Dynamic Spatial Coding within the Dorsal Frontoparietal Network during a Visual Search Task

To what extent are the left and right visual hemifields spatially coded in the dorsal frontoparietal attention network? In many experiments with neglect patients, the left hemisphere shows a contralateral hemifield preference, whereas the right hemisphere represents both hemifields. This pattern of spatial coding is often used to explain the right-hemispheric dominance of lesions causing hemispatial neglect. However, pathophysiological mechanisms of hemispatial neglect are controversial because recent experiments on healthy subjects produced conflicting results regarding the spatial coding of visual hemifields. We used an fMRI paradigm that allowed us to distinguish two attentional subprocesses during a visual search task. Either within the left or right hemifield subjects first attended to stationary locations (spatial orienting) and then shifted their attentional focus to search for a target line. Dynamic changes in spatial coding of the left and right hemifields were observed within subregions of the dorsal front-parietal network: During stationary spatial orienting, we found the well-known spatial pattern described above, with a bilateral hemifield representation in the right hemisphere and a contralateral preference in the left hemisphere. However, during search, the right hemisphere had a contralateral preference and the left hemisphere equally represented both hemifields. This finding leads to novel perspectives regarding models of visuospatial attention and hemispatial neglect.     

Maintenance and manipulation in spatial working memory: dissociations in the prefrontal cortex

Two experiments were conducted to compare thec ries of the functional organization of spatial working memory within the human prefrontal cortex. In Experiment I, memory set size for locations was parametrically varied, allowing for the assessment of BOLD signal across maintenance requirements. In the sec ond experiment, manipulation of spatial information held in working memory was contrasted with simple maintenance of that information. Both experiment evoked significant activity in a distributed spatia working memory network. Although dorsolateral prefrontal activation increased monotonically with memory set size, this region was differentially engaged in task conditions involving explicit manipulation of in ternal representations. Activation in the superior frontal sulcal region was associated with maintenance of spatial information, increasing with memory se size. In contrast, ventrolateral prefrontal activation was present only at the highest memory set size, possibly due to the differential use of organizational strategies with more complex stimuli. These results sup port claims that the dorsolateral prefrontal cortex is involved in the manipulation of internal representa tions and that the superior frontal sulcal region is involved in the maintenance of spatial information but they suggest a complex role for the ventrolatera prefrontal region.     

Common neural substrates for visual working memory and attention

Humans are severely limited in their ability to memorize visual information over short periods of time. Selective attention has been implicated as a limiting factor. Here we used functional magnetic resonance imaging to test the hypothesis that this limitation is due to common neural resources shared by visual working memory (WM) and selective attention. We combined visual search and delayed discrimination of complex objects and independently modulated the demands on selective attention and WM encoding. Participants were presented with a search array and performed easy or difficult visual search in order to encode one or three complex objects into visual WM. Overlapping activation for attention-demanding visual search and WM encoding was observed in distributed posterior and frontal regions. In the right prefrontal cortex and bilateral insula blood oxygen-level-dependent activation additively increased with increased WM load and attentional demand. Conversely, several visual, parietal and premotor areas showed overlapping activation for the two task components and were severely reduced in their WM load response under the condition with high attentional demand. Regions in the left prefrontal cortex were selectively responsive to WM load. Areas selectively responsive to high attentional demand were found within the right prefrontal and bilateral occipital cortex. These results indicate that encoding into visual WM and visual selective attention require to a high degree access to common neural resources. We propose that competition for resources shared by visual attention and WM encoding can limit processing capabilities in distributed posterior brain regions.     

Interference resolution in spatial working memory

Anterior cingulate and lateral inferior prefrontal cortex (PFC) are considered important for conflict monitoring and interference resolution in many verbal tasks. We studied interference resolution in a spatial working memory task using event-related fMRI. The task required participants to ignore two locations from a set of four initially held in working memory and to remember the remaining two locations as the target set for a subsequent recognition test. Familiarity of nontarget probes was manipulated by drawing the probe from either the ignored locations [high familiarity (HF)] or locations not used in the present trial [low familiarity (LF)]. Precentral sulcus (PrCS) and superior parietal lobe (SPL), two regions commonly associated with motor planning and spatial attention, showed heightened activity in response to increased level of interference from nontargets of high familiarity. Our finding suggests that interference resolution in the spatial domain may involve a different subset of the working memory system from that in the verbal domain.     

Age differences in mental scanning of locomotor maps

PURPOSE: To investigate whether the metric quality of mental spatial representations of the environment changes with aging, by means of mental scanning of locomotor space. METHOD: Twenty young adults (age 21.1-30.8) and 20 old adults (age 60.4-76.4) were submitted to a battery of psychometric tests to assess the effects of aging on general cognitive functions and specific visuo-spatial abilities, and to determine which cognitive factors correlated with scanning performance. The mental scanning task followed. Participants firstly had to learn a path comprising six positions by walking and observing these; secondly, they had to use mental imagery to scan from a first to a second position, according to different distances. RESULTS: Aging reduces the validity of the metric information incorporated by mental spatial representations. Attentional and working memory resources, ability to manipulate spatial information and a general factor of abstract visuo-spatial reasoning strongly correlated with scanning performance. Finally, aging seemed to have a selective effect on cognitive functioning. CONCLUSIONS: Age differences emerge when demanding storage and processing of information are simultaneously required, as predicted by the Limited Resources view of aged cognition.     

Control processes during selective long-term memory retrieval

In our daily life, we often need to selectively remember information related to the same retrieval cue in a consecutive manner (e.g., ingredients from a recipe). To investigate such selection processes during cued long-term memory (LTM) retrieval, we used a paradigm in which the retrieval demands were systematically varied from trial to trial and analyzed, by means of behavior and slow cortical EEG potentials (SCPs), the retrieval processes in the current trial depending on those of the previous trial. We varied whether the retrieval cue, the type of to-be-retrieved association (feature), or retrieval load was repeated or changed from trial to trial. The behavioral data revealed a benefit of feature repetition, probably due to trial-by-trial feature priming. SCPs further showed an effect of cue change with a mid-frontal maximum, suggesting increased control demands when the cue was repeated, as well as a parietal effect of retrieval-load change, indicating increased activation of posterior neural resources when focusing on a single association after all learned associations had been activated previously, compared to staying with single associations across trials. These effects suggest the existence of two distinct types of dynamic (trial-by-trial) control processes during LTM retrieval: (1) medial frontal processes that monitor or regulate interference within a set of activated associations, and (2) posterior processes regulating attention to LTM representations. The present study demonstrates that processes mediating selective LTM retrieval can be successfully studied by manipulating the history of processing demands in trial sequences.     

精神分裂症患者视觉数字和视觉空间N-back任务下工作记忆的相关研究

目的:通过不同记忆负荷的视觉数字和视觉空间n-back任务,了解精神分裂症患者的工作记忆能力的受损情况。方法:选取精神分裂症患者33例(患者组)及正常对照50例(对照组),分别完成视觉数字、视觉空间两种n-back范式任务,以信号检测论中的辨别力指标来评价受试者的工作记忆能力。结果:在各范式和记忆负荷下,患者组的辨别力低于对照组(P0.05)。随着记忆负荷的增大,无论是患者组还是对照组的辨别力都有显著下降。视觉数字n-back记忆负荷与分组的交互作用不显著(P=0.223),视觉空间n-back记忆负荷与分组的交互作用显著(P=0.000)。结论:精神分裂症患者存在工作记忆能力受损,且其工作记忆能力的损害独立于精神症状之外;精神分裂症患者工作记忆的受损对视觉空间工作记忆负荷的增加表现得更为敏感。     

Spatial maps in frontal and prefrontal cortex

Though the function of prefrontal cortex has been extensively investigated, little is known about the internal organization of individual prefrontal areas. Functional magnetic resonance imaging was used to show that some frontal and prefrontal cortical areas represent visual space in orderly, reproducible, topographic maps. The map-containing areas partly overlap dorsolateral prefrontal areas engaged by working memory tasks. These maps may be useful for attending to task-relevant objects at various spatial locations, an aspect of the executive control of attention.     

Neural correlates of simple and complex mental calculation

Some authors proposed that exact mental calculation is based on linguistic representations and relies on the perisylvian language cortices, while the understanding of proximity relations between numerical quantities implicates the parietal cortex. However, other authors opposed developmental arguments to suggest that number sense emerges from nonspecific visuospatial processing areas in the parietal cortex. Within this debate, the present study aimed at revealing the functional anatomy of the two basic resolution strategies involved in mental calculation, namely arithmetical fact retrieval and actual computation, questioning in particular the respective role of language and/or visuospatial cerebral areas. Regional cerebral blood flow was measured with positron emission tomography while subjects were at rest (Rest), read digits (Read), retrieved simple arithmetic facts from memory (i.e., 2 x 4, Retrieve), and performed mental complex calculation (i.e., 32 x 24, Compute). Compared to Read, Retrieve engaged a left parieto-premotor circuit representing a developmental trace of a finger-counting representation that mediates, by extension, the numerical knowledge in adult. Beside this basic network, Retrieve involved a naming network, including the left anterior insula and the right cerebellar cortex, while it did not engage the perisylvian language areas, which were deactivated as compared to Rest. In addition to this retrieval network, Compute specifically involved two functional networks: a left parieto-frontal network in charge of the holding of the multidigit numbers in visuospatial working memory and a bilateral inferior temporal gyri related to the visual mental imagery resolution strategy. Overall, these results provide strong evidence of the involvement of visuospatial representations in different levels of mental calculation.     

Dissociable neural circuits for encoding and retrieval of object locations during active navigation in humans

Several cortical and subcortical circuits have been implicated in object location memory and navigation. Uncertainty remains, however, about which neural circuits are involved in the distinct processes of encoding and retrieval during active navigation through three-dimensional space. We used functional magnetic resonance imaging (fMRI) to measure neural responses as participants learned the location of a single target object relative to a small set of landmarks. Following a delay, the target was removed and participants were required to navigate back to its original position. The relative and absolute locations of landmarks and the target object were changed on every trial, so that participants had to learn a novel arrangement for each spatial scene. At encoding, greater activity within the right hippocampus and the parahippocampal gyrus bilaterally predicted more accurate navigation to the hidden target object in the retrieval phase. By contrast, during the retrieval phase, more accurate performance was associated with increased activity in the left hippocampus and the striatum bilaterally. Dividing participants into good and poor navigators, based upon behavioural performance, revealed greater striatal activity in good navigators during retrieval, perhaps reflecting superior procedural learning in these individuals. By contrast, the poor navigators showed stronger left hippocampal activity, suggesting reliance on a less effective verbal or symbolic code by this group. Our findings suggest separate neural substrates for the encoding and retrieval stages of object location memory during active navigation, which are further modulated by participants' overall navigational ability.     

Age differences in mental scanning of locomotor maps

Purpose.?To investigate whether the metric quality of mental spatial representations of the environment changes with aging, by means of mental scanning of locomotor space.

Method.?Twenty young adults (age 21.1?–?30.8) and 20 old adults (age 60.4?–?76.4) were submitted to a battery of psychometric tests to assess the effects of aging on general cognitive functions and specific visuo-spatial abilities, and to determine which cognitive factors correlated with scanning performance. The mental scanning task followed. Participants firstly had to learn a path comprising six positions by walking and observing these; secondly, they had to use mental imagery to scan from a first to a second position, according to different distances.

Results.?Aging reduces the validity of the metric information incorporated by mental spatial representations. Attentional and working memory resources, ability to manipulate spatial information and a general factor of abstract visuo-spatial reasoning strongly correlated with scanning performance. Finally, aging seemed to have a selective effect on cognitive functioning.

Conclusions.?Age differences emerge when demanding storage and processing of information are simultaneously required, as predicted by the Limited Resources view of aged cognition.     

The neural networks underlying endogenous auditory covert orienting and reorienting

Auditory information communicated through vocalizations, music, or sounds in the environment is commonly used to orient and direct attention to different locations in extrapersonal space. The neural networks subserving attention to auditory space remain poorly understood in comparison to our knowledge about attention in the visual system. The present study investigated whether a parietal-prefrontal right-hemisphere network controls endogenous orienting and reorienting of attention to the location of sounds just as it does for visual-spatial information. Seventeen healthy adults underwent event-related functional magnetic resonance imaging (FMRI) while performing an endogenous auditory orienting task, in which peripheral cues correctly (valid) or incorrectly (invalid) specified the location of a forthcoming sound. The results showed that a right precuneus and bilateral temporal-frontal network mediated the reorienting of auditory attention at both short and long stimulus onset asynchronies (SOAs). In contrast, the more automatic stage of auditory reorienting at the shorter SOA was associated with activation in a bilateral inferior parietal-frontal oculomotor network. These findings suggest that the reorienting of auditory attention is generally supported by a similar inferior parietal-frontal network as visual attention, but in both hemispheres. However, peripheral auditory cues also appear to elicit an automatic orienting response to the spatial location of a sound followed by a period of reduced processing of information that occurs in the same location later in time.     

Cognitive Bias Modification of Attention is Less Effective Under Working Memory Load

Cognitive bias modification for attentional bias (CBM-A) attempts to alleviate anxiety by training an attentional bias away from threat. Several authors have argued that CBM-A in fact trains top-down, reactive counteraction of the tendency to orient towards threat. Imposing a working memory (WM) load during training should therefore limit its efficacy, since WM resources are required for goal-driven control of attention. Twenty-eight subclinical high-anxious participants completed two sessions of CBM-A or placebo training: one under a high WM load, and one under a low WM load. Attentional bias was assessed after each training. CBM-A produced an attentional bias away from threat under low load, but not under high load. These results suggest CBM-A trains top-down counteraction of orienting to threat. It also suggests the administration of CBM-A in the home environment may be affected by everyday worries and distractions.     

Distinct spatial patterns of brain activity associated with memory storage and search

The time it takes for a human participant to decide whether a given stimulus is an element of a remembered set increases approximately linearly with the number of elements in the set. Here we tested for and detected a spatial pattern of brain activity whose magnitude of expression during this memory search process correlates with set size. We then tested the idea that memory search simply involves a re-activation of neurons involved in remembering the set by statistically comparing the patterns of brain activity corresponding to memory search and set size dependent working memory maintenance. These patterns were significantly different, suggesting that memory search and working memory maintenance are mediated by distinct neural mechanisms.     

Working memory load modulates the auditory "What" and "Where" neural networks

Working memory for sound identity (What) and sound location (Where) has been associated with increased neural activity in ventral and dorsal brain regions, respectively. To further ascertain this domain specificity, we measured fMRI signals during an n-back (n=1, 2) working memory task for sound identity or location, where stimuli selected randomly from three semantic categories (human, animal, and music) were presented at three possible virtual locations. Accuracy and reaction times were comparable in both "What" and "Where" tasks, albeit worse for the 2-back than for the 1-back condition. The analysis of fMRI data revealed greater activity in ventral and dorsal brain regions during sound identity and sound location, respectively. More importantly, there was an interaction between task and working memory load in the inferior parietal lobule (IPL). Within the right IPL, there were two sub-regions modulated differentially by working memory load: an anterior ventromedial region modulated by location load and a posterior dorsolateral region modulated by category load. These specific changes in neural activity as a function of working memory load reveal domain-specificity within the parietal cortex.     

Unity and diversity of tonic and phasic executive control components in episodic and working memory

The present study aimed to delineate the extent to which unitary executive functions might be shared across the separate domains of episodic and working memory. A mixed blocked/event-related functional magnetic resonance imaging (fMRI) design was employed to assess sustained (tonic control) and transient (phasic control) brain responses arising from incrementing executive demand (source versus item episodic memory - vis-à-vis - two-back versus one-back working memory) using load-dependent activation overlaps as indices of common components. Although an extensive portion of the regional load effects constituted differential control modulations in both sustained and transient responses, commonalities were also found implicating a subset of executive core mechanisms consistent with unitary or domain general control. 'Unitary' control modulations were temporally dissociated into (1) shared tonic components involving medial and lateral prefrontal cortex, striatum, cerebellum and superior parietal cortex, assumed to govern enhanced top-down context processing, monitoring and sustained attention throughout task periods and (2) stimulus-synchronous phasic components encompassing posterior intraparietal sulcus, hypothesized to support dynamic shifting of the 'focus of attention' among internal representations. Taken together, these results converge with theoretical models advocating both unity and diversity among executive control processes.     

Spatial working memory deficits in obsessive compulsive disorder are associated with excessive engagement of the medial frontal cortex

Recent studies have shown that obsessive compulsive disorder (OCD) is associated with a specific deficit in spatial working memory, especially when task difficulty (i.e., working memory load) is high. It is not clear whether this deficit is associated with dysfunction of the brain system that subserves spatial working memory, or whether it is associated with a more generalized effect on executive functions. In contrast to studies in healthy volunteers and schizophrenia, spatial working memory in OCD has not been investigated before using functional neuroimaging techniques. We conducted a functional MRI study in 11 treatment-free female patients with OCD and 11 for sex-, age-, education-, and handedness pairwise-matched healthy controls in order to assess performance on a parametric spatial n-back task as well as the underlying neuronal substrate and its dynamics. Patients with OCD performed poorly at the highest level of task difficulty and engaged the same set of brain regions as the matched healthy controls. In this set, the effect of difficulty on magnitude of brain activity was the same in patients and in controls except for a region covering the anterior cingulate cortex. In this region activity was significantly elevated in patients with OCD at all levels of the parametric task. These findings do not provide evidence for a deficit of the spatial working memory system proper, but suggest that the abnormal performance pattern may be secondary to another aspect of executive dysfunctioning in OCD.     

Content-specific activation during associative long-term memory retrieval

We tested whether visual stimulus material that is assumed to be processed in different cortical networks during perception (i.e., faces and spatial positions) is also topographically dissociable during long-term memory recall. With an extensive overlearning procedure, 12 participants learned paired associates of words and faces and words and spatial positions. Each word was combined with either one or two positions or one or two faces. fMRI was recorded several days later during a cued recall test, in which two words were presented and the participants had to decide whether these were linked to each other via a common mediator, i.e., a face or a position. This paradigm enforces retrieval from long-term memory without confounding recall with perceptual processes. A network of cortical areas was found to be differently activated during recall of positions and faces, including regions along the dorsal and ventral visual pathways, such as the parietal and precentral cortex for positions and the left prefrontal, temporal (including fusiform gyrus) and posterior cingulate cortex for faces. In a subset of these areas, the BOLD response was found to increase monotonically with the number of the to-be-re-activated associations. These results show that material-specific cortical networks are systematically activated during long-term memory retrieval that overlap with areas also activated by positions and faces during perceptual and working memory tasks.     

fMRI BOLD response to increasing task difficulty during successful paired associates learning

We used functional magnetic resonance imaging (fMRI) to assess cortical activations associated with increasing task difficulty (TD) in a visuospatial paired associates learning task. Encoding and retrieval were examined when 100% successful retrieval of three, four, or six object-location pairs had been attained (thus ensuring that performance was matched across subjects). As memory load increased, in general, the number of attempts taken to achieve 100% successful retrieval increased, while the number of trials correctly completed on the first attempt decreased. By modelling parametric variations in working memory load with BOLD signal changes we were able to identify brain regions displaying linear and nonlinear responses to increasing load. During encoding, load-independent activations were found in occipitoparietal cortices (excluding the precuneus for which linear load dependency was demonstrated), anterior cingulate, and cerebellum, while linear load-dependent activations in these same regions were found during retrieval. Nonlinear load-dependent responses, as identified by categorical contrasts between levels of load, were found in the right DLPFC and left inferior frontal gyrus. The cortical response to increasing cognitive demands or TD appears to involve the same, rather than an additional, network of brain regions "working harder."     

Neuropsychological function in children following admission to paediatric intensive care: a pilot investigation

OBJECTIVE: Little is known about neuropsychological status following acute severe paediatric illness. This pilot study explored the effects on memory function of severe acute paediatric illness and associations between memory functioning and psychiatric sequelae. DESIGN AND SETTING: Case control study of children after paediatric intensive care unit (PICU) discharge and healthy volunteers. PATIENTS AND PARTICIPANTS: 16 PICU discharged children comprised of 11 boys and 5 girls (mean age 9.44 +/- 2.85 years) tested a mean of 4.8 +/- 1.4 months following hospital discharge, and 16 age- and sex-matched controls. MEASUREMENTS AND RESULTS: Visual-spatial memory and attention were assessed using the CANTAB battery (visual memory) and verbal memory with the Children Memory Scale; Intelligence Quotient was tested using the Wechsler Abbreviated Scale of Intelligence. Emotional and behavioural function was measured with the Strengths and Difficulties Questionnaire and Impact of Event Scales. Children admitted to PICU displayed statistically poorer performance on tests of spatial memory (spatial working memory) sustained attention (rapid visual information) and verbal memory (word pairs learning and delayed recognition). Septic illness was specifically associated with poorer pattern recognition memory on the CANTAB. There were significant correlations in the PICU group between cognitive functioning and emotional/behaviour scores. CONCLUSIONS: Our results suggest impaired memory and attention in children following acute severe paediatric illness, a specific deficit in children with septic illness and links between memory anomalies and emotional/behavioural problems. The findings and their clinical significance require replication and clarification in a larger sample.