-regulation: A multidisciplinary review and integrative conceptual framework

This review examines mechanisms contributing to the intergenerational transmission of self-regulation. To provide an integrated account of how self-regulation is transmitted across generations, we draw from over 75 years of accumulated evidence, spanning case studies to experimental approaches, in literatures covering developmental, social, and clinical psychology, and criminology, physiology, genetics, and human and animal neuroscience (among others). First, we present a taxonomy of what self-regulation is and then examine how it develops--overviews that guide the main foci of the review. Next, studies supporting an association between parent and child self-regulation are reviewed. Subsequently, literature that considers potential social mechanisms of transmission, specifically parenting behavior, interparental (i.e., marital) relationship behaviors, and broader rearing influences (e.g., household chaos) is considered. Finally, evidence that prenatal programming may be the starting point of the intergenerational transmission of self-regulation is covered, along with key findings from the behavioral and molecular genetics literatures. To integrate these literatures, we introduce the self-regulation intergenerational transmission model, a framework that brings together prenatal, social/contextual, and neurobiological mechanisms (spanning endocrine, neural, and genetic levels, including gene-environment interplay and epigenetic processes) to explain the intergenerational transmission of self-regulation. This model also incorporates potential transactional processes between generations (e.g., children's self-regulation and parent-child interaction dynamics that may affect parents' self-regulation) that further influence intergenerational processes. In pointing the way forward, we note key future directions and ways to address limitations in existing work throughout the review and in closing. We also conclude by noting several implications for intervention work.

A dual-task paradigm for behavioral and neurobiological studies in nonhuman primates

BACKGROUND

The dual-task paradigm is a procedure in which subjects are asked to perform two behavioral tasks concurrently, each of which involves a distinct goal with a unique stimulus-response association. Due to the heavy demand on subject's cognitive abilities, human studies using this paradigm have provided detailed insights regarding how the components of cognitive systems are functionally organized and implemented. Although dual-task paradigms are widely used in human studies, they are seldom used in nonhuman animal studies.

NEW METHOD

We propose a novel dual-task paradigm for monkeys that requires the simultaneous performance of two cognitively demanding component tasks, each of which uses an independent effector for behavioral responses (hand and eyes). We provide a detailed description of an optimal training protocol for this paradigm, which has been lacking in the existing literature.

RESULTS

An analysis of behavioral performance showed that the proposed dual-task paradigm (1) was quickly learned by monkeys (less than 40 sessions) with step-by-step training protocols, (2) produced specific behavioral effects, known as dual-task interference in human studies, and (3) achieved rigid and independent control of the effectors for behavioral responses throughout the trial.

COMPARISON WITH EXISTING METHODS

The proposed dual-task paradigm has a scalable task structure, in that each of the two component tasks can be easily replaced by other tasks, while preserving the overall structure of the paradigm.

CONCLUSIONS

This paradigm should be useful for investigating executive control that underlies dual-task performance at both the behavioral and neuronal levels.

Maintenance of non-consciously presented information engages the prefrontal cortex

Conscious processing is generally seen as required for flexible and willful actions, as well as for tasks that require durable information maintenance. Here we present research that questions the assumption that only consciously perceived information is durable (>500 ms). Using the attentional blink (AB) phenomenon, we rendered otherwise relatively clearly perceived letters non-conscious. In a first experiment we systematically manipulated the delay between stimulus presentation and response, for the purpose of estimating the durability of non-conscious perceptual representations. For items reported not seen, we found that behavioral performance was better than chance across intervals up to 15 s. In a second experiment we used fMRI to investigate the neural correlates underlying the maintenance of non-conscious perceptual representations. Critically, the relatively long delay period demonstrated in experiment 1 enabled isolation of the signal change specifically related to the maintenance period, separate from stimulus presentation and response. We found sustained BOLD signal change in the right mid-lateral prefrontal cortex, orbitofrontal cortex, and crus II of the cerebellum during maintenance of non-consciously perceived information. These findings are consistent with the controversial claim that working-memory mechanisms are involved in the short-term maintenance of non-conscious perceptual representations.