Impaired orientation processing in hemispatial neglect

Patients with hemispatial neglect show deficits in size perception. We investigated how this effect would be modulated by a change in object orientation. Seven right-hemisphere-lesioned patients, with and without neglect, and a control group, were asked to indicate which one of two bilaterally presented lines was longer, shorter or the same. Depending on the participant's response, the length was increased or decreased in a staircase-like procedure. Line orientation was varied over separate blocks. All neglect patients judged a line on the left as shorter, predominantly for horizontal lines and lines rotated by 30 degrees. Moreover, the magnitude of the distortion effect varied considerably between patients from as little as 2% objective underestimation to as much as 20%.     

Impaired size processing for single objects after hemispatial neglect.

Some patients with hemispatial neglect show deficits in horizontal size perception. Most previous studies investigating this effect required the relative comparison of two horizontal stimuli. We examined whether the effect would also be present for single stimuli which would reflect an impairment in the computation of absolute horizontal length. Ten neglect patients, five with and five without hemianopia, and two control groups were asked to verbally judge the length of horizontal lines varying in length (3, 3.5, 4 inches) and spatial location (extreme left to extreme right). Three of the ten neglect patients judged a single object on the left as significantly smaller than a single object on the right, thus demonstrating a size processing deficit. This deficit was unrelated to hemianopia.     

Space re-exploration in hemispatial neglect

Exploration of the space around us is a fundamental part of human behaviour. When it breaks down there is an important opportunity to understand its underlying mechanisms. Here we show that many right-hemisphere patients with left neglect re-explore rightward locations, failing to keep track of them during search. Importantly, such re-exploration occurred despite leftward stimuli being indistinguishable in peripheral vision, so it is unlikely to result from implicit processing of neglected targets. Revisits generally occurred after visits to other targets and are therefore not immediate perseverations. Finally, manipulating the visual salience of found targets altered the degree of neglect, but not revisit rates. Space exploration appears to be modulated both by the ability to keep track of spatial locations and by stimulus salience.     

Selective attention in hemispatial neglect

To determine how increasing demands on visual selective attention affect the symptoms of hemispatial neglect, we studied patients with right hemispheric lesions on a cancellation task requiring various degrees of focused attention. In the target only condition, the patients were to cancel all stimuli. In the target-nontarget condition, discriminating targets from nontargets did not require close scrutiny, whereas in the target-foil condition, discriminating targets from foils required greater attention to detail. Our findings indicate that increasing demands on visual selective attention adversely affect both exploration of the left side of space and visual discrimination.     

Gaze-dependent hemianopia without hemispatial neglect

A hemispheric field is the space defined by the midsagittal plane of the body. Hemianopia, in the absence of hemispheric spatial neglect, has been thought to be purely retinotopic. Confrontation testing of visual fields in eccentric positions of gaze in hemianopic patients permits the discrimination of hemispheric from retinotopic deficits. In the primary position of gaze, a patient with an ischemic lesion of the right occipital and temporal lobes, who was without unilateral spatial neglect, was unable to detect finger movement, name objects, or identify shapes or colors in the left retinotopic hemifield, but when gazing 30 degrees to the right (so that the left retinotopic field was in the right hemispheric field), he performed nearly as well in the left retinotopic field as in the right. The mechanism of this gaze-dependent hemifield visual impairment is unknown, but we discuss four possibilities: (1) eccentric gaze enhances an alternative visual system; (2) the lesion led to modality-specific hemispheric inattention that was corrected by directing intention (plan to act) and gazing to the right; (3) the lesion led to modality specific inattention that was corrected by compensatory mechanisms that may normally enhance attentive vigilance in central space during eccentric gaze; and (4) the neuronal substrate for hemispheric vision was destroyed, whereas that for retinotopic vision was preserved.     

Mechanisms underlying hemispatial neglect.

If patients with left-sided hemispatial neglect bisect lines incorrectly because hemianopia or sensory hemiinattention prevents them from seeing how far the line extends to the left, a strategy that ensures their seeing the left side of the line in their normal field should improve performance. If patients have hemispatial hypokinesia, moving the line toward the normal half of body space should improve performance. Six patients with left-sided neglect from right hemisphere infarctions were required to identify a letter at either the right or the left end of a line before bisecting that line. The task was given with the lines placed at either the right, the center, or the left of the body midline. Performance in trials when subjects were required to look to the left before bisecting a line did not differ from when they were required to look right. Performance was significantly better when the line was placed to the right side of the body than to the left. These observations support the hypothesis that patients with hemispatial neglect have hemispatial hypokinesia. An alternative hypothesis is that these subjects had a hemispatial memory defect. Although they saw the left side of the line in their normal field, they were incapable of forming a stable trace and performed as if they did not see the left side of the line.     

Immediate and delayed reaching in hemispatial neglect

Milner and Goodale (The visual brain in action, Oxford: Oxford University Press, 1995; The visual brain in action, 2nd ed., Oxford: Oxford University Press, 2006) propose a model of vision that makes a distinction between vision for perception and vision for action. One strong claim of the model is that the dorsal stream's control of action is designed for dealing with target stimuli in the ‘here and now’, yet when time is allowed to pass and a reaction has to be made on the basis of a visual memory, the ventral stream is required for successful performance.Regarding the syndrome of hemispatial neglect, Milner and Goodale further claim that the visual dorsal stream is relatively spared in these patients. In the current study we tested whether neglect patients would indeed be unimpaired in immediate pointing, yet show inaccurate pointing in a condition where a delay is interposed between the presentation of the stimulus and the response signal (in particular in left space). We tested the ability of nine neglect patients (and healthy and right hemisphere no neglect control groups) to perform reaches towards immediate and delayed targets, placed in left, central and right locations. Neglect patients showed no accuracy impairments when asked to perform an immediate action. Conversely, when pointing towards remembered leftward locations, they markedly overshoot the target or failed to initiate a reach altogether. These results confirm that patients with neglect are not specifically impaired when performing ‘here and now’ actions, but rather present deficits when the visuomotor task taps into more perceptual ‘off-line’ representations thought to depend on ventral visual stream activation.     

Changing attentional demands in left hemispatial neglect.

Seven variations of a letter cancellation test were used to examine how varying attentional demands affect hemispatial neglect in patients with right hemisphere lesions. While the 14 targets always remained in the same location, the number of distractors (zero, nine, 28, or 82) as well as their complexity (one letter or nine different letters) were varied. The percentage of targets canceled in the left hemispace was linearly related to the number of distractors. There were no differences between the complexity conditions. In a second study, the same 14 targets were presented but the distractors (zero, 14, or 41) were all placed on the right. Increasing the number of distractors on the right increased neglect on both sides of the space. Taken together, these results suggest that, while the limited attentional resources of the left hemisphere are biased toward the right hemispace, the absence of contralateral attentional demands allows these resources to be directed ipsilaterally.     

Residual perceptual distortion in 'recovered' hemispatial neglect.

In most neglect patients, line bisection errors become smaller on repeated tests over the months following the lesion. We have tried to determine in two typical patients whether this is because of a real reduction in the perceptual distortions that appear to underlie line bisection errors in neglect, or whether it reflects a learned behavioural strategy to counteract those perceptual biases. We tested the patients on two occasions (2 and 12 months post-stroke), on line bisection and also on the 'Landmark' task. The data indicated that at the first testing session both patients showed strong 'perceptual' neglect, making large rightward errors in the standard bisection task and uniformly leftward pointing in the Landmark task. On the second occasion, as expected, both patients showed a marked recovery when tested with the line bisection task, making only very small errors. In contrast, their landmark performance was still markedly biased in the same direction as before. These findings suggest that despite their apparent recovery on the bisection task, both patients still experience some form of perceptual distortion of horizontal lines. It is suggested that the Landmark task may provide a sensitive means for identifying real recovery of the underlying perceptual deficit.     

Prism adaptation improves visual search in hemispatial neglect

Visuomotor prism adaptation has been found to induce a lateral bias of spatial attention in chronic hemispatial neglect patients. Here, two experiments were conducted to explore the effects of 10° prism adaptation on visual search tasks and standard visual inattention tests. Baselines and intervention effects were measured on separate days for all patients. The first experiment explored whether prism adaptation affects performance on a time restricted visual search task (maximum 3500 ms presentation followed by visual and auditory feedback). No positive effects of prism adaptation were found on accuracy in visual search nor on traditional neglect tests. These results accord well with previous studies showing that increased cognitive load can lead to prism de-adaptation or unchanged performance following prism adaptation. Response times in visual search became faster following intervention but this was not the case for the standard neglect tests. In the second experiment, the same single-featured search task was used, but the participants had unlimited search time and received no feedback on their response. This time, the patients showed accuracy improvements in visual search and all four on regular neglect tests. Therapeutic effects lasted for at least 90-120 min. Response times on all tasks became faster after prism adaptation. The results are consistent with studies showing effects of prism adaptation on neuropsychological neglect tests and other attentional tasks that are not speeded or time restricted, where feedback is not provided, or are performed following non-feedback-based tasks. The current findings show that prism adaptation improves visual search in neglect and that these beneficial effects can disappear with feedback.     

Line quadrisection errors in patients with hemispatial neglect

Patients with left unilateral neglect misbisect lines toward the right. To discriminate between contralesional unawareness and ipsilesional hyperattention hypotheses for this ipsilesional bias, we performed the line quadrisection test on 18 patients with and 25 without neglect, and 24 normal controls. Overall the patients with neglect were unbiased when performing the left quadrisection task, but erred rightward on the right quadrisection task. These results suggest that the ipsilesional bisection errors produced by patients with neglect are primarily influenced by ipsilesional hyperattention rather than contralesional unawareness. However, further analyses showed heterogeneity of performance in left quadrisection, which can be explained by multiple factors that include the top down attention to left space associated with left quadrisection, the orienting to the salience of the line's left end, and distorted mental representation, in addition to ipsilesional hyperattention.     

Left hemispatial neglect with a splenial lesion

ABSTRACT

With injury of the anterior two-thirds of the corpus callosum, each hemisphere’s attentional bias to contralateral hemispace becomes manifest with each hand deviating ipsilaterally during line bisection tasks. Patients with infarctions in the right posterior cerebral artery distribution with occipital and splenial damage can also exhibit spatial neglect. The goal of this report is to learn the role of the splenium of the corpus callosum in mediating visuospatial attention. A right-handed woman with Marchiafava-Bignami disease and damage to the splenium of her corpus callosum without evidence of a mesial frontal, parietal, or occipital injury was assessed for spatial neglect with line bisections. When bisecting lines in her left hemispace with her right hand, she deviated to the right, but revealed no major deviations when the line was place in the midline, in right hemispace, or when bisecting lines with her left hand. This patient provides evidence that damage to the splenium can induce a special form of asymmetrical spatial neglect. This asymmetry might be related to the disconnected right hemisphere’s ability to allocate attention to both right and left hemispaces with the disconnected left hemisphere’s ability to allocate attention to the right but not left hemispace.     

Line quadrisection errors in patients with hemispatial neglect

Patients with left unilateral neglect misbisect lines toward the right. To discriminate between contralesional unawareness and ipsilesional hyperattention hypotheses for this ipsilesional bias, we performed the line quadrisection test on 18 patients with and 25 without neglect, and 24 normal controls. Overall the patients with neglect were unbiased when performing the left quadrisection task, but erred rightward on the right quadrisection task. These results suggest that the ipsilesional bisection errors produced by patients with neglect are primarily influenced by ipsilesional hyperattention rather than contralesional unawareness. However, further analyses showed heterogeneity of performance in left quadrisection, which can be explained by multiple factors that include the top down attention to left space associated with left quadrisection, the orienting to the salience of the line's left end, and distorted mental representation, in addition to ipsilesional hyperattention.     

Bilateral impairment of concurrent saccade programming in hemispatial neglect

When healthy observers make a saccade that is erroneously directed toward a distracter stimulus, they often produce a corrective saccade within 100 ms after the end of the primary saccade. Such short inter-saccadic intervals indicate that programming of the secondary saccade has been initiated prior to the execution of the primary saccade and hence that the two saccades have been programmed concurrently. Here we show that concurrent saccade programming is bilaterally impaired in left spatial neglect, a strongly lateralized disorder of visual attention resulting from extensive right cerebral damage. Neglect patients were asked to make saccades to targets presented left or right of fixation while disregarding a distracter presented in the opposite hemifield. We examined those experimental trials on which participants first made a saccade to the distracter, followed by a secondary (corrective) saccade to the target. Compared to healthy and right-hemisphere damaged control participants the proportion of secondary saccades directing gaze to the target instead of bringing it even closer to the distracter was bilaterally reduced in neglect patients. In addition, the characteristic reduction of secondary saccade latency observed in both control groups was absent in neglect patients, whether the secondary saccade was directed to the left or right hemifield. This pattern is consistent with a severe, bilateral impairment of concurrent saccade programming in left spatial neglect.     

The neural basis of visuomotor deficits in hemispatial neglect

We have recently reported that patients with hemispatial neglect demonstrate increased terminal errors when performing delayed leftward reaches and that right-brain damaged patients (irrespective of neglect) take longer to complete their movements [Rossit, S., Muir, K., Reeves, I., Duncan, G., Birschel, P., & Harvey, M. (2009). Immediate and delayed reaching in hemispatial neglect. Neuropsychologiaa 47, 1563-1573]. Here we conducted an initial voxel-based lesion-symptom analysis to examine the neural basis of such deficits in 21 right-brain damaged subjects with 11 patients suffering from hemispatial neglect (2 more than in Rossit et al. [Rossit S., Muir K., Reeves I., Duncan, G., Birschel, P., & Harvey, M. (2009). Immediate and delayed reaching in hemispatial neglect. Neuropsychologia 47, 1563-1573] and 10 control patients without the condition. We found that the accuracy impairments in delayed leftward reaches were associated with damage to occipito-temporal areas. In contrast, the movement time slowing was related to more anterior lesions in the frontal lobe. These findings agree with the view that neglect affects actions thought to depend on the processing carried out by the ventral visual stream. In addition, we suggest that the timing impairments which are not neglect-specific maybe be driven by frontal structures.     

Vestibular hemispatial neglect: patterns and possible mechanism

Recent reports have suggested that hemispatial neglect may be a vestibular disorder at the cortical level, based on the similarities of symptoms and neural correlates between the two phenomena. If this is the case, peripheral vestibulopathy may lead to hemispatial neglect. However, the etiology of hemispatial neglect in patients with unilateral peripheral vestibulopathy remains unclear. The aims of the present study were to investigate the following: (1) if unilateral peripheral vestibulopathy might cause hemispatial neglect, and if so, (2) whether hemispatial neglect in unilateral peripheral vestibulopathy might be induced by horizontal bias for eye position and body orientation or whether it is secondary to vestibular cortical dysfunction following unilateral peripheral vestibulopathy. Twenty-five consecutive patients with acute vestibular neuritis were recruited at the Dizziness Clinic of Pusan National University Hospital. All participants underwent neglect testing and measurements of horizontal bias for eye position and head and body orientation. Hemispatial neglect occurred in 32 % of patients with unilateral peripheral vestibulopathy. The frequency of contralesional neglect was equal to that of ipsilesional neglect. All patients with hemispatial neglect showed abnormal performance in bisection tasks. The incidence and severity of the horizontal bias of eye position and head and body orientation did not differ between patients with or without hemispatial neglect. Our study demonstrates that hemispatial neglect can develop after acute unilateral peripheral vestibulopathy. Hemispatial neglect after acute unilateral peripheral vestibulopathy may be attributed to damaged vestibular subnuclei, which receive afferents from both peripheral vestibular end organs and the vestibulocerebellum and project to the ipsilateral or contralateral thalamus and vestibular cortex.     

左侧大脑半球病变所致右侧空间疏忽一例报告

目的 探讨左半球病变引起右侧空间疏忽的可能机制。方法对１例左半球病变及诊断为经皮质感觉性失语且右利的患者,用Ａｌｂｅｒｔ删除试验及Ｓｃｈｅｎｋｅｎｂｅｒｇ二等分试验,临摹及摆方块检查其视空间功能及有无疏忽。结果（１）患者有视空间功能障碍及右半侧空间疏忽。（２）患者语言障碍恢复快,视空间障碍及右侧疏忽恢复慢。结论 本例患者的视空间功能及对双侧空间注意的优势半球可能在左侧。     

Non-lateralised deficits in anti-saccade performance in patients with hemispatial neglect

We tested patients suffering from hemispatial neglect on the anti-saccade paradigm to assess voluntary control of saccades. In this task participants are required to saccade away from an abrupt onset target. As has been previously reported, in the pro-saccade condition neglect patients showed increased latencies towards targets presented on the left and their accuracy was reduced as a result of greater undershoot. To our surprise though, in the anti-saccade condition, we found strong bilateral effects: the neglect patients produced large numbers of erroneous pro-saccades to both left and right stimuli. This deficit in voluntary control was present even in patients whose lesions spared the frontal lobes. These results suggest that the voluntary control of action is supported by an integrated network of cortical regions, including more posterior areas. Damage to one or more components within this network may result in impaired voluntary control.