Neural Circuits for Spatial Attention and Unilateral Neglect

Object-based inhibition of return in three-dimensional space: From simple drawings to real objects

Cued to an object in space, inhibition of the attended location can spread to the entire object. Although object-based inhibition of return (IOR) studies in a two-dimensional plane have been documented, the IOR has not been explored when objects cross depth in three-dimensional (3D) space. In the present study, we used a virtual reality technique to adapt the double-rectangle paradigm to a 3D space, and manipulated the cue validity and target location to examine the difference in object-based IOR between far and near spaces under different object representations. The study showed that the object-based IOR of simple drawings existed only in near space, whereas object-based IOR of real objects existed only in far space at first, and as the object similarity decreases, it appeared in both far and near spaces.

Physically real and virtual reality exposed line bisection response patterns: visuospatial attention allocation in virtual reality

INTRODUCTION

To understand the nature of hemispatial attention allocation in virtual reality (VR), a line bisection task (LBT) was administered both in a real environment and a virtual environment to assess the rate of pseudoneglect. The mental construction of real and virtual environments was assumed to increase visuospatial activity in right hemisphere-related cognitive processes; an alteration in the activity that manifests in the direction and rate of line bisection lateral error.

METHODS

In the present study, fifty-one right-handed healthy college students were recruited. They performed a line bisection task in real and virtual environments.

RESULTS

The obtained data showed that LBT errors in real and VR environments were correlated and individually consistent. Furthermore, a leftward LBT error was found in the physically real environment, however, in a VR the line bisection bias drifted towards the right hemispace. Participants with a lower right-handedness score showed a lower rate of left LBT bias in a real environment, but in VR, their LBT error showed a stronger rightwards error.

DISCUSSION

Participants showed an individually consistent pattern in both real and VR environments, but VR-induced visuospatial reality construction was associated with rightward LBT bias in a virtual environment.

Spatial Attention Disregard in Children With Hemiplegic Cerebral Palsy

IMPORTANCE

Children with hemiplegic cerebral palsy (CP) demonstrate spatial attention disregard, but the rehabilitation approach to CP is traditionally motor oriented.

OBJECTIVE

To explore spatial attention disregard in children with hemiplegic CP and its relationship to their motor performance in daily activities.

DESIGN

Cross-sectional study.

SETTING

Community.

PARTICIPANTS

Twenty-five children with hemiplegic CP and 25 age-matched typically developing children.

OUTCOMES AND MEASURES

For spatial attention performance, the Random Visual Stimuli Detection Task; for developmental disregard, the Observatory Test of Capacity, Performance, and Developmental Disregard; and for motor performance, the Melbourne Assessment 2.

RESULTS

Children with hemiplegic CP evidenced spatial attention disregard on their more affected sides, and this phenomenon was correlated with developmental disregard.

CONCLUSIONS AND RELEVANCE

Children with hemiplegic CP demonstrate developmental disregard in both the motor and the visual-spatial attention domains. Including evaluation of and intervention for visual-spatial attention for children with hemiplegic CP in the traditionally motor-oriented rehabilitation approach is recommended.

WHAT THIS ARTICLE ADDS

This research provides evidence that children with hemiplegic CP demonstrate disregard in the domain of visual-spatial attention. The findings suggest that evaluation of and intervention for visual-spatial attention should be included in CP rehabilitation in addition to the traditionally motor-oriented approach.

Mild Parkinsonian Signs in a Hospital-based Cohort of Mild Cognitive Impairment Types: A Cross-sectional Study

Background:

Mild Parkinsonian Signs (MPS) have been associated with Mild Cognitive Impairment (MCI) types with conflicting results.

Objective:

To investigate the association of individual MPS with different MCI types using logistic ridge regression analysis, and to evaluate for each MCI type, the association of MPS with caudate atrophy, global cerebral atrophy, and the topographical location of White Matter Hyperintensities (WMH), and lacunes.

Methods:

A cross-sectional study was performed among 1,168 subjects with different types of MCI aged 45-97 (70,52 ± 9,41) years, who underwent brain MRI. WMH were assessed through two visual rating scales. The number and location of lacunes were also rated. Atrophy of the caudate nuclei and global cerebral atrophy were assessed through the bicaudate ratio, and the lateral ventricles to brain ratio, respectively. Apolipoprotein E (APOE) genotypes were also assessed. Using the items of the motor section of the Unified Parkinson’s Disease Rating Scale, tremor, rigidity, bradykinesia, and gait/balance/axial dysfunction were evaluated.

Results:

Bradykinesia, and gait/balance/axial dysfunction were the MPS more frequently encountered followed by rigidity, and tremor. MPS were present in both amnestic and non-amnestic MCI types, and were associated with WMH, lacunes, bicaudate ratio, and lateral ventricles to brain ratio.

Conclusion:

MPS are present in both amnestic and non-amnestic MCI types, particularly in those multiple domain, and carrying the APOE ε4 allele. Cortical and subcortical vascular and atrophic processes contribute to MPS. Long prospective studies are needed to disentangle the contribution of MPS to the conversion from MCI to dementia.

EPS Mid-Career Award 2004: Brain mechanisms of attention

There are many varieties of “attention”, to some extent separate yet working together to produce coherent perception, thought, and behaviour. Using data from human behaviour, functional neuroimaging, and single-cell recording in the behaving monkey, I consider different levels of attention and their basis in physiological mechanisms of biased competition. Beginning with visual attention, I suggest that processing is competitive in many brain systems that code visual input. Competition is biased towards stimuli that match task requirements and is integrated between systems coding different object properties. The result is flexible, object-based attentional selection. In the second part of the paper, I describe recent experiments on attentional competition within and between sensory modalities. Though competition is often modality specific, more global levels of interference are also easy to demonstrate. In the third part of the paper, I move to frontoparietal cortex and to a pattern of similar brain regions recruited by many different cognitive demands. This multiple-demand (MD) pattern, I suggest, reflects neurons with highly flexible response properties, adapting to represent the information and events of many different tasks. Biased competition in MD regions may play a central role in broad attentional capacity limits and attentional focusing. More generally, I suggest that biased competition is characteristic of many different cognitive domains and brain systems. Coherent “attention” develops as different systems converge to work on related cognitive content.

Reducing Neglect in Adult Hemiplegia: Recent Findings and Implications for Treatment

Neglect is a disorder of attentional and intentional processes most commonly observed following a cerebrovascular accident. Recent research indicates that neglect can be reduced by priming neural circuits unaffected by cerebral lesions; the more effective remediation strategies are those that prime neural circuits that are under some voluntary control, although new strategies that influence involuntary attentional mechanisms show therapeutic potential. The implications of recent studies involving single-case and group remediation efforts are discussed with respect to treatment and the theoretical context for understanding disorders of attention.

[Treatment of neglect: new therapy approaches]

Neglect is a supramodal, clinically relevant disorder, which occurs in the different sensory modalities as well as in the mental representation. The different manifestations of neglect show interindividual and intraindividual variation. Different spatial and attention-related interventions lead to varying degrees of improvement in neglect symptoms but are often not stable in time or transferable to activities of daily living. Representational deficits or body-related aspects to modify personal neglect are rarely a priority in current therapies. Because of the multisensory and motor deficits from our point of view the future of neglect rehabilitation lies in a combined therapy of visual exploration, motor imagery with intensive motor therapy of motor sensory deficits and probably best reinforced by continued neck muscle vibration or allocation of attention to the neglected side.

The effect of voluntary trunk rotation and half-field eye-patching for patients with unilateral neglect in stroke: a randomized controlled trial

Objective : To investigate the effectiveness of voluntary trunk rotation and half-field eye-patching to treat patients with unilateral neglect in stroke.

Design : Pre—post, day 60 follow-up, single-blinded randomized controlled trial.

Setting : Single-centre inpatient rehabilitation hospital.

Subjects : Sixty subacute patients with right hemisphere stroke having unilateral neglect within eight weeks post stroke consented to participate between November 2003 and July 2005. They were randomly assigned to three comparison groups.

Interventions : Nineteen patients received daily experimental training in voluntary trunk rotation (TR) for 1 hour five times a week for 30 days. Twenty patients received the same kind of treatment together with half-field eye-patching

(TR + EP). Fifteen patients in the control group received conventional training with the same contact time.

Main outcome measures : Patients were assessed on days 0, 30 and 60 using the Behavioural Inattention Test, the Clock Drawing Test, and the Functional Independence Measure.

Results : No significant differences between voluntary trunk rotation (TR), voluntary trunk rotation and half-field eye-patching (TR + EP) and controls were found in functional performance and neglect measures at day 30 (P = 0.042—0.994) and follow-up (P = 0.052—0.911) at P = 0.005 using Bonferroni correction.

Conclusions : The results of this study do not support the use of voluntary trunk rotation alone or with half-field eye-patching to improve functional performance or reduce unilateral neglect in subacute patients with stroke.

Specialization of the motor system in infancy: from broad tuning to selectively specialized purposeful actions

In executing purposeful actions, adults select sufficient and necessary limbs. But infants often move goal-irrelevant limbs, suggesting a developmental process of motor specialization. Two experiments with 9- and 12-month-olds revealed gradual decreases in extraneous movements in non-acting limbs during unimanual actions. In Experiment 1, 9-month-olds produced more extraneous movements in the non-acting hand/arm and feet/legs than 12-month-olds. In Experiment 2, analysis of the spatiotemporal dynamics of infants' movements revealed developmental declines in the spatiotemporal coupling of movements between acting and non-acting arms. We also showed that the degree of specialization in infants' unimanual actions is associated with individual differences in motor experience and visual attention, indicating the experience-dependent and broad functional nature of these developmental changes. Our study provides important new insights into motor development: as in cognitive domains, motor behaviours are initially broadly tuned to their goal, becoming progressively specialized during the first year of life.

Effects of contralesional robot-assisted hand training in patients with unilateral spatial neglect following stroke: a case series study

Background

A reduction of hemispatial neglect due to stroke has been associated with activation of the contralesional hand in the contralesional hemispace. Robot-assisted upper limb training was found to effectively improve paretic arm function in stroke patients. To date no proof of concept of robot-assisted hemispatial neglect therapy has been reported in literature. This study aimed to determine whether robot-assisted left (contralesional) hand activation alone could lead to an improvement in hemispatial neglect following stroke.

Methods

Three stroke patients with right brain injury underwent a 2-week training program of robotic left hand activation with the Gloreha® hand rehabilitation glove, which provides repetitive, passive mobilization of the fingers. Outcomes were assessed using the Line Crossing test, the Bells test, the Sentence Reading test, the Saccadic Training, the Sustained Attention to Response Task, and the Purdue Pegboard test.

Results

Changes were observed after treatment as follows. Line Crossing test: all patients showed improved performance (6.7%, 89.5% and 80% increase in lines crossed) with two patients reaching normal performance levels. Bells test: one patient improved performance (50% increase), while one patient showed no change and one patient declined (−10.3% change); no patient reached normal performance levels. Sentence Reading test: all patients showed improved performance (800%, 57.1% and 42.9% increase in number of sentences read) with no patient reaching normal performance level. Saccadic Training: all patients showed improved performance (−62.8%, −15.5% and −9.7% change of the left hemifield reaction time). Sustained Attention to Response Task: all patients showed improved performance (−20.5%, −5.8% and −10% change of the reaction time) with two patients reducing incorrect responses (−42.9% and −73.3%) and one patient increasing them (9.1%). Purdue Pegboard test: all patients showed improved performance (100%, 27.3% and 75% change in the left + right + both hands sub-item score).

Conclusions

Some caution is warranted when interpreting our results, as the responses to the intervention were variable and might have been due to a placebo effect or fluctuating clinical conditions. However, robot-assisted hemispatial neglect therapy might be useful in stroke patients. Larger-scale investigations are needed to confirm our preliminary findings.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-0003-11-160) contains supplementary material, which is available to authorized users.

Neglect-like characteristics of developmental disregard in children with cerebral palsy revealed by event related potentials

BACKGROUND

Children with unilateral Cerebral Palsy (CP) often show diminished awareness of the remaining capacity of their affected upper limb. This phenomenon is known as Developmental Disregard (DD). DD has been explained by operant conditioning. Alternatively, DD can be described as a developmental delay resulting from a lack of use of the affected hand during crucial developmental periods. We hypothesize that this delay is associated with a general delay in executive functions (EF) related to motor behavior, also known as motor EFs.

METHODS

Twenty-four children with unilateral CP participated in this cross-sectional study, twelve of them diagnosed with DD. To test motor EFs, a modified go/nogo task was presented in which cues followed by go- or nogo-stimuli appeared at either the left or right side of a screen. Children had to press a button with the hand corresponding to the side of stimulus presentation. Apart from response accuracy, Event-Related Potentials (ERPs) extracted from the ongoing EEG were used to register covert cognitive processes. ERP N1, P2, N2, and P3 components elicited by cue-, go-, and nogo-stimuli were further analyzed to differentiate between different covert cognitive processes.

RESULTS

Children with DD made more errors. With respect to the ERPs, the P3 component to go-stimuli was enhanced in children with DD. This enhancement was related to age, such that younger children with DD showed stronger enhancements. In addition, in DD the N1 component to cue- and go-stimuli was decreased.

CONCLUSIONS

The behavioral results show that children with DD experience difficulties when performing the task. The finding of an enhanced P3 component to go-stimuli suggests that these difficulties are due to increased mental effort preceding movement. As age in DD mediated this enhancement, it seems that this increased mental effort is related to a developmental delay. The additional finding of a decreased N1 component in DD furthermore suggests a general diminished visuo-spatial attention. This effect reveals that DD might be a neuropsychological phenomenon similar to post-stroke neglect syndrome that does not resolve during development. These findings suggest that therapies aimed at reducing neglect could be a promising addition to existing therapies for DD.

Mapping the functional neuroanatomy of spatial neglect and human parietal lobe functions: progress and challenges

Spatial neglect is generally defined by various deficits in processing information from one (e.g., left) side of space contralateral to focal (e.g., right) hemisphere damage. Although classically associated with parietal lobe functions, there is now compelling evidence that neglect can follow lesions in many different cortical and subcortical sites, suggesting a dysfunction in distributed brain networks. In addition, neglect is likely to result from a combination of distinct deficits that co-occur due to concomitant damage affecting juxtaposed brain areas and their connections, but the exact nature of core deficits and their neural substrates still remains unclear. The present review describes recent progress in identifying functional components of the neglect syndrome and relating them to distinct subregions of parietal cortex. A comprehensive understanding of spatial neglect will require a more precise definition of cognitive processes implicated in different behavioral manifestations, as well as meticulous mapping of these processes onto specific brain circuits, while taking into account functional changes in activity that may arise in structurally intact areas subsequent to damage in distant portions of the relevant networks.

Crossing the hands disrupts tactile spatial attention but not motor attention: evidence from event-related potentials

During covert shifts of tactile spatial attention both somatotopic and external reference frames are employed to encode hand location. When participants cross their hands these frames of references produce conflicting spatial codes which disrupt tactile attentional selectivity. Because attentional shifts are triggered not only in Attention tasks but also during covert movement preparation, the present study aimed at investigating the reference frame employed during such 'motor shifts of attention'. Event related brain potentials (ERPs) were recorded during a Motor task where a visual cue (S1) indicated the relevant hand for a manual movement prior to a tactile Go/Nogo stimulus (S2). For comparison, we ran a tactile Attention task where the same cue (S1) now indicated the relevant hand for a tactile discrimination (S2). Both tasks were performed under uncrossed and crossed hands conditions. In both Attention and Motor tasks similar lateralized components were observed following S1 presentation. Anterior and posterior ERP components indicative of covert attention shifts were exclusively guided by an external reference frame, while a later central negativity operated according to a somatotopic reference frame in both tasks. In the Motor task, this negativity reflected selective activation of the motor cortex in preparation for movement execution. In the Attention task, this component might reflect activity in the somatosensory cortex in preparation for the subsequent tactile discrimination. The presence of multiple and conflicting spatial codes resulted in disruption of tactile attentional selection in the Attention task where attentional modulations of tactile processing were delayed and attenuated with crossed hands as indicated by the analysis of ERPs elicited by S2. In contrast, attentional modulations of S2 processing in the Motor task were largely unaffected by the hand posture manipulation, suggesting that motor attention employs primarily one spatial coordinate system.

Isolating the internal in endogenous attention

Neuroimaging studies have provided evidence that a bilateral frontal-parietal network is involved in voluntary attentional control. However, because those studies used instructive cue stimuli, some of the activity may have been due to interactions between cue processing and voluntary orienting. Here, we show that self-initiated voluntary orienting, in the absence of any cue stimulus, evokes activity in this frontal-parietal network. In contrast to the typical symmetric activity observed with cued attentional shifts, self-initiated shifts showed a hemispheric asymmetry consistent with studies of unilateral neglect patients. Specifically, the right hemisphere was equally involved in orienting to either visual field, whereas the left hemisphere was biased toward the contralateral field. Our data show that the asymmetry of attentional control can be revealed in neuroimaging of healthy subjects, when voluntary orienting is effectively isolated.

Motor neglect: Implications for movement and rehabilitation following stroke

PURPOSE

The syndrome of unilateral neglect following stroke is associated with poor outcome and presents significant challenges to those providing therapy for affected individuals. In contrast to a number of reviews which have recently appeared in therapy and rehabilitation journals relating to sensory aspects of neglect, this review focuses on 'motor neglect'.

SEARCH STRATEGY

We searched the clinical and scientific literature for papers concerning motor neglect. The search included the databases Web of Science, Pubmed and Cinahl, primarily using the term 'motor neglect'. There was also a large degree of secondary searching involved.

DISCUSSION

Motor neglect refers to the under-utilization of a limb opposite a brain lesion that cannot be fully explained by primary sensory and motor deficits. The paper discusses classical descriptions of motor neglect and highlights the difficulties in disentangling motor neglect from hemiparesis. The related problem of motor extinction is introduced as a useful clinical measure of neglect-related movement difficulties and a significant clinical problem in its own right.

CONCLUSION

Motor neglect is a relatively under-recognized deficit which may have a significant impact on patient performance and recovery following stroke. We conclude with a discussion of the implications of motor neglect for rehabilitation, including the relative contributions that may be made by Constraint-induced movement therapy and Bilateral movement therapy in managing patients with neglect-related movement problems.

Crossing the pillars of Hercules: the role of spatial frames of reference in error making

This paper introduces a perspective accounting for errors that may occur as a result of the human interaction with the three-dimensional (3D) space. Particularly, we are interested in errors that are caused by cognitively crossing the boundary between peripersonal and extrapersonal space. Several behavioural, neurophysiological, and neuropsychological perspectives are reviewed in order to develop the theoretical framework on which our ideas are founded. We argue that cognitively crossing the boundary implies switching from one frame of reference to another, which, as a consequence, implies specific costs. The results of five experiments (overall involving 76 participants) support our hypothesis, suggesting the existence of different action-oriented spatial frames of reference.

Functional Recovery After Hemiplegia in Patients With Neglect

Background and Purpose— The aim of this study was to verify whether the presence of anosognosia (A) affects the rehabilitative prognosis of hemiplegic subjects with neglect (N).

Methods— This study was carried out on 30 patients with left hemiplegia: 15 patients had neglect (group N) and 15 had neglect and anosognosia (group N+A). Mean age was 68.2±6.3 in group N (9 men and 6 women) and 72.1±6.4 in group N+A (7 men and 8 women). The average interval from onset of stroke to admission for rehabilitation was 23 and 23.6 days, respectively, in group N and in group N+A. Patients were assessed through the Mesulam test, Bisiach test, Wechsler Adult Intelligence Scale, Fugl-Meyer scale, Functional Independence Measure (FIM), and Rankin scale.

Results— Before rehabilitation, cognitive FIM scores of patients of group N were significantly higher than those of group N+A ( P =0.001), whereas motor FIM scores and total FIM scores did not differ between the 2 groups. After rehabilitation, cognitive FIM scores ( P =0.000) and even motor ( P =0.009) and total FIM scores ( P =0.000) were statistically higher in group N than in group N+A. Effectiveness ( P =0.005) and efficiency ( P =0.012) in the motor FIM scores of group N were significantly greater than those of group N+A. Disability was lower in group N ( P =0.040).

Conclusions— Our study shows that the presence of anosognosia worsens the rehabilitation prognosis in hemiplegic subjects who also have neglect.

The nature and contribution of space- and object-based attentional biases to free-viewing perceptual asymmetries

Two experiments investigated the contribution of space- and object-based coordinates to previously reported leftward perceptual biases (pseudoneglect) at various locations across visual space. Neurologically intact participants (n = 34 and 27) made luminance discriminations between two left/right mirror-reversed luminance gradients (greyscales task), which were variously displaced around the midline in the participants' left and right hemispaces. The orientations of the stimuli were manipulated so that object- and space-based coordinates were congruent or incongruent. Experiment 1 confirmed the presence of a leftward object-based perceptual bias. The bias was moderated, however, by overattention to the more central stimulus. This central spatial effect could have resulted from the use of task-specific strategies, which were controlled by presenting the stimuli sequentially in Experiment 2. The findings of Experiment 1, a leftward object-based bias and a central spatial bias, were replicated. Overall, the results indicate a leftward object-based bias and a central spatial bias, both of which are relevant for the allocation of attention. The results are discussed with reference to a variety of models of the distribution of attention across space.

Selective rotation of egocentric spatial representation following right putaminal hemorrhage

Although the role of frontoparietal cortex in spatial egocentric processing is well established, recent animal-lesion and human functional imaging studies have suggested that the neostriatum may also be a critical modulator in the processing of body-centred spatial orientation. We describe here a patient with right putamen-centred hemorrhage who exhibited a consistent counterclockwise rotation of approximately 90 degrees when drawing and writing from memory. A more detailed assessment with a series of representational clock tests demonstrated that the rotation was present only in tasks requiring the use of egocentric cues. In the absence of external cues the patient would adopt and maintain a stable but incorrectly-oriented egocentric representation of the imagined or recollected object. By contrast, performance could be rectified by presentation of correctly-oriented stimuli. These findings suggest that the putamen is part of a circuit underlying egocentric, as opposed to allocentric, representation of space in humans.

Facial affect recognition in criminal psychopaths

Prior studies provide consistent evidence of deficits for psychopaths in processing verbal emotional material but are inconsistent regarding nonverbal emotional material. To examine whether psychopaths exhibit general versus specific deficits in nonverbal emotional processing, 34 psychopaths and 33 nonpsychopaths identified with Hare's (R. D. Hare, 1991) Psychopathy Checklist--Revised were asked to complete a facial affect recognition test. Slides of prototypic facial expressions were presented. Three hypotheses regarding hemispheric lateralization anomalies in psychopaths were also tested (right-hemisphere dysfunction, reduced lateralization, and reversed lateralization). Psychopaths were less accurate than nonpsychopaths at classifying facial affect under conditions promoting reliance on right-hemisphere resources and displayed a specific deficit in classifying disgust. These findings demonstrate that psychopaths exhibit specific deficits in nonverbal emotional processing.

Fast responses to neglected targets in visual search reflect pre-attentive processes: an exploration of response times in visual neglect

AE is a patient who suffered a right hemisphere stroke resulting in visual neglect symptoms. In the first experiment, AE neglected a single visual target that was present in half of the trials and appeared in variable and unpredictable positions on the computer screen. The contrast of the target to the screen's background was also varied. AE demonstrated severe neglect for left-sided targets, and yet his RTs to targets reported incorrectly as absent were faster than correct rejections and even right-sided hits. AEs fast "neglect" responses seem to indicate that the target was detected but that he remained unaware of its presence. Counter intuitively, his fast misses got faster as the discriminability of the target decreased. The possibility that fast responses to neglected targets reflected a guessing strategy, used proportionally to the degree of uncertainty of a target presence, was examined. AEs fast misses were indeed faster at lower level of contrast of the stimulus, but his error rate did not tend to approach the chance level as the guessing model would predict. In a second experiment, AE searched for the letter Z, present on half of the trials, among variable sets of distractor letters. In one condition the distractors were all O's and therefore differed from the target by an elementary feature. In the other condition, the distractors were various letters that differed from the target by combinations of features. The key finding was that fast responses to neglected targets occurred only in the simple feature search task and not in the complex features (conjunction) task. We interpret these findings as indicating that AEs pre-attentive processing can detect pop-out targets on the left-hand side, but that the attentional search is faulty and is aborted early. Hence, the patient's attentional system has an "early start" when "pop-out" forms are present, but can also fail to "grab" the detected target; consequently, by not attending to a stimulus, the patient remains unaware of its presence and will quickly respond "no" to present targets.

Line bisection in normal adults: direction of attentional bias for near and far space

The direction of attentional bias in forty normal adults was assessed using a computer generated line bisection task. A 4 (viewing distance)x4 (line length)x2 (cursor starting position) repeated measures factorial design was employed. As predicted, differences in bisection performance as a function of viewing distance were observed. The findings confirmed that scanning direction (contingent upon the starting position of the cursor), but not line length, significantly modulated this effect. The direction of bias across near and far space was further clarified yielding a progressive shift from a leftward bias in near space to a rightward bias in far space. A significant interaction of distance, line length and starting position revealed differential effects for left and right starting positions as a function of viewing distance and line length. More specifically, a leftward start witnessed deviations shifting progressively from left-to-right as distance and line length increased though no comparable pattern was observed for rightward starts. The results provide important behavioural support for the suggestion that dissociated neural systems may be responsible for attending and acting in near and far space and that other lateralised functions (such as scanning strategies) can influence hemispheric activation. The findings have relevant theoretical implications as well as important implications for the clinical assessment of unilateral neglect using a standard line bisection task, both of which are discussed.

Head-centred meridian effect on auditory spatial attention orienting

Six experiments examined the issue of whether one single system or separate systems underlie visual and auditory orienting of spatial attention. When auditory targets were used, reaction times were slower on trials in which cued and target locations were at opposite sides of the vertical head-centred meridian than on trials in which cued and target locations were at opposite sides of the vertical visual meridian or were not separated by any meridian. The head-centred meridian effect for auditory stimuli was apparent when targets were cued by either visual (Experiments 2, 3, and 6) or auditory cues (Experiment 5). Also, the head-centred meridian effect was found when targets were delivered either through headphones (Experiments 2, 3, and 5) or external loud-speakers (Experiment 6). Conversely, participants showed a visual meridian effect when they were required to respond to visual targets (Experiment 4). These results strongly suggest that auditory and visual spatial attention systems are indeed separate, as far as endogenous orienting is concerned.

Do neck-proprioceptive and caloric-vestibular stimulation influence covert visual attention in normals, as they influence visual neglect?

Neck-proprioceptive and caloric-vestibular stimulation have been shown to ameliorate the spatial bias exhibited by patients suffering from unilateral visual neglect. These interventions might in principle have their effect by biasing covert attention towards the neglected side. If so, the same interventions should also modulate covert attention in neurologically-intact subjects. However, we demonstrate here that neither neck-proprioception (vibration of left neck muscles) nor caloric-vestibular stimulation (injection of iced water into the left ear) affect covert visual attention in healthy individuals. These results from normals may distinguish between different accounts for unilateral neglect in patients. In particular, they argue against explanations of neglect solely in terms of a pathological misperception of body orientation within an otherwise normal neural representation of space.

Attention switching in depth using random-dot autostereograms: attention gradient asymmetries

Random-dot autostereograms (RDASs) were used to investigate attention shifts along the sagittal plane in distractor-free tasks of high perceptual load. In three experiments using a same/different comparison task, the shape of the gradient over five different depths was examined and the conditions under which the gradient is and is not observed were compared. When the target set consisted of five similar objects, a robust asymmetric depth gradient was observed. When the target set consisted of two dissimilar objects, no gradient was observed. The results support a hypothesis of a viewer-centered asymmetric attention gradient in the depth plane that is dependent on perceptual or attentional load defined by target-set discriminability.

Effects of hand-used on unilateral spatial neglect: a case study

It was originally claimed that left hand use on line bisection tasks reduced the extent of left neglect by Halligan and Marshall. However, in a following study, the same research group failed to reproduce this left hand amelioration effect if the left hand was initially placed on the right side, and they claimed that spatio-motor cueing was more important in reducing unilateral neglect than the hand used. The present study concerns with the validity of these two theoretical views on modification of unilateral neglect, i.e., hemispheric activation and spatio-motor cueing. A patient with left unilateral neglect and a slight left hemiparesis participated in three experiments. Under conventional testing condition, line bisection performed with the right hand showed more severe left neglect than when performed with the left hand. These hand effects were modified by changing a starting position of the patient's hand when bisecting horizontal lines. However, under body-fixed condition, effects in hand-used as well as starting position were again significant. The results suggest that not only spatio-motor cueing but also differential hemispheric activation can exert a profound effect on unilateral neglect.

Lateralization of brain mechanisms underlying automatic and controlled forms of spatial orienting of attention

Unilateral Spatial Neglect (USN) can be considered as a defect in spatial orienting of attention toward the half space contralateral to the damaged hemisphere. Since the classical work of Brain, W.R. Brain 64:224-272 (1941), several authors have emphasized that this defect is definitely more frequent and severe in patients with right brain damage, but the reason for this hemispheric asymmetry has remained controversial. Several investigations conducted with different experimental paradigms in different laboratories have shown that only automatic orienting of attention toward stimuli arising in the half space contralateral to the damaged hemisphere is disrupted in USN, whereas controlled, volitional orienting can be more or less completely spared. The hypothesis that at the level of the right hemisphere, spatial orienting of attention may be mainly prompted by automatic mechanisms may, therefore, be advanced. By contrast, the left hemisphere might play a leading role in mechanisms underlying volitional orienting of attention. According to this model, recovery from USN could be due to substitution of the lost automatic orienting mechanisms with the spread volitional orienting mechanisms subserved by the intact left hemisphere.

Spatial attention in the mental architecture: evidence from neuropsychology

Using neuropsychological evidence, this paper examines whether spatial attention functions as a domain-specific module or as a more general-purpose central processor. Data are presented from two spatial attention cuing tasks completed by subjects, with an acquired attentional deficit, and control subjects. In both tasks, an arrow indicated with high probability the side of response (response task) or the side of space on which the stimulus would appear (visuospatial task). In the response task, the stimuli appeared foveally and the response component was lateralized, and in the visuospatial task, the stimuli were lateralized and the response component remained constant in the midline. Only the neglect subjects showed a disproportionate increase in reaction time on both the response and visuospatial tasks when the arrow cued the subject to the ipsilateral side and the stimulus or response was on the side of space contralateral to the lesion. The substantial association across the two tasks suggests that a common underlying internal spatial representation subserves perception and action. While this finding is consistent with Fodor's view of a cross-domain processor, it does not meet all of his criteria of a central processor. We conclude, therefore, that the posterior attentional mechanism is strictly neither a module nor a central processor. Rather, these results suggest that a common attentional mechanism may subserve behavior in domains that are tightly coupled.

Residual rightward attentional bias after apparent recovery from right hemisphere damage: implications for a multicomponent model of neglect

Unilateral neglect may be a multicomponent attentional disorder consisting of an initial automatic orienting of attention toward the ipsilesional side and a subsequent impairment in contralesionally reorienting attention, both of which are superimposed on a generalised reduction in attention resources. It has been hypothesised that patients' ability to reorient attention contralesionally may recover relatively quickly, but that the ipsilesional attention bias may be relatively persistent. This hypothesis was tested by consecutively examining 13 patients who had had a right hemisphere stroke, and who had left unilateral neglect. They were examined once shortly after the stroke and again 12 months later, using a battery of standard clinical and experimental tasks. Patients initially showed a strong and consistent rightward attentional bias in addition to a failure to reorient their attention leftward. After 12 months patients continued to show an abnormal ipsilesional attentional bias, though most were now able to fully reorient their attention toward the contralesional side. These results suggest that restitution of the capacity to reorient attention contralesionally may underlie the apparent recovery from clinical signs of unilateral neglect. The presence of a residual ipsilesional attentional bias in most patients, however, may, at least in part, account for the poor functional outcomes in some apparently "recovered" patients.

Spatiomotor cueing in unilateral left neglect: three case studies of its therapeutic effects

Limb activation contralateral to a cerebral lesion seems to reduce visual neglect, though whether this is due to perceptual cueing or hemispheric activation is controversial. Three case studies are presented which attempt to use this experimental finding therapeutically in the rehabilitation of unilateral left neglect. The first study used a combination of perceptual anchoring training with left arm activation procedures and produced improvements. The second used the same method, but stimulated left arm activation using an avoidance conditioning procedure, again with positive results. The third case treatment focused on cueing for left arm activation without explicit instructions for perceptual anchoring, with positive results.

Use of left vs right hand in responding to lateralized stimuli in unilateral neglect

Previous research suggests that contralesional limb activation may reduce the degree of visual neglect shown by hemi-inattentive subjects. The present study examines whether minimal assisted-left-limb activation (pressing a response key with the hemiplegic hand, aided by the ipsilesional hand) resulted in lowered response latencies on the neglected side in a group of six patients showing unilateral left visual neglect. The study was carried out using a simple computerized test of speed of detection of lateralized stimuli, with responses being made on a key located at the body midline. In five of the subjects, there was no evidence of relatively faster response times to contralesional stimuli when the contralesional limb was involved in the responses. In one of these five subjects, there was no hemiplegia, and hence full use of the contralesional limb. Only in one subject did an interaction appear between the side of presentation of the stimuli and limb used. In this case, a small but significant tendency appeared for the subject to make relatively faster responses to left-sided stimuli when using the left hand to make responses.