Attention,spatial representation,and visual neglect: simulating emergent attention and spatial memory in the selective attention for identification model (SAIM)

The selective attention for identification model (SAIM) is presented. This uses a spatial window to select visual information for recognition, binding parts to objects and generating translation-invariant recognition. The model provides a qualitative account of both normal and disordered attention. Simulations of normal attention demonstrate 2-object costs and effects of object familiarity on selection, global precedence, spatial cueing, and inhibition of return. When lesioned, SAIM demonstrated either view- or object-centered neglect or spatial extinction, depending on the type and extent of lesion. The model provides a framework to unify (a) object- and space-based theories of normal selection, (b) dissociations within the syndrome of unilateral neglect, and (c) attentional and representational accounts of neglect.     

Understanding the effects of moving visual stimuli on unilateral neglect following stroke

Moving visual stimuli have been shown to reduce unilateral neglect (ULN), however, the mechanisms underlying these effects remain poorly understood. This study compared lateralised and non-lateralised moving visual stimuli to investigate whether the spatial characteristics or general alerting properties of moving visual stimuli are responsible for reducing neglect. Post-stroke left neglect patients as well as healthy and patient control subjects were tested on a computerised line bisection task under six visual stimulus conditions. The key finding was that, relative to the no stimulus condition, leftward moving and left-sided moving visual stimuli shifted neglect patients' bisection errors leftward while the non-lateralised random moving visual stimuli did not reduce neglect patients' rightward bisection errors. The results provide evidence that spatial characteristics rather than general alerting properties of moving visual stimuli reduce rightward bisection errors in ULN. Moreover, the pattern of findings strongly supports the notion that moving visual stimuli reduce neglect by capturing attention and drawing it to a spatial location rather than by activating the attentional system via superior collicular neurons.     

Dopaminergic and cholinergic modulations of visual-spatial attention and working memory: insights from molecular genetic research and implications for adult cognitive development

Attention and working memory are fundamental for selecting and maintaining behaviorally relevant information. Not only do both processes closely intertwine at the cognitive level, but they implicate similar functional brain circuitries, namely the frontoparietal and the frontostriatal networks, which are innervated by cholinergic and dopaminergic pathways. Here we review the literature on cholinergic and dopaminergic modulations of visual-spatial attention and visual working memory processes to gain insights on aging-related changes in these processes. Some extant findings have suggested that the cholinergic system plays a role in the orienting of attention to enable the detection and discrimination of visual information, whereas the dopaminergic system has mainly been associated with working memory processes such as updating and stabilizing representations. However, since visual-spatial attention and working memory processes are not fully dissociable, there is also evidence of interacting cholinergic and dopaminergic modulations of both processes. We further review gene-cognition association studies that have shown that individual differences in visual-spatial attention and visual working memory are associated with acetylcholine- and dopamine-relevant genes. The efficiency of these 2 transmitter systems declines substantially during healthy aging. These declines, in part, contribute to age-related deficits in attention and working memory functions. We report novel data showing an effect of dopamine COMT gene on spatial updating processes in older but not in younger adults, indicating potential magnification of genetic effects in old age.     

Visual exploration pattern in hemineglect

The analysis of eye movement parameters in visual neglect such as cumulative fixation duration, saccade amplitude, or the numbers of saccades has been used to probe attention deficits in neglect patients, since the pattern of exploratory eye movements has been taken as a strong index of attention distribution. The current overview of the literature of visual neglect has its emphasis on studies dealing with eye movement and exploration analysis. We present our own results in 15 neglect patients. The free exploration behavior was analyzed in these patients presenting 32 naturalistic color photographs of everyday scenes. Cumulative fixation duration, spatial distribution of fixations in the horizontal and vertical plane, the number and amplitude of exploratory saccades was analyzed and compared with the results of an age-matched control group. A main result of our study was that in neglect patients, fixation distribution of free exploration of natural scenes is not only influenced by the left–right bias in the horizontal direction but also by the vertical direction.     

Space and the parietal cortex

Current views of the parietal cortex have difficulty accommodating the human inferior parietal lobe (IPL) within a simple dorsal versus ventral stream dichotomy. In humans, lesions of the right IPL often lead to syndromes such as hemispatial neglect that are seemingly in accord with the proposal that this region has a crucial role in spatial processing. However, recent imaging and lesion studies have revealed that inferior parietal regions have non-spatial functions, such as in sustaining attention, detecting salient events embedded in a sequence of events and controlling attention over time. Here, we review these findings and show that spatial processes and the visual guidance of action are only part of the repertoire of parietal functions. Although sub-regions in the human superior parietal lobe and intraparietal sulcus contribute to vision-for-action and spatial functions, more inferior parietal regions have distinctly non-spatial attributes that are neither conventionally 'dorsal' nor conventionally 'ventral' in nature.     

Attention accesses multiple reference frames: evidence from visual neglect

Research with normal participants has demonstrated that mechanisms of selective attention can simultaneously gain access to internal representations of spatial information defined with respect to both location- and object-based frames of reference. The present study demonstrates that patients with unilateral spatial neglect following a right-hemisphere lesion are poorer at detecting information on the contralateral left side in both location- and object-based spatial coordinates simultaneously. Moreover, the extent of the neglect is modulated by the probability of a target's appearing in either reference frame; as the probability of sampling a target in a particular frame of reference increases, so does the severity of neglect in the frame. These findings suggest that attention can be flexibly and strategically assigned to a reference frame depending on the contingencies of the task.     

发展性阅读障碍的视觉空间注意加工能力

鉴于阅读起始于基础视觉加工阶段, 越来越多的研究者开始关注阅读障碍者的视觉空间注意加工能力。视觉空间注意是指个体对视觉刺激的空间位置的注意, 可通过线索提示、视觉搜索和视觉注意广度等视觉任务来考察。大量国内外研究发现, 发展性阅读障碍者在视觉空间注意任务下表现出行为和神经活动方面的异常。其中的神经机制问题不仅反映在与视觉空间注意有关的顶叶区域激活异常, 还存在于脑区间功能连接异常(如顶叶区域与字形加工区的功能连接)。未来研究还需利用横断和追踪研究探讨阅读障碍与视觉空间注意能力发展关系的内在机制, 以及探究语言特性对阅读障碍者视觉空间注意缺陷的可能调节作用。     

Spatial probability cuing and right hemisphere damage

In this experiment we studied statistical learning, inter-trial priming, and visual attention. We assessed healthy controls and right brain damaged (RBD) patients with and without neglect, on a simple visual discrimination task designed to measure priming effects and probability learning. All participants showed a preserved priming effect for item color. Contrary to healthy controls and RBD participants without neglect, RBD participants with neglect did not show positional priming and both RBD groups learned the underlying spatial probability distribution of target locations to a lesser degree. To see if the latter deficiency could be improved, we tested a patient with long standing chronic spatial neglect on three separate days and observed improved identification times for left sided, high probability, targets. In summary, we found preservation of priming per se in people with spatial neglect. However, this was only clearly demonstrable for color priming and not for positional priming. Associated with this impairment was a difficulty in learning the overall statistical structure of target locations. In a patient with severe persistent neglect we were able to demonstrate that the deficit in statistical learning was not absolute, as this subject improved his identification times for targets appearing in high probability regions of the test display.     

Lexical processes and eye movements in neglect dyslexia

Neglect dyslexia is a disturbance in the allocation of spatial attention over a letter string following unilateral brain damage. Patients with this condition may fail to read letters on the contralesional side of an orthographic string. In some of these cases, reading is better with words than with non-words. This word superiority effect has received a variety of explanations that differ, among other things, with regard to the spatial distribution of attention across the letter string during reading. The primary goal of the present study was to explore the interaction between attention and lexical processes by recording eye movements in a patient (F.C.) with severe left neglect dyslexia who was required to read isolated word and non-word stimuli of various length. F.C.'s ocular exploration of orthographic stimuli was highly sensitive to the lexical status of the letter string. We found that: (1) the location to which F.C. directed his initial saccade (obtained approximately 230 ms post-stimulus onset) differed between word and non-word stimuli; (2) the patient spent a greater amount of time fixating the contralesional side of word than non-word strings. Moreover, we also found that F.C. failed to identify the left letters of a string despite having fixated them, thus showing a clear dissociation between eye movement responses and conscious access to orthographic stimuli. Our data suggest the existence of multiple interactions between lexical, attentional and eye movement systems that occur from very initial stages of visual word recognition.     

Spatial probability cuing and right hemisphere damage

In this experiment we studied statistical learning, inter-trial priming, and visual attention. We assessed healthy controls and right brain damaged (RBD) patients with and without neglect, on a simple visual discrimination task designed to measure priming effects and probability learning. All participants showed a preserved priming effect for item color. Contrary to healthy controls and RBD participants without neglect, RBD participants with neglect did not show positional priming and both RBD groups learned the underlying spatial probability distribution of target locations to a lesser degree. To see if the latter deficiency could be improved, we tested a patient with long standing chronic spatial neglect on three separate days and observed improved identification times for left sided, high probability, targets. In summary, we found preservation of priming per se in people with spatial neglect. However, this was only clearly demonstrable for color priming and not for positional priming. Associated with this impairment was a difficulty in learning the overall statistical structure of target locations. In a patient with severe persistent neglect we were able to demonstrate that the deficit in statistical learning was not absolute, as this subject improved his identification times for targets appearing in high probability regions of the test display.     

Spatial analysis after perinatal stroke: patterns of neglect and exploration in extra-personal space

This study was conducted to determine whether school-aged children who had experienced a perinatal stroke demonstrate evidence of persistent spatial neglect, and if such neglect was specific to the visual domain or was more generalized. Two studies were carried out. In the first, 38 children with either left hemisphere (LH) or right hemisphere (RH) damage and 50 age-matched controls were given visual cancellation tasks varying in two factors: target stimuli and stimulus array. In the second study, tactile neglect was evaluated in 41 children with LH or RH damage and 72 age-matched controls using a blindfolded manual exploration task. On the visual cancellation task, LH subjects omitted more target stimuli on the right, but also on the left, compared with controls. Children with RH lesions also produced a larger number of omissions on both the left and right sides than controls, but with poorer performance on the left. On the manual exploration task, LH children required significantly longer times to locate the target on both sides of the board than did controls. RH children had significantly prolonged search times on the left side, but not on the right, compared with controls. In both tasks, LH subjects employed unsystematic search strategies more often than both control and RH children. The search strategy of RH children also tended to be erratic when compared to controls, but only in the random arrays of the visual cancellation tasks; structure of the target stimuli improved their organization. These results demonstrate that children with early LH brain damage display bilateral difficulties in visual and tactile modalities; a pattern that is in contrast to that seen in adults with LH damage. This may result from disorganized search strategies or other subtle spatial or attentional deficits. Results of performance of RH children suggests the presence of contralateral neglect in both the visual and tactile modalities; a finding that is similar to the neglect in adult stroke patients with RH lesions. The fact that deficits in spatial attention and organizational strategies are present after very early focal damage to either the LH or the RH broadens our understanding of the differences in functional lateralization between the immature and mature brain. These results also add to evidence for limitations to plasticity in the developing brain. Our findings may have therapeutic and rehabilitative implications for the management of children with early focal brain lesions.     

Systematic analysis of deficits in visual attention

A variety of impairments in visual attention can follow damage to the brain. The authors develop systematic methods for analyzing such impairments in terms of C. Bundesen's (1990) Theory of Visual Attention and apply these in a group of 9 patients with parietal lobe lesions and variable spatial neglect. In whole report, patients report letters from brief, vertical arrays in left or right visual field. The results show substantial, largely bilateral impairments in processing capacity, implying a major nonlateralized aspect to neglect. In partial report, arrays contain 1 or 2 letters in red and/or green. The task is to report only those letters in a specified target color. In addition to the expected bias against left-sided letters, patients show striking, bilateral preservation of top-down control, or attentional priority for targets. The results show how differentiation of attentional impairments can be informed by a theory of normal function.     

Two spatially separated attention systems in the visual field: evidence from inhibition of return

It has been demonstrated that the human visual field shows some functional inhomogeneities, in particular when the central and perifoveal regions are compared to the more peripheral regions. The present study examined this inhomogeneity by examining the effect of stimulus eccentricity on inhibition of return (IOR), a phenomenon that biases our attention towards novel locations against returning it back to previously attended locations. Eighteen subjects were examined in a visual detection task, in which a target appeared randomly following a nonpredictive spatial cue in the visual field. The eccentricities of the cues and targets were systematically manipulated from 5° to 30° with 5° increments. Results showed that response times to targets that appeared at cued locations were significantly slower than those at uncued locations for all stimulus eccentricities, demonstrating the IOR effects. However, response times at cued locations increased significantly when stimulus eccentricity shifted from 15° to 20°, leading to a much stronger IOR effect at more peripheral regions compared to central and perifoveal regions, indicating a functional dissociation between these two regions of the visual field. Possible neural mechanisms underlying this dissociation are discussed, and two attention systems modulating the two functional regions of the visual field are put forward to best account the present finding implicating in particular midbrain mechanism.     

Remembering the past and imagining the future: a neural model of spatial memory and imagery

The authors model the neural mechanisms underlying spatial cognition, integrating neuronal systems and behavioral data, and address the relationships between long-term memory, short-term memory, and imagery, and between egocentric and allocentric and visual and ideothetic representations. Long-term spatial memory is modeled as attractor dynamics within medial-temporal allocentric representations, and short-term memory is modeled as egocentric parietal representations driven by perception, retrieval, and imagery and modulated by directed attention. Both encoding and retrieval/imagery require translation between egocentric and allocentric representations, which are mediated by posterior parietal and retrosplenial areas and the use of head direction representations in Papez's circuit. Thus, the hippocampus effectively indexes information by real or imagined location, whereas Papez's circuit translates to imagery or from perception according to the direction of view. Modulation of this translation by motor efference allows spatial updating of representations, whereas prefrontal simulated motor efference allows mental exploration. The alternating temporal-parietal flows of information are organized by the theta rhythm. Simulations demonstrate the retrieval and updating of familiar spatial scenes, hemispatial neglect in memory, and the effects on hippocampal place cell firing of lesioned head direction representations and of conflicting visual and ideothetic inputs.     

Attention-referenced visual representations: evidence from impaired visual localization

Spatial representations in the visual system were probed in 4 experiments involving A. H., a woman with a developmental deficit in localizing visual stimuli. Previous research (M. McCloskey et al., 1995) has shown that A. H.'s localization errors take the form of reflections across a central vertical or horizontal axis (e.g., a stimulus 30 degrees to her left localized to a position 30 degrees to her right). The present experiments demonstrate that A. H.'s errors vary systematically as a function of where her attention is focused, independent of how her eyes, head, or body are oriented, or what potential reference points are present in the visual field. These results suggest that the normal visual system constructs attention-referenced spatial representations, in which the focus of attention defines the origin of a spatial coordinate system. A more general implication is that some of the brain's spatial representations take the form of coordinate systems.     

Spatial distortion induced by imperceptible visual stimuli

Previous studies have explored the effects of attention on spatial representation. Specifically, in the attentional repulsion effect, a transient visual cue that captures attention has been shown to alter the perceived position of a target stimulus to the direction away from the cue. The effect is also susceptible to retrospective influence, that attention appears to attract the target when the cue appears afterwards. This study examined the necessity of visual awareness of the cue in these phenomena. We found that when the cues were rendered invisible by backward visual masks, both repulsion and attraction effects were weakened but still observed. The results suggest that the effects possibly depend on processes that are not necessarily associated with conscious visual awareness of the cues. We conjecture that attentional shift produced by the weak, invisible cues may play a role in spatial distortion; but other possible accounts including non-attentional ones are also discussed.     

The effect of spatial attention on invisible stimuli

The influence of selective attention on visual processing is widespread. Recent studies have demonstrated that spatial attention can affect processing of invisible stimuli. However, it has been suggested that this effect is limited to low-level features, such as line orientations. The present experiments investigated whether spatial attention can influence both low-level (contrast threshold) and high-level (gender discrimination) adaptation, using the same method of attentional modulation for both types of stimuli. We found that spatial attention was able to increase the amount of adaptation to low- as well as to high-level invisible stimuli. These results suggest that attention can influence perceptual processes independent of visual awareness.     

Interference in immediate spatial memory

It has been suggested that maintenance in visuospatial immediate memory involves implicit motor processes that are analogous to the articulatory loop in verbal memory. An alternative account, which is explored here, is that maintenance is based on shifts of spatial attention. In four experiments, subjects recalled spatial memory span items after an interval, and in a fifth experiment, digit span was recalled after an interval. The tasks carried out during the interval included touching visual targets, repeating heard words, listening to tones from spatially sepa-rated locations, pointing to these tones, pointing to visual targets, and categorizing spatial tar-gets as being from the left or right. Spatial span recall was impaired if subjects saw visual tar-gets or heard tones, and this impairment was increased if either a motor response or a categorical response was made. Repeating words heard in different spatial locations did not impair recall, but reading visually presented words did interfere. For digit span only, the tasks involving a verbal response impaired recall. The results are interpreted within a framework in which active spatial attention is involved in maintaining spatial items in order in memory, and is interfered with by any task (visual, auditory, perceptual, motor) that also makes demands on spatial attention.     

Transcranial alternating brain stimulation at alpha frequency reduces hemispatial neglect symptoms in stroke patients

Background/ObjectiveNon-invasive brain stimulation techniques such as transcranial alternating current stimulation (tACS) may help alleviate attention deficits in stroke patients with hemispatial neglect by modulating oscillatory brain activity. We applied high-definition (HD)-tACS at alpha frequency over the contralesional hemisphere to support unilateral oscillatory alpha activity and correct for the pathologically altered attention bias in neglect patients.MethodsWe performed a within-subject, placebo-controlled study in which sixteen stroke patients with hemispatial neglect underwent 10 Hz (alpha) as well as sham (placebo) stimulation targeting the contralesional posterior parietal cortex. Attentional bias was measured with a computerized visual detection paradigm and two standard paper-and-pencil neglect tests.ResultsWe revealed a significant shift of attentional resources after alpha-HD-tACS, but not sham tACS, toward the ipsilateral and thus contralesional hemifield leading to a reduction in neglect symptoms, measured with a computerized visual detection paradigm and a widely used standard paper and pencil neglect tests.ConclusionsWe showed a significant alpha-HD-tACS-induced shift of attentional resources toward the contralesional hemifield, thus leading to a reduction in neglect symptoms. Importantly, HD-tACS effects persisted after the stimulation itself had ended. This tACS protocol, based on intrinsic oscillatory processes, may be an effective and well-tolerated treatment option for neglect.     

Auditory and multisensory aspects of visuospatial neglect

Spatial neglect is a common neurological syndrome in which awareness of contralesional space is disrupted after unilateral (typically right) stroke. Although most research has focused on visual aspects of neglect, there is increasing evidence that neglect can often be multisensory. Here we focus on auditory disturbances that can co-occur with visual neglect. Patients selected for showing visual neglect often also show deficits in localization of single sounds; and also in the detection or identification of contralesional sounds, particularly in the presence of an ipsilesional competitor. Moreover, auditory and visual aspects of neglect often correlate in their severity across patients. These results are considered in relation to recent neuroimaging findings in the normal brain, showing that the brain regions typically damaged in neglect patients might contain multimodal representations of space.