Maintaining and shifting attention within left or right hemifield

Positron emission tomography (PET) was used to examine two questions: (i) which structures of the intact human brain change their activity with the direction of attention to left or right visual field; and (ii) how does activity in these structures, and in parietal cortex in particular, depend on the frequency of attentional shifts? Subjects were required to discriminate the orientation of peripheral gratings. The two main experimental variables were the attended hemifield (left or right) and the proportion of trials requiring a shift within that hemifield (20% or 80%). A detection control condition was also included. Behaviourally, subjects were less accurate and significantly slower when a trial required a shift than when it did not. Ventral and lateral occipital areas showed significantly higher blood flow levels contralateral to the direction of attention. Replicating previous work, there was also a significant main effect of the direction of attention in left lateral prefrontal cortex: blood flow levels were higher during leftward attention in comparison both to baseline and to rightward attention. This left frontal effect reached significance in single subjects in whom several activation sites could be distinguished within left middle and inferior frontal gyrus. Right and left parietal cortex were activated during both left- and right-field attention conditions, with a tendency for higher activity levels when attention was directed contralaterally. Contrary to the experimental hypothesis, however, parietal regions were not activated differentially by high versus low numbers of attentional shifts. The current experiment confirms that left frontal convexity is sensitive to manipulations of the direction of visuospatial attention. The results do not indicate a specific role of parietal cortex in attentional shifting.     

Visual function and brain organization in non-decussating retinal-fugal fibre syndrome

Functional neuroimaging, psychophysical and electrophysiologicalinvestigations were performed in a patient with non-decussatingretinal–fugal fibre syndrome, an inborn achiasmatic statein which the retinal projections of each eye map entirely tothe ipsilateral primary visual cortex. Functional magnetic resonanceimaging (fMRI) studies showed that for monocularly presentedsimple visual stimuli, only the ipsilateral striate cortex wasactivated. Within each hemisphere's striate cortex, the representationof the two hemifields overlapped extensively. Despite this grossmiswiring, visual functions that require precise geometricalinformation (such as vernier acuity) were normal, and therewas no evidence for the confounding of visual information betweenthe overlapping ipsi-lateral and contralateral representations.Contrast sensitivity and velocity judgments were abnormal, buttheir dependence on the orientation and velocity of the targetssuggests that this deficit was due to ocular instabilities,rather than the miswiring per se. There were no asymmetriesin performance observed in visual search, visual naming or illusorycontour perception. fMRI analysis of the latter two tasks undermonocular viewing conditions indicated extensive bilateral activationof striate and prestriate areas. Thus, the remarkably normalvisual behavior achieved by this patient is a result of boththe plasticity of visual pathways, and efficient transfer ofinformation between the hemispheres.     

Fractionating attentional control using event-related fMRI

Despite numerous functional neuroimaging and lesion studies of human executive function, the precise neuroanatomical correlates of specific components of attentional control remain controversial. Using a novel approach that focused upon volunteer behavior rather than experimental manipulations, specific components of attentional shifting were fractionated, and their neural correlates differentiated using event-related fMRI. The results demonstrate that the ventrolateral prefrontal cortex is involved in switching attention "between" stimulus dimensions, whereas the posterior parietal cortex mediates changes in stimulus-response mapping. Furthermore, reversals based on negative feedback activated the lateral orbitofrontal cortex, whereas positive feedback modulated activity in the medial orbital frontal cortex. Finally, the dorsolateral prefrontal cortex was active throughout solution search. These findings support the hypothesis that lateral prefrontal, orbital, and parietal areas form a supervisory network that controls the focus of attention and suggests that these regions can be fractionated in terms of their specific contributions.     

Parieto-frontal interactions in visual-object and visual-spatial working memory: evidence from transcranial magnetic stimulation.

This study aimed to investigate whether transcranial magnetic stimulation (TMS) can induce selective working memory (WM) deficits of visual-object versus visual-spatial information in normal humans. Thirty-five healthy subjects performed two computerized visual n-back tasks, in which they were required to memorize spatial locations or abstract patterns. In a first series of experiments, unilateral or bilateral TMS was delivered on posterior parietal and middle temporal regions of both hemispheres after various delays during the WM task. Bilateral temporal TMS increased reaction times (RTs) in the visual-object, whereas bilateral parietal TMS selectively increased RTs in the visual-spatial WM task. These effects were evident at a delay of 300 ms. Response accuracy was not affected by bilateral or unilateral TMS of either cortical region. In a second group of experiments, bilateral TMS was applied over the superior frontal gyrus (SFG) or the dorsolateral prefrontal cortex (DLPFC). TMS of the SFG selectively increased RTs in the visual-spatial WM task, whereas TMS of the DLPFC interfered with both WM tasks, in terms of both accuracy and RTs. These effects were evident when TMS was applied after a delay of 600 ms, but not one of 300 ms. These findings confirm the segregation of WM buffers for object and spatial information in the posterior cortical regions. In the frontal cortex, the DLPFC appears to be necessary for WM computations regardless of the stimulus material.     

Directing attention to locations and to sensory modalities: multiple levels of selective processing revealed with PET

We used positron emission tomography (PET) to investigate the neural correlates of selective attention in humans. We examined the effects of attending to one side of space versus another (spatial selection) and to one sensory modality versus another (intermodal selection) during bilateral, bimodal stimulation of vision and touch. Attention toward one side resulted in greater activity in several contralateral areas. In somatosensory cortex, these spatial attentional modulations were found only when touch was relevant. In the intraparietal sulcus, spatial attentional effects were multimodal, independent of the modality attended. In occipital areas, spatial modulations were also found during both visual and tactile attention, indicating that tactile attention can affect activity in visual cortex; but occipital areas also showed more activity overall during visual attention. This suggests that while spatial attention can exert multimodal influences on visual areas, these still maintain their specificity for the visual modality. Additionally, irrespective of the attended side, attending to vision activated posterior parietal and superior premotor cortices, while attending to touch activated the parietal operculi. We conclude that attentional selection operates at multiple levels, with attention to locations and attention to modalities showing distinct effects. These jointly contribute to boost processing of stimuli at the attended location in the relevant modality.     

Retinotopy and attention in human occipital, temporal, parietal, and frontal cortex

Novel mapping stimuli composed of biological motion figures were used to study the extent and layout of multiple retinotopic regions in the entire human brain and to examine the independent manipulation of retinotopic responses by visual stimuli and by attention. A number of areas exhibited retinotopic activations, including full or partial visual field representations in occipital cortex, the precuneus, motion-sensitive temporal cortex (extending into the superior temporal sulcus), the intraparietal sulcus, and the vicinity of the frontal eye fields in frontal cortex. Early visual areas showed mainly stimulus-driven retinotopy; parietal and frontal areas were driven primarily by attention; and lateral temporal regions could be driven by both. We found clear spatial specificity of attentional modulation not just in early visual areas but also in classical attentional control areas in parietal and frontal cortex. Indeed, strong spatiotopic activity in these areas could be evoked by directed attention alone. Conversely, motion-sensitive temporal regions, while exhibiting attentional modulation, also responded significantly when attention was directed away from the retinotopic stimuli.     

Distinct causal influences of parietal versus frontal areas on human visual cortex: evidence from concurrent TMS-fMRI

It has often been proposed that regions of the human parietal and/or frontal lobe may modulate activity in visual cortex, for example, during selective attention or saccade preparation. However, direct evidence for such causal claims is largely missing in human studies, and it remains unclear to what degree the putative roles of parietal and frontal regions in modulating visual cortex may differ. Here we used transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) concurrently, to show that stimulating right human intraparietal sulcus (IPS, at a site previously implicated in attention) elicits a pattern of activity changes in visual cortex that strongly depends on current visual context. Increased intensity of IPS TMS affected the blood oxygen level-dependent (BOLD) signal in V5/MT+ only when moving stimuli were present to drive this visual region, whereas TMS-elicited BOLD signal changes were observed in areas V1-V4 only during the absence of visual input. These influences of IPS TMS upon remote visual cortex differed significantly from corresponding effects of frontal (eye field) TMS, in terms of how they related to current visual input and their spatial topography for retinotopic areas V1-V4. Our results show directly that parietal and frontal regions can indeed have distinct patterns of causal influence upon functional activity in human visual cortex.     

大鼠健侧C7移位后大脑皮层可塑性变化的fMRI研究

目的研究大鼠健侧C7移位后跨大脑两半球可塑性的动态变化。方法SD大鼠42只,随机分为正常对照组和手术组,手术组又分为术后2、8、16、20、28、40周组,每组6只,左侧肢体为患侧(全臂丛神经根性撕脱伤),右侧肢体为健侧。建立全臂丛神经根性撕脱伤模型,一期施行健侧C7神经根移位术,通过BOLD成像技术来观察移位术后双侧大脑皮层可塑性变化。结果术后2周、8周刺激患肢前爪,fMRI显示双侧大脑皮层无激活区出现;术后16周在同侧皮层出现激活区,对侧皮层无激活区出现;术后20周同侧皮层有激活区,激活区较16周激活区大,对侧皮层仍无激活区出现;术后28周对侧皮层和同侧皮层均有激活区出现,对侧皮层为1个激活区,同侧皮层多个激活区;术后40周同侧皮层无激活区出现,对侧皮层激活区高度集中,只有一个。结论健侧C7移位后可以成功实现跨正常大脑两半球的功能重组,但其机制与神经通路尚不明确。     

Neural basis for priming of pop-out during visual search revealed with fMRI

Maljkovic and Nakayama first showed that visual search efficiency can be influenced by priming effects. Even "pop-out" targets (defined by unique color) are judged quicker if they appear at the same location and/or in the same color as on the preceding trial, in an unpredictable sequence. Here, we studied the potential neural correlates of such priming in human visual search using functional magnetic resonance imaging (fMRI). We found that repeating either the location or the color of a singleton target led to repetition suppression of blood oxygen level-dependent (BOLD) activity in brain regions traditionally linked with attentional control, including bilateral intraparietal sulci. This indicates that the attention system of the human brain can be "primed," in apparent analogy to repetition-suppression effects on activity in other neural systems. For repetition of target color but not location, we also found repetition suppression in inferior temporal areas that may be associated with color processing, whereas repetition of target location led to greater reduction of activation in contralateral inferior parietal and frontal areas, relative to color repetition. The frontal eye fields were also implicated, notably when both target properties (color and location) were repeated together, which also led to further BOLD decreases in anterior fusiform cortex not seen when either property was repeated alone. These findings reveal the neural correlates for priming of pop-out search, including commonalities, differences, and interactions between location and color repetition. fMRI repetition-suppression effects may arise in components of the attention network because these settle into a stable "attractor state" more readily when the same target property is repeated than when a different attentional state is required.     

Effects of enriched environment and fluid percussion injury on dendritic arborization within the cerebral cortex of the developing rat

We have recently demonstrated that fluid percussion injury (FPI) sustained early in life prevents the neural plasticity response associated with rearing in an enriched environment (EE). In order to determine if this reduction in plasticity capacity is reflected in alterations in dendritic arborization, the present study examined dendritic changes in response to EE, FPI, and FPI followed by EE. Twenty postnatal day 19-20 rat pups were subjected to FPI or sham injury and were subsequently housed in EE (17 days) or standard conditions. Brains were processed according to the Golgi-Cox method and were analyzed using dendritic density (Sholl) and dendritic branching analyses in frontal, parietal, and occipital cortices. Rearing in EE induced an increase in dendritic density, primarily within the occipital cortex. FPI induced an increase in dendritic density, primarily in regions remote from the injury site, namely contralateral parietal cortex and ipsilateral and contralateral occipital cortex. In injured animals subsequently housed in EE, FPI appeared to inhibit the experience-dependent dendritic density effects of EE. However, an unexpected enhancement of dendritic density was seen in the ipsilateral occipital cortex, indicating a unique response of this region based on its distance-specific sensitivity to injury-induced plasticity and its region-specific sensitivity to experience-dependent plasticity. These results suggest that dendritic changes mediate the anatomical and behavioral changes characteristic of impaired developmental plasticity following FPI, and that these changes are dependent on location within the cerebral cortex.     

Bilateral activation of fronto-parietal networks by incrementing demand in a working memory task

Working memory (WM) is known to activate the prefrontal cortex. In the present study we hypothesized that when additional contingencies are added to the instruction of a WM task, this would increase the WM load and result in the activation of additional prefrontal areas. With positron emission tomography we measured regional cerebral blood flow in nine subjects performing a control task and two delayed matching to sample tasks, in which the subjects were matching colours and patterns to a reference picture. The second of the two delayed matching tasks had a more complex instruction than the first, with additional contingencies of how to alternate between the matching of colours and patterns. This task thus required the subjects not only to remember a stimulus to match but also to perform this matching according to a specified plan. Both delayed matching tasks activated cortical fields in the middle frontal gyrus, the frontal operculum, upper cingulate gyrus, inferior parietal cortex and cortex lining the intraparietal sulcus, all in the left hemisphere. When alternated delayed matching was compared to simple delayed matching, increases were located in the right superior and middle frontal gyrus and the right anterior inferior parietal cortex. The increased demand during alternated matching thus resulted in bilateral activation of both dorsolateral prefrontal and inferior parietal cortex. The area in the inferior parietal cortex has previously been coactivated with the dorsolateral prefrontal cortex in several WM tasks, irrespective of the sensory modality of the stimuli, and during tasks involving planning.      

Anatomical physiology of spatial extinction

Neurologically intact volunteers participated in a functional magnetic resonance imaging experiment that simulated the unilateral (focal) and bilateral (global) stimulations used to elicit extinction in patients with hemispatial neglect. In peristriate areas, attentional modulations were selectively sensitive to contralaterally directed attention. A higher level of mapping was observed in the intraparietal sulcus (IPS), inferior parietal lobule (IPL), and inferior frontal gyrus (IFG). In these areas, there was no distinction between contralateral and ipsilateral focal attention, and the need to distribute attention globally led to greater activity than either focal condition. These physiological characteristics were symmetrically distributed in the IPS and IFG, suggesting that the effects of unilateral lesions in these 2 areas can be compensated by the contralateral hemisphere. In the IPL, the greater activation by the bilateral attentional mode was seen only in the right hemisphere. Its contralateral counterpart displayed equivalent activations when attention was distributed to the right, to the left, or bilaterally. Within the context of this experiment, the IPL of the right hemisphere emerged as the one area where unilateral lesions can cause the most uncompensated and selective impairment of global attention (without interfering with unilateral attention to either side), giving rise to the phenomenon of extinction.     

Competition in working memory reduces frontal guidance of visual selection

Working memory (WM) representations can bias visual selection to matching stimuli in the field. WM biases can, however, be modulated by the level of cognitive load, with WM guidance reduced as memory load increases. Here, we used functional magnetic resonance imaging to distinguish between competing hypotheses for the reduction of WM guidance under load: 1) poor neural representations of memory contents under high load, 2) strategic control at high loads to direct attention away from search distracters matching the WM content, and 3) reduction of frontal top-down biasing of visual areas with increasing memory loads. We show that matching between WM contents and the visual display appeared to be well represented in visual areas under high memory loads, despite a lack of WM guidance at the behavioral level. There was little engagement of "cognitive control" areas in the prefrontal cortex during search at high loads. More importantly, WM guidance at low loads engaged a set of frontal regions in the superior and inferior ventral frontal cortex. Functional connectivity analyses revealed frontal regions working in concert with occipital areas at low memory loads, but this coupling was disrupted by increased memory load. We discuss the implications for understanding the mechanisms supporting the interplay between WM and attention.     

Mechanical noxious stimuli cause bilateral activation of parietal operculum in callosotomized subjects

The patterns of cortical activation evoked by tactile and mechanical painful stimulation in six normal subjects and three patients with complete resection of the corpus callosum are described and compared, with emphasis on the parietal operculum. Stimulus-related cortical activation was investigated by functional magnetic resonance imaging. In both groups, painful stimulation activated the first somatosensory, insular and cingulate cortices in the contralateral hemisphere, and the parietal opercular cortex in both hemispheres. Comparison between the two patterns of cortical activation demonstrated that ipsilateral activation by unilateral painful stimulation is at least partially independent of the corpus callosum and suggests a different organization of the pain and touch systems.     

Active Representation of Shape and Spatial Location in Man

Neural activity during the delay period of spatial delayed response(DR) and delayed matching (DM) tasks was investigated by positronemission tomography. A distributed cortical system was activatedin each condition. The bilateral dorsolateral prefrontal cortex(DLPFC) was activated in the delay period of both tasks; activationwas of higher significance on the right in the DR task and theleft in the DM task, and extended to the anterolateral prefrontalcortex in the DM condition. Active representation of spatiallocation in the DR task was associated with co-activation ofthe medial and lateral parietal cortex and the extrastriatevisual cortex. Active representation of shape in the DM taskwas associated with co-activation of the medial and lateralparietal cortex and the inferior temporal cortex. Response-relatedactivity was observed in both tasks. Activation of anteriorcingulate, inferior frontal, lateral premotor and rostral inferiorparietal cortex was observed in the DR condition, a task characterizedby preparation of a movement to a predetermined location. Incontrast, preparation to move to an undetermined location inthe DM task was associated with activation predominantly inrostral SMA.     

Unilateral deficits in visual perception and learning after unilateral inferotemporal cortex lesions in macaques

This study adapted the method of partial lesions, combined with controlled fixation, to study the perceptual role of macaque inferotemporal (IT) cortex. Unilateral lesions were made in IT cortex of three monkeys, without section of the corpus callosum, and visual function was tested ipsilateral and contralateral to the lesion. The observed changes were compared to the effects of bilateral lesions of IT cortex in one monkey, the approach used in most previous studies. Unilateral lesions produced far less profound, although more selective, loss on the tested visual abilities than did bilateral lesions. All three monkeys with unilateral lesions showed decreased chromatic sensitivity, but sparing of achromatic sensitivity, and severely disrupted learning and performance of visual matching to sample, and in all cases, the visual loss was contralateral to the site of the lesion. Unexpectedly, the magnitude of the contralateral loss was not increased by later section of the corpus callosum and anterior commissure in one of the monkeys, a lesion that removes interhemispheric input to contralateral from ipsilateral temporal cortex neurons. These results support physiological findings that show that the response of IT cortex neurons is dominated by the contralateral visual field, despite the bilateral activation many IT neurons receive. Comparison to earlier studies of lesions of area V4, which provides input to IT cortex, shows that V4 and IT lesions produce qualitatively different effects.     

Distribution of activity across the monkey cerebral cortical surface, thalamus and midbrain during rapid, visually guided saccades

To examine the distribution of visual and oculomotor activity across the macaque brain, we performed functional magnetic resonance imaging (fMRI) on awake, behaving monkeys trained to perform visually guided saccades. Two subjects alternated between periods of making saccades and central fixations while blood oxygen level dependent (BOLD) images were collected [3 T, (1.5 mm)3 spatial resolution]. BOLD activations from each of four cerebral hemispheres were projected onto the subjects' cortical surfaces and aligned to a surface-based atlas for comparison across hemispheres and subjects. This surface-based analysis revealed patterns of visuo-oculomotor activity across much of the cerebral cortex, including activations in the posterior parietal cortex, superior temporal cortex and frontal lobe. For each cortical domain, we show the anatomical position and extent of visuo-oculomotor activity, including evidence that the dorsolateral frontal activation, which includes the frontal eye field (on the anterior bank of the arcuate sulcus), extends anteriorly into posterior principal sulcus (area 46) and posteriorly into part of dorsal premotor cortex (area 6). Our results also suggest that subcortical BOLD activity in the pulvinar thalamus may be lateralized during voluntary eye movements. These findings provide new neuroanatomical information as to the complex neural substrates that underlie even simple goal-directed behaviors.     

Reversible Deficit in Haptic Delay Tasks from Cooling Prefrontal Cortex

The main purpose of this study was to explore the role of dorsolateralprefrontal cortex in skilled and sequential haptic performance.Monkeys were trained to perform a delayed matching-to-sampletask that required thememorization of three-dimensional objectsperceived either by palpation (haptically) or by sight. At thestart of a trial the animal was allowed to touch or view anobject, the sample; after a period of delay, during which theobject remained out of touch and out of sight, the animal waspresented with two side-by-side objects—- one of themthe sample—-for either tactile or visual recognition,and the choice of the sample (correct match) was rewarded. Threevariants of the task were used: (1) visual sample, haptic match;(2) haptic sample, visual match; and (3) haptic sample, hapticmatch. The temporary bilateral cooling of dorsolateral prefrontalcortex to 15°C induced a reversible deficit in performanceof all three tasks. Cooling to the same degree a portion ofposterior parietal cortex of equivalent size did not significantlyalter either performance or reaction time. These findings indicatethat the functional integrity of the dorsolateral prefrontalcortex is important for performance of sequential behavior dependenton haptic skill. Further, the results suggest that the roleof this cortex in active memory, already well documented forspatially and nonspatially defined visual information, extendsalso to tactile information and associated motor acts.     

A contralateral preference in the lateral occipital area: sensory and attentional mechanisms

Here we examined the level of the lateral occipital (LO) area within the processing stream of the ventral visual cortex. An important determinant of an area's level of processing is whether it codes visual elements on both sides of the visual field, as do higher visual areas, or prefers those in the contralateral visual field, as do early visual areas. The former would suggest that LO, on one side, combines bilateral visual elements into a whole, while the latter suggests that it codes only the parts of forms. We showed that LO has a relative preference for visual objects in the contralateral visual field. LO responses were influenced by attention. However, relative changes in LO activity caused by changes in object location were preserved even when attention was shifted away from the objects to moving random dot patterns on the opposite side. Our data offer a new view on LO as an intermediate, but not a high-level, visual area in which neurons are driven by visual input and spatial attention in a multiplicative fashion.     

Tactile attention tasks enhance activation in somatosensory regions of parietal cortex: a positron emission tomography study.

We used positron emission tomography to study cortical regions mediating tactile attention. Cues selectively directed subjects to attend to the roughness or duration of contact with embossed gratings that rubbed against a single fingertip with controlled speed and force. The task required discriminating between paired gratings that differed in tactile features of roughness and/or length. For different blocks of trials, cues directed attention to one tactile feature or indicated a divided attention strategy to a change in either feature. All attention conditions unambiguously activated several somatosensory foci in the parietal cortex, including somatotopically appropriate portions of the primary somatosensory cortex in the postcentral gyrus (S1) and the secondary somatosensory region (S2) within parietal opercular regions. There was no evidence for separate processing foci for selective and divided attention strategies, or for selectively attending to roughness versus stimulus duration. We observed a greater magnitude blood flow change in S2 versus S1 during attention tasks, which suggests that S2 might actually influence S1 activity. Despite these differences, modulation of S1 and S2 supports concepts of early selection in tactile attention. There were also examples of non-sensory foci in frontal cortex, anterior cingulate gyrus and bilateral superior parietal regions at the fundus of the postcentral sulcus. Posterior parietal regions observed in this study did not overlap foci seen in studies of visual attention. Thus, the posterior parietal region may be subdivided into modality-specific subregions, each of which processes information needed to attend to a specific modality. These non-sensory areas may constitute a network that provides a source of modulating influences on the earlier stage, sensory areas.