Neural measures reveal individual differences in controlling access to working memory

The capacity of visual short-term memory is highly limited, maintaining only three to four objects simultaneously. This extreme limitation necessitates efficient mechanisms to select only the most relevant objects from the immediate environment to be represented in memory and to restrict irrelevant items from consuming capacity. Here we report a neurophysiological measure of this memory selection mechanism in humans that gauges an individual's efficiency at excluding irrelevant items from being stored in memory. By examining the moment-by-moment contents of visual memory, we observe that selection efficiency varies substantially across individuals and is strongly predicted by the particular memory capacity of each person. Specifically, high capacity individuals are much more efficient at representing only the relevant items than are low capacity individuals, who inefficiently encode and maintain information about the irrelevant items present in the display. These results provide evidence that under many circumstances low capacity individuals may actually store more information in memory than high capacity individuals. Indeed, this ancillary allocation of memory capacity to irrelevant objects may be a primary source of putative differences in overall storage capacity.     

Capacity limit of visual short-term memory in human posterior parietal cortex

At any instant, our visual system allows us to perceive a rich and detailed visual world. Yet our internal, explicit representation of this visual world is extremely sparse: we can only hold in mind a minute fraction of the visual scene. These mental representations are stored in visual short-term memory (VSTM). Even though VSTM is essential for the execution of a wide array of perceptual and cognitive functions, and is supported by an extensive network of brain regions, its storage capacity is severely limited. With the use of functional magnetic resonance imaging, we show here that this capacity limit is neurally reflected in one node of this network: activity in the posterior parietal cortex is tightly correlated with the limited amount of scene information that can be stored in VSTM. These results suggest that the posterior parietal cortex is a key neural locus of our impoverished mental representation of the visual world.     

Dissociable neural mechanisms supporting visual short-term memory for objects

Using visual information to guide behaviour requires storage in a temporary buffer, known as visual short-term memory (VSTM), that sustains attended information across saccades and other visual interruptions. There is growing debate on whether VSTM capacity is limited to a fixed number of objects or whether it is variable. Here we report four experiments using functional magnetic resonance imaging that resolve this controversy by dissociating the representation capacities of the parietal and occipital cortices. Whereas representations in the inferior intra-parietal sulcus (IPS) are fixed to about four objects at different spatial locations regardless of object complexity, those in the superior IPS and the lateral occipital complex are variable, tracking the number of objects held in VSTM, and representing fewer than four objects as their complexity increases. These neural response patterns were observed during both VSTM encoding and maintenance. Thus, multiple systems act together to support VSTM: whereas the inferior IPS maintains spatial attention over a fixed number of objects at different spatial locations, the superior IPS and the lateral occipital complex encode and maintain a variable subset of the attended objects, depending on their complexity. VSTM capacity is therefore determined both by a fixed number of objects and by object complexity.     

Neural organization for the long-term memory of paired associates

Most of our long-term memories of episodes or objects are organized so that we can retrieve them by association. Clinical neuropsychologists assess human memory by the paired-associate learning test, in which a series of paired words or figures is presented and the subject is then asked to retrieve the other pair member associated with each cue. Patients with lesions of the temporal lobe show marked impairment in this test. In our study, we trained monkeys in a pair-association task using a set of computer-generated paired patterns. We found two types of task-related neurons in the anterior temporal cortex. One type selectively responded to both pictures of the paired associates. The other type, which had the strongest response to one picture during the cue presentation, exhibited increasing activity during the delay period when the associate of that picture was used as a cue. These results provide new evidence that single neurons acquire selectivity for visual patterns through associative learning. They also indicate neural mechanisms for storage and retrieval in the long-term memory of paired associates.     

Discrete fixed-resolution representations in visual working memory

Limits on the storage capacity of working memory significantly affect cognitive abilities in a wide range of domains, but the nature of these capacity limits has been elusive. Some researchers have proposed that working memory stores a limited set of discrete, fixed-resolution representations, whereas others have proposed that working memory consists of a pool of resources that can be allocated flexibly to provide either a small number of high-resolution representations or a large number of low-resolution representations. Here we resolve this controversy by providing independent measures of capacity and resolution. We show that, when presented with more than a few simple objects, human observers store a high-resolution representation of a subset of the objects and retain no information about the others. Memory resolution varied over a narrow range that cannot be explained in terms of a general resource pool but can be well explained by a small set of discrete, fixed-resolution representations.     

An event-related potential study of working memory in children

Working memory refers to temporary storage and manipulation of “on-line” information in the brain,which is central to a large range of cognitive capabili- ties[1]. Visual working memory involves processes such as maintaining, updating and manipulating b…     

Dissociation between mental imagery and object recognition in a brain-damaged patient.

Visual imagery is the creation of mental representations that share many features with veridical visual percepts. Studies of normal and brain-damaged people reinforce the view that visual imagery and visual perception are mediated by a common neural substrate and activate the same representations. Thus, brain-damaged patients with intact vision who have an impairment in perception should have impaired visual imagery. Here we present evidence to the contrary from a patient with severely impaired object recognition (visual object agnosia) but with normal mental imagery. He draws objects in considerable detail from memory and uses information derived from mental images in a variety of tasks. In contrast, he cannot identify visually presented objects, even those he has drawn himself. He has normal visual acuity and intact perception of equally complex material in other domains. We conclude that rich internal representations can be activated to support visual imagery even when they cannot support visually mediated perception of objects.     

Neuronal correlate of visual associative long-term memory in the primate temporal cortex

In human long-term memory, ideas and concepts become associated in the learning process. No neuronal correlate for this cognitive function has so far been described, except that memory traces are thought to be localized in the cerebral cortex; the temporal lobe has been assigned as the site for visual experience because electric stimulation of this area results in imagery recall and lesions produce deficits in visual recognition of objects. We previously reported that in the anterior ventral temporal cortex of monkeys, individual neurons have a sustained activity that is highly selective for a few of the 100 coloured fractal patterns used in a visual working-memory task. Here I report the development of this selectivity through repeated trials involving the working memory. The few patterns for which a neuron was conjointly selective were frequently related to each other through stimulus-stimulus association imposed during training. The results indicate that the selectivity acquired by these cells represents a neuronal correlate of the associative long-term memory of pictures.     

A neural representation of depth from motion parallax in macaque visual cortex

Perception of depth is a fundamental challenge for the visual system, particularly for observers moving through their environment. The brain makes use of multiple visual cues to reconstruct the three-dimensional structure of a scene. One potent cue, motion parallax, frequently arises during translation of the observer because the images of objects at different distances move across the retina with different velocities. Human psychophysical studies have demonstrated that motion parallax can be a powerful depth cue, and motion parallax seems to be heavily exploited by animal species that lack highly developed binocular vision. However, little is known about the neural mechanisms that underlie this capacity. Here we show, by using a virtual-reality system to translate macaque monkeys (Macaca mulatta) while they viewed motion parallax displays that simulated objects at different depths, that many neurons in the middle temporal area (area MT) signal the sign of depth (near versus far) from motion parallax in the absence of other depth cues. To achieve this, neurons must combine visual motion with extra-retinal (non-visual) signals related to the animal's movement. Our findings suggest a new neural substrate for depth perception and demonstrate a robust interaction of visual and non-visual cues in area MT. Combined with previous studies that implicate area MT in depth perception based on binocular disparities, our results suggest that area MT contains a more general representation of three-dimensional space that makes use of multiple cues.     

Neuronal correlate of pictorial short-term memory in the primate temporal cortex

It has been proposed that visual-memory traces are located in the temporal lobes of the cerebral cortex, as electric stimulation of this area in humans results in recall of imagery. Lesions in this area also affect recognition of an object after a delay in both humans and monkeys, indicating a role in short-term memory of images. Single-unit recordings from the temporal cortex have shown that some neurons continue to fire when one of two or four colours are to be remembered temporarily. But neuronal responses selective to specific complex objects, including hands and faces, cease soon after the offset of stimulus presentation. These results led to the question of whether any of these neurons could serve the memory of complex objects. We report here a group of shape-selective neurons in an anterior ventral part of the temporal cortex of monkeys that exhibited sustained activity during the delay period of a visual short-term memory task. The activity was highly selective for the pictorial information to be memorized and was independent of the physical attributes such as size, orientation, colour or position of the object. These observations show that the delay activity represents the short-term memory of the categorized percept of a picture.     

视觉神经表征中底-顶和顶-底的双向激活——来自人与猴的实验证据

长期以来,建立在坚实生理学证据之上的底-顶加工说和特征检测理论在视觉研究中占主导地位。对于顶-底加工人们只能靠一般常识,即知识或经验通过激活记忆中的神经表征影响视觉过程。但是近年来,来自人和猴的研究为顶-底的加工提供了实验证据。 首先位于猴腹侧加工系统内的物体和面孔视觉记忆表征,提供了神经编码是怎样创立、组织和再激活的最佳实验证据。联想性编码是通过学习由一些具有特殊功能的神经元建立的,这些神经元具有将时间性关联刺激的表征联系起来的能力。其次,不仅来自视网膜的底-顶信号,而且来自前额叶的顶-底信号都能触发联想性编码的提取,既可以作为有意识回忆的神经基础,又是顶-底加工影响视觉过程的基础。脑损伤病人研究、具有高时间分辨率的人类功能性核磁共振成像(functional magnetic resonance imaging, fMRI)和猴fMRI研究以及猴细胞电生理分析相结合,将进一步加强人们对视觉脑机制的全面理解。     

视听工作记忆的相位同步与跨频耦合研究

为了深入了解视听工作记忆中的神经机制和节律耦合规律,采集了视听工作记忆任务脑电数据,通过时频分析确定关键节律及核心电极,并使用多变量模式分析确定关键时间段。theta相位同步在记忆呈现期至关重要,其中额颞叶同步参与听觉记忆,额顶叶同步参与视觉记忆,而theta和alpha的跨频耦合只在特定记忆模态的相关脑区有显著增强。工作记忆任务相关脑区通过远端theta同步协调中央执行功能,alpha同步协调记忆存储缓冲功能,theta-alpha耦合整合不同模态功能间的信息,揭示了视听模态工作记忆的部分神经机制。     

Small modulation of ongoing cortical dynamics by sensory input during natural vision

During vision, it is believed that neural activity in the primary visual cortex is predominantly driven by sensory input from the environment. However, visual cortical neurons respond to repeated presentations of the same stimulus with a high degree of variability. Although this variability has been considered to be noise owing to random spontaneous activity within the cortex, recent studies show that spontaneous activity has a highly coherent spatio-temporal structure. This raises the possibility that the pattern of this spontaneous activity may shape neural responses during natural viewing conditions to a larger extent than previously thought. Here, we examine the relationship between spontaneous activity and the response of primary visual cortical neurons to dynamic natural-scene and random-noise film images in awake, freely viewing ferrets from the time of eye opening to maturity. The correspondence between evoked neural activity and the structure of the input signal was weak in young animals, but systematically improved with age. This improvement was linked to a shift in the dynamics of spontaneous activity. At all ages including the mature animal, correlations in spontaneous neural firing were only slightly modified by visual stimulation, irrespective of the sensory input. These results suggest that in both the developing and mature visual cortex, sensory evoked neural activity represents the modulation and triggering of ongoing circuit dynamics by input signals, rather than directly reflecting the structure of the input signal itself.     

A neurological dissociation between perceiving objects and grasping them

Studies of the visual capacity of neurological patients have provided evidence for a dissociation between the perceptual report of a visual stimulus and the ability to direct spatially accurate movements toward that stimulus. Some patients with damage to the parietal lobe, for example, are unable to reach accurately towards visual targets that they unequivocally report seeing. Conversely, some patients with extensive damage to primary visual cortex can make accurate pointing movements or saccades toward a stimulus presented in their 'blind' scotoma. But in investigations of visuomotor control in patients with visual disorders, little consideration has been given to complex acts such as manual prehension. Grasping a three-dimensional object requires knowledge not only of the object's spatial location, but also of its form, orientation and size. We have examined a patient with a profound disorder in the perception of such object qualities. Our quantitative analyses demonstrate strikingly accurate guidance of hand and finger movements directed at the very objects whose qualities she fails to perceive. These data suggest that the neural substrates for the visual perception of object qualities such as shape, orientation and size are distinct from those underlying the use of those qualities in the control of manual skills.     

Using Quantum Associative Memory to Simulate the Brain Functions

To simulate the brain functions, a quantum associative memory combined with information preprocessing by a sparse coding model is presented. The sparse coding scheme is used to simulate the information transformation from retina up to primary visual cortex (V1) along the visual path and the quantum associative memory is used to simulate the pattern processing functions of the brain such as the pattern storing, forgetting and retrieving. Experimental results show that the model exhibits good associative ability on face recognition. Considering the huge storage capacity, mass parailel-distributed processing ability and oscillatory phenomena of the quantum system, this model might he a biological plausible implementation.     

神经系统中的复杂性研究

神经系统面临的一个基本科学问题是要回答神经系统中微观到宏观的机制.微观运动形式以神经元突触活动为基础,宏观行为包括视觉、听觉等基本认知功能,以及大脑思维的高级认知.微观与宏观的差异是巨大的,其过渡的形式困扰着神经科学,迄今未获解决.系统科学复杂性研究的出现,为这个科学问题提供了新的角度和方法.本文讨论了神经系统信息传递和转换中的突变行为,通过视知觉、概念形成、记忆等研究案例,阐明了复杂性研究对神经系统的处理方案与结果.并讨论了学习过程中几种最基本的突变形式.     

Time-dependent reorganization of brain circuitry underlying long-term memory storage.

Retrograde amnesia observed following hippocampal lesions in humans and animals is typically temporally graded, with recent memory being impaired while remote memories remain intact, indicating that the hippocampal formation has a time-limited role in memory storage. However, this claim remains controversial because studies involving hippocampal lesions tell us nothing about the contribution of the hippocampus to memory storage if this region was present at the time of memory retrieval. We therefore used non-invasive functional brain imaging using (14C)2-deoxyglucose uptake to examine how the brain circuitry underlying long-term memory storage is reorganized over time in an intact brain. Regional metabolic activity in the brain was mapped in mice tested at different times for retention of a spatial discrimination task. Here we report that increasing the retention interval from 5 days to 25 days resulted in both decreased hippocampal metabolic activity during retention testing and a loss of correlation between hippocampal metabolic activity and memory performance. Concomitantly, a recruitment of certain cortical areas was observed. These results indicate that there is a time-dependent reorganization of the neuronal circuitry underlying long-term memory storage, in which a transitory interaction between the hippocampal formation and the neocortex would mediate the establishment of long-lived cortical memory representations.     

Characteristics of fMRI BOLD signal and its neurophysiological mechanism

The functional magnetic resonance imaging (fMRI) based on blood oxygen level dependent (BOLD) contrast has emerged as one of the most potent noninvasive tools for mapping brain function and has been widely used to explore physiological, pathological changes and mental activity in the brain. Exploring the nature and property of BOLD signal has recently attracted more attentions. Despite that great progress has been made in investigation of the characteristics and neurophysiological basis, the exact nature of BOLD signal remains unclear. In this paper we discuss the characteristics of BOLD signals, the nonlinear BOLD response to external stimuli and the relation between BOLD signals and neural electrophysiological recordings. Furthermore, we develop our new opinions regarding nonlinear BOLD response and make some perspectives on future study.     

Characteristics of fMRI BOLD signal and its neurophysiological mechanism

The functional magnetic resonance imaging (fMRI) based on blood oxygen level dependent (BOLD) contrast has emerged as one of the most potent noninvasive tools for mapping brain function and has been widely used to explore physiological, pathological changes and mental activity in the brain. Exploring the nature and property of BOLD signal has recently attracted more attentions. Despite that great progress has been made in investigation of the characteristics and neurophysiological basis, the exact nature of BOLD signal remains unclear. In this paper we discuss the characteristics of BOLD signals, the nonlinear BOLD response to external stimuli and the relation between BOLD signals and neural electrophysiological recordings. Furthermore, we develop our new opinions regarding nonlinear BOLD response and make some perspectives on future study.     

基于域理论的自适应谐振神经网络研究

在自适应谐振理论和域理论的基础上提出了一种基于域理论的自适应谐振神经网络算法ＦＴＡＲＴ。该算法将自适应谐振理论和域理论的优点有机结合,采用了独特的解决样本间冲突和动态扩大分类区域的方法,不需人为设置陷层神经元,学习速度快,精度高。此外,还提出了一种从ＦＴＡＲＴ网络中抽取符号规则的方法,即基于统计的产生－测试法,实验结果表明,使用该方法抽取出的符号规则可理解性好、预测精度高,可以很好地描述ＦＴＡＲＴ