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.     

工作记忆刷新及其训练效应——经颅直流电刺激的作用

工作记忆刷新是执行功能的重要成分之一。以往研究结合n-back任务和经颅直流电刺激（transcranial direct current stimulation ,tDCS）技术探究刷新能力,发现tDCS技术能够一定程度上增强刷新训练效果并引起大脑神经元相关活动的变化,其增强效果能够维持一定的时间,且能够迁移到其他认知任务之中。但tDCS的干预效果在各项研究中存在一定的差异性。其作用效果会受到训练任务与训练方式、电流强度、电极布局和训练时间间隔等因素的影响。未来研究可以探索tDCS结合刷新训练的最佳刺激参数,以获得最优化的干预效果。     

Developmental differences in prefrontal activation during working memory maintenance and manipulation for different memory loads

The ability to keep information active in working memory is one of the cornerstones of cognitive development. Prior studies have demonstrated that regions which are important for working memory performance in adults, such as dorsolateral prefrontal cortex (DLPFC), ventrolateral prefrontal cortex (VLPFC), and superior parietal cortex, become increasingly engaged across school-aged development. The primary goal of the present functional MRI study was to investigate the involvement of these regions in the development of working memory manipulation relative to maintenance functions under different loads. We measured activation in DLPFC, VLPFC, and superior parietal cortex during the delay period of a verbal working memory task in 11-13-year-old children and young adults. We found evidence for age-related behavioral improvements in working memory and functional changes within DLPFC and VLPFC activation patterns. Although activation profiles of DLPFC and VLPFC were similar, group differences were most pronounced for right DLPFC. Consistent with prior studies, right DLPFC showed an interaction between age and condition (i.e. manipulation versus maintenance), specifically at the lower loads. This interaction was characterized by increased activation for manipulation relative to maintenance trials in adults compared to children. In contrast, we did not observe a significant age-dependent load sensitivity. These results suggest that age-related differences in the right DLPFC are specific to working memory manipulation and are not related to task difficulty and/or differences in short-term memory capacity.     

右侧背外侧前额叶在视觉工作记忆中的因果性作用

以往的影像学研究表明右侧背外侧前额叶皮层(DLPFC)在视觉工作记忆中发挥重要作用, 然而缺乏因果性的证据。本研究旨在考察右侧DLPFC的激活与视觉工作记忆容量的因果关系, 并探讨这一关系受到记忆负荷的调节及其神经机制。被试接受经颅直流电刺激之后完成视觉工作记忆变化检测任务, 根据被试在虚假刺激情况下从负荷4到负荷6任务记忆容量的增量将被试分为低记忆增长潜力组(简称低潜力组)和高记忆增长潜力组(简称高潜力组), 结果发现正性电刺激右侧DLPFC相对于虚假电刺激显著提升了高潜力组被试在低记忆负荷(负荷4)下的记忆容量及其对应的提取阶段的脑电指标SPCN成分。表明右侧DLPFC在视觉工作记忆的提取阶段发挥重要的因果性作用; 正性经颅直流电刺激右侧DLPFC可使工作记忆容量高潜力被试获得更多的脑活动增益, 并导致更好的行为提升效果。     

Trait anxiety and the neural efficiency of manipulation in working memory

The present study investigates the effects of trait anxiety on the neural efficiency of working memory component functions (manipulation vs. maintenance) in the absence of threat-related stimuli. For the manipulation of affectively neutral verbal information held in working memory, high- and low-anxious individuals (N = 46) did not differ in their behavioral performance, yet trait anxiety was positively related to the neural effort expended on task processing, as measured by BOLD signal changes in fMRI. Higher levels of anxiety were associated with stronger activation in two regions implicated in the goal-directed control of attention--that is, right dorsolateral prefrontal cortex (DLPFC) and left inferior frontal sulcus--and with stronger deactivation in a region assigned to the brain's default-mode network--that is, rostral-ventral anterior cingulate cortex. Furthermore, anxiety was associated with a stronger functional coupling of right DLPFC with ventrolateral prefrontal cortex. We interpret our findings as reflecting reduced processing efficiency in high-anxious individuals and point out the need to consider measures of functional integration in addition to measures of regional activation strength when investigating individual differences in neural efficiency. With respect to the functions of working memory, we conclude that anxiety specifically impairs the processing efficiency of (control-demanding) manipulation processes (as opposed to mere maintenance). Notably, this study contributes to an accumulating body of evidence showing that anxiety also affects cognitive processing in the absence of threat-related stimuli.     

Integrating rewards and cognition in the frontal cortex

Research indicates that the dorsolateral prefrontal cortex (DLPFC) contributes to working memory and executive control, whereas the ventral frontal cortex (VFC) contributes to affective and motivational processing. Few studies have examined both the functional specificity and the integration of these regions. We did so using fMRI and a verbal working memory task in which visual cues indicated whether recall performance on an upcoming trial would be linked to a monetary reward. On the basis of prior findings obtained in delayed response tasks performed by nonhuman primates, we hypothesized that (1) VFC would show an increase only in response to a cue indicating potential for a monetary reward; (2) DLPFC would show sustained activity across a delay interval for all trials, though activity in rewarded trials would be enhanced; and (3) regions engaged in speech-based rehearsal would be relatively insensitive to monetary incentive. Our hypotheses about DLPFC and rehearsal-related regions were confirmed. In VFC regions, we failed to observe statistically significant effects of reward when the cue or delay epochs of the task were examined in isolation. However, an unexpected and significant deactivation was observed in VFC during the delay epoch; furthermore, a post hoc voxelwise analysis indicated a complex interaction between (1) the cue and delay epochs of the task and (2) the reward value of the trials. The pattern of activation and deactivation across trial types suggests that VFC is sensitive to reward cues, and that portions of DLPFC and VFC may work in opposition during the delay epoch of a working memory task in order to facilitate task performance.     

背外侧前额叶对主动遗忘负性社会反馈的作用：针对抑郁症的TMS研究

抑郁症患者的负性心境可能源于其抑制功能障碍。患者在主动遗忘负性材料时无法有效调用背外侧前额叶(the dorsolateral prefrontal cortex, DLPFC)等负责抑制控制的额叶脑网络。同时, 患者对社会信息的加工比对非社会信息的加工存在更明显的认知神经障碍, 很难主动遗忘对自己不利的社会反馈信息。为了提高抑郁症患者对负性社会反馈的主动遗忘能力, 本研究采用经颅磁刺激技术(transcranial magnetic stimulation, TMS), 考察抑郁症患者在左侧(n = 32)或右侧DLPFC (n = 30)被激活后其记忆控制能力的改变。结果表明, 当患者的DLPFC被TMS激活时, 他们对社会拒绝的回忆正确率与健康对照组(n = 31)无差异, 且TMS激活右侧DLPFC还改善了患者对他人的社会态度。本研究是采用TMS提高抑郁症患者主动遗忘能力的首次尝试, 研究结果不但支持了DLPFC与记忆控制功能的因果关系, 还为临床治疗抑郁症、创伤后应激障碍、药物成瘾等患者的记忆控制缺陷提供了明确的神经靶点。     

右侧背外侧前额叶皮质在时间知觉中的作用:基于经颅磁刺激的研究

将10 Hz的重复经颅磁刺激(r TMS)施加于右背外侧前额叶皮质(r DLPFC),探讨该区域在时间加工模型中的作用。实验一采用时间复制任务,通过比较基线和后测条件下时间估计行为的差异,探究高频r TMS离线施加于r DLPFC之后对时间知觉的影响。实验二采用毫秒和秒两种范围的时距,探究r DLPFC在不同范围的时间知觉中的作用。结果发现r TMS施加于r DLPFC导致对1.5 s高估,对600 ms的估计无显著影响,提示r DLPFC在涉及工作记忆加工过程的秒范围的时间知觉中有重要作用。     

Working memory load reduces the late positive potential and this effect is attenuated with increasing anxiety

Emotion regulation decreases the processing of arousing stimuli, as indexed by the late positive potential (LPP), an electrocortical component that varies in amplitude with emotional arousal. Emotion regulation increases activity in the prefrontal areas associated with cognitive control, including the dosolateral prefrontal cortex (DLPFC). The present study manipulated working memory load, known to activate the DLPFC, and recorded the LPP elicited by aversive and neutral IAPS pictures presented during the retention interval. The LPP was larger on low-load compared to high-load trials, and on trials with aversive compared to neutral pictures. These LPP data suggest that emotional content and working memory load have opposing effects on attention to distracting stimuli. State anxiety was associated with reduced modulation of the LPP by working memory load. Results are discussed in terms of competition for attention between emotion and cognition and suggest a relationship between DLPFC activation and the allocation of attentional resources to distracting visual stimuli–a relationship that may be disrupted with increasing anxiety.     

Dissociation of the neural systems for working memory maintenance of verbal and nonspatial visual information

Working memory for names and faces was investigated to ascertain whether verbal and nonspatial visual information is maintained in working memory by separate neural systems. The subjects performed a delayed match-to-sample task for famous or unfamous faces and names and a sensorimotor control task. Several occipital, temporal, parietal, and prefrontal areas were activated during all memory delays, in comparison with the control delays. Greater delay activity for unfamous faces than for names was obtained in the right fusiform gyrus, right inferior frontal gyrus (IFG), right IFG/ precentral gyrus, and right medial superior frontal gyrus, whereas greater delay activity for unfamous names than for faces was observed in the precuneus, left insula/postcentral gyrus, and left IFG/ precentral gyrus. There was no significant difference in the prefrontal activity in the comparison between famous faces and names. Greater delay activity for famous names than for faces was obtained in visual association and parietal areas. The results indicate that there is a functional dissociation based on information type within the neural system that is responsible for working memory maintenance of verbal and nonspatial visual information.     

The effect of motivation on working memory: an fMRI and SEM study

This study investigated the effective connectivity between prefrontal regions of human brain supporting motivational influence on working memory. Functional magnetic resonance imaging (fMRI) and structural equation modeling (SEM) were used to examine the interaction between the lateral orbitofrontal (OFC), medial OFC, and dorsolateral prefrontal (DLPFC) regions in the left and right hemisphere during performance of the verbal 2-back working memory task under two reinforcement conditions. The "low-motivation" condition was not associated with monetary reinforcement, while the "high-motivation" condition involved the probability of winning a certain amount of money. In the "low-motivation" condition, the OFC regions in both hemispheres positively influenced the left DLPFC activity. In the "high-motivation" condition, the connectivity in the network including the right OFC regions and left DLPFC changed from positive to negative, whereas the positive connectivity in the network composed of the left OFC and left DLPFC became slightly enhanced compared with the "low-motivation" condition. However, only the connection between the right lateral OFC and left DLPFC showed a significant condition-dependent change in the strength of influence conveyed through the pathway. This change appears to be the functional correlate of motivational influence on verbal working memory.     

Language switching in the bilingual brain: What’s next?

Recent work using functional neuroimaging with early bilinguals has found little evidence for separate neural systems for each language during picture naming (Hernandez, A. E., Dapretto, M., Mazziotta, J., & Bookheimer, S. (2001). Language switching and language representation in Spanish–English bilinguals: An fMRI study. Neuroimage, 14, 510–520). However, switching between languages in early bilinguals during picture naming shows increased activity in the Dorsolateral Prefrontal Cortex (DLPFC) suggesting the importance of maintaining goal related information in order to bias subsequent response selection (Braver, T. S., Barch, D. M., Kelley, W. M., Buckner, R. L., Cohen, N. J., Miezin, F. M., et al. (2001). Direct comparison of prefrontal cortex regions engaged by working and long-term memory tasks. Neuroimage, 14, 48–59; Cohen, J. D., Braver, T. S., & O’Reilly, R. C. (1996). A computational approach to prefrontal cortex, cognitive control and schizophrenia: Recent developments and current challenges. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences, 351, 1515–1527; O’Reilly, R. C., Braver, T. S., & Cohen, J. D. (1999). A biologically based computational model of working memory. In E. Akira Miyake, E. Priti Shah & et al. (Eds.), Models of working memory: Mechanisms of active maintenance and executive control. (pp. 375–411): New York, NY, USA). The current study set out to test early bilinguals using a picture naming paradigm. Results revealed increased activity in the DLPFC and the superior parietal lobule during language switching compared to naming of pictures in a single language. Increased activity was also observed between early learned first and second languages. The results from single language conditions revealed differences in areas devoted to language processing such as the Superior Temporal Gyrus. However, increased activity in brain areas devoted to memory, somatosensory processing and emotion were also observed. Taken together these results replicate previous studies on language switching. They also extend studies on the neural bases of bilingualism by suggesting that early bilinguals’ representation of the two languages may be mediated by neural systems not typically associated with language. The article ends by considering future directions in understanding the brain bases of language switching and single language processing in bilinguals.     

Anxiety and cognitive efficiency: Differential modulation of transient and sustained neural activity during a working memory task

According to the processing-efficiency hypothesis (Eysenck, Derakshan, Santos, & Calvo, 2007), anxious individuals are thought to require greater activation of brain systems supporting cognitive control (e.g.,dorsolateral prefrontal cortex; DLPFC) in order to maintain equivalent performance to nonanxious subjects. A recent theory of cognitive control (Braver, Gray, & Burgess, 2007) has proposed that reduced cognitive efficiency might occur as a result of changes in the temporal dynamics of DLPFC recruitment. In this study, we used a mixed blocked/ event-related fMRI design to track transient and sustained activity in DLPFC while high- and low-anxious participants performed a working memory task. The task was performed after the participants viewed videos designed to induce neutral or anxiety-related moods. After the neutral video, the high-anxious participants had reduced sustained but increased transient activation in working memory areas, in comparison with low-anxious participants. The high-anxious group also showed extensive reductions in sustained activation of "default-network" areas (possible deactivation). After the negative video,the low-anxiety group shifted their activation dynamics in cognitive control regions to resemble those of the high-anxious group. These results suggest that reduced cognitive control in anxiety might be due to a transient, rather than sustained, pattern of working memory recruitment. Supplementary information for this study may be found at www.psychonomic.org/archive.     

Integrating orbitofrontal cortex into prefrontal theory: common processing themes across species and subdivisions

Currently, many theories highlight either representational memory or rule representation as the hallmark of prefrontal function. Neurophysiological findings in the primate dorsolateral prefrontal cortex indicate that both features may characterize prefrontal processing. Neurons in the dorsolateral prefrontal cortex encode information in working memory, and this information is represented when relevant to the rules governing performance in a task. In this review, we discuss recent reports of encoding in primate and rat orbitofrontal regions indicating that these features also characterize activity in the orbitofrontal subdivision of the prefrontal cortex. These data indicate that (1) neural activity in the orbitofrontal cortex links the current incentive value of reinforcers to cues, rather than representing the physical features of cues or associated reinforcers; (2) this incentive-based information is represented in the orbitofrontal cortex when it is relevant to the rules guiding performance in a task; and (3) incentive information is also represented in the orbitofrontal cortex in working memory during delays when neither the cues nor reinforcers are present. Therefore, although the orbitofrontal cortex appears to be uniquely specialized to process incentive or motivational information, it may be integrated into a more global framework of prefrontal function characterized by representational encoding of performance-relevant information.     

振动触觉频率信息的工作记忆容量及存储机制

工作记忆可以同时保存多个信息并且容量有限, 这一内在机制是工作记忆研究的重点问题。视觉和言语等研究领域都发现工作记忆能够存储多个信息单元, 但对振动触觉工作记忆是否能存储多个频率信息目前尚无相关研究。由于振动触觉频率刺激和视觉刺激具有不同的神经编码机制, 以及振动频率信息是通过躯体感觉产生的模拟的、单维的、参数化信息, 振动触觉工作记忆容量及其加工存储机制的研究也必不可少。首先, 本项目将采用新的实验范式, 探究不同的刺激呈现方式以及不同反应报告方式下, 振动触觉工作记忆的容量及其认知机制。其次, 本项目也将同时运用功能磁共振成像(fMRI)技术, 来阐述振动触觉工作记忆加工存储的神经机制。探究基于触觉频率信息的参数工作记忆容量及其神经机制是完善工作记忆模型的重要补充, 将有助于提高我们对工作记忆系统的理解, 并为视觉、听觉、触觉多模态感知觉信息的跨通道研究奠定基础。     

Recognition memory and prefrontal cortex: dissociating recollection and familiarity processes using rTMS

Recognition memory can be supported by both the assessment of the familiarity of an item and by the recollection of the context in which an item was encountered. The neural substrates of these memory processes are controversial. To address these issues we applied repetitive transcranial magnetic stimulation (rTMS) over the right and left dorsolateral prefrontal cortex (DLPFC) of healthy subjects performing a remember/know task. rTMS disrupted familiarity judgments when applied before encoding of stimuli over both right and left DLPFC. rTMS disrupted recollection when applied before encoding of stimuli over the right DLPFC. These findings suggest that the DLPFC plays a critical role in recognition memory based on familiarity as well as recollection.     

Binding in short-term visual memory

The integration of complex information in working memory, and its effect on capacity, shape the limits of conscious cognition. The literature conflicts on whether short-term visual memory represents information as integrated objects. A change-detection paradigm using objects defined by color with location or shape was used to investigate binding in short-term visual memory. Results showed that features from the same dimension compete for capacity, whereas features from different dimensions can be stored in parallel. Binding between these features can occur, but focused attention is required to create and maintain the binding over time, and this integrated format is vulnerable to interference. In the proposed model, working memory capacity is limited both by the independent capacity of simple feature stores and by demands on attention networks that integrate this distributed information into complex but unified thought objects.     

Neural correlates of learning and working memory in the primate posterior parietal cortex

The posterior parietal cortex has been traditionally associated with coordinate transformations necessary for interaction with the environment and with visual-spatial attention. More recently, involvement of posterior parietal cortex in other cognitive functions such as working memory and task learning has become evident. Neurophysiological experiments in non-human primates and human imaging studies have revealed neural correlates of memory and learning at the single neuron and at the brain network level. During working memory, posterior parietal neurons continue to discharge and to represent stimuli that are no longer present. This activation resembles the responses of prefrontal neurons, although important differences have been identified in terms of the ability to resist stimulation by distracting stimuli, which is more evident in the prefrontal than the posterior parietal cortex. Posterior parietal neurons also become active during tasks that require the organization of information into larger structured elements and their activity is modulated according to learned context-dependent rules. Neural correlates of learning can be observed in the mean discharge rate and spectral power of neuronal spike trains after training to perform new task sets or rules. These findings demonstrate the importance of posterior parietal cortex in brain networks mediating working memory and learning.     

An attractor network model of serial recall

We present a neural network model of verbal working memory which attempts to illustrate how a few simple assumptions about neural computation can shed light on cognitive phenomena associated with the serial recall of verbal material. We assume that neural representations are distributed, that neural connectivity is massively recurrent, and that synaptic efficacy is modified based on the correlation between pre- and post-synaptic activity (Hebbian learning). Together these assumptions give rise to emergent computational properties that are relevant to working memory, including short-term maintenance of information, time-based decay, and similarity-based interference. We instantiate these principles in a specific model of serial recall and show how it can both simulate and explain a number of standard cognitive phenomena associated with the task, including the effects of serial position, word length, articulatory suppression (and its interaction with word length), and phonological similarity.     

工作记忆对时间加工的影响

非时间信息加工与时间加工的关系是复杂的, 存在双向干扰或单向干扰, 结果的不一致与任务所需的注意资源及工作记忆都有关。工作记忆的中央执行系统在时间加工中起主要作用, 同时进行的非时间任务对中央执行功能的需求越多, 两种任务之间的干扰程度越大。语言环和视空间模板对时间加工的影响与非时间信息的类型有关。工作记忆作为一个整体以工作记忆容量为指标, 计时成绩也表现出与工作记忆容量有关的年龄、智力等方面的差异。时间加工和工作记忆在额叶、顶叶、基底神经节等皮质存在共同的神经机制。未来应该丰富工作记忆的研究内容, 结合时间加工的分段性探讨工作记忆影响时间加工的具体进程及神经机制, 并力求在应用方面取得一定的成果。