Conflict and integration of spatial attention between disconnected hemispheres

OBJECTIVES: To clarify how the disconnected hemispheres perceive a line and bisect it with successful or unsuccessful integration of spatial attention. METHODS: Eye movements were recorded when a patient with an extensive callosa infarction bisected horizontal lines. The lesion extended into the left cingulate gyrus. RESULTS: When the patient bisected lines with the right hand, the gaze was initially directed rightward and shifted further to the right side with the execution of manual response, which resulted in rightward errors. Shortly after bisection, rapid ocular searches occurred to the left side, whereas the rightward errors did not decrease throughout the trials. When using the left hand, there was no deviation of the gaze before presentation of lines. In the first few trials, the patient bisected the line with a leftward error and then searched rapidly to the right side. The subsequent bisections were almost accurate, as the subjective midpoint was placed near the point of the initial fixation that fell around the true centre. Ocular searching was mostly absent during and after line bisection. CONCLUSIONS: In callosa disconnection, left unilateral spatial neglect may appear when use of the right hand induces a rightward bias in the attentional control of the left hemisphere and damage to its cingulate gyrus inhibits interhemispheric integration of attention. Resultant rightward errors of line bisection often cause interhemispheric conflict of attention, as the right hemisphere perceives the longer extent on the left side. By contrast, the disconnected but intact right hemisphere may bisect a line accurately by integrating attention to the extents perceived in the left and right visual fields.     

Can free-viewing perceptual asymmetries be explained by scanning,pre-motor or attentional biases?

Judgments of relative magnitude between the left and right sides of a stimulus are generally weighted toward the features contained on the left side. This leftward perceptual bias could be the result of, (a) left-to-right scanning biases, (b) pre-motor activation of the right hemisphere, or (c) a left hemispatial attentional bias. The relative merits of these explanations of perceptual asymmetry were investigated. In Experiment 1, English and Hebrew readers made luminance judgements for two left/right mirror-reversed luminance gradients (greyscales task). Despite different reading/scanning habits, both groups exhibited a leftward perceptual bias. English and Hebrew readers also performed a line bisection task. Scanning biases were controlled by asking participants to follow a marker as it moved left-to-right or right-to-left and then stop it as it reached the midpoint of the line. Despite controlling scanning, a leftward bias was observed in both groups. In Experiment 2, peripheral spatial cues were presented prior to the greyscales stimuli. English readers showed a reduction in the leftward bias for right-sided cues as compared to left-sided and neutral cues. Right-side cues presumably overcame a pre-existing leftward attentional bias. In both experiments, pre-motor activation was controlled using bimanual responses. Despite this control, a leftward bias was observed throughout the study. The data support the attentional bias account of leftward perceptual biases over the scanning and pre-motor activation accounts. Whether or not unilateral hemispheric activation provides an adequate account of this attentional bias is discussed.     

What's in a line? Verbal,facial, and emotional influences on the line bisection task

ABSTRACT

In Bryden and MacRae's [(1988). Dichotic laterality effects obtained with emotional words. Neuropsychiatry, Neuropsychology, and Behavioral Neurology, 1(3), 171–176] dichotic listening task, attending to verbal (left hemisphere) or emotional (right hemisphere) auditory stimuli can result in opposite patterns of behaviour. We examined whether performance on the line bisection task might also be influenced in opposite ways by left- and right-lateralized functions. The line bisection task is a simple and effective measure of visuospatial bias. Pseudoneglect, a leftward bias, is typically found on this task, and appears to result from right hemisphere dominance for spatial processing. We investigated how emotion, verbal, and facial processing impacted line bisection performance, while also examining the influence of hand-use. Line type (face, word, and solid), valence (positive, negative, and neutral), and hand-use (left, right, and both) were manipulated. Results indicated that face and word lines decreased and increased the extent of pseudoneglect, respectively, and that valence accentuated these results. These results were in the opposite direction from our predictions. Hand-use had little influence. We discuss the impact that visual scanning, and local and global processing, may have had on line bisection performance. Until future research clarifies how lateralized functions affect line bisections, we suggest caution in adapting the line bisection task as a general measure of relative hemispheric activation.     

Ipsilesional versus contralesional neglect depends on attentional demands

Right hemisphere injuries often produce contralesional hemispatial neglect (CN). In contrast to CN, some patients with right hemisphere damage can also show so-called ipsilesional neglect (IN). Previous reports found that patients tend to show IN on line bisection tasks but CN on other tasks such as target cancellation. To learn why these two tasks induce different spatial biases in patients with right hemisphere injury, conventional (i.e. solid) line bisection was compared with two novel bisection tasks consisting of horizontally aligned strings of characters. The subjects' task was to mark a target character that was at or closest to the true midpoint of the simulated line. Four of the 5 patients showed a dissociation whereby IN occurred for solid lines while CN was observed on character lines. The two patients assessed with an antisaccade paradigm showed a "visual grasp" for leftward stimuli. The present results suggest that neglect on line bisection may reflect two opposing forces, an approach behavior or "visual grasp" toward left hemispace and an attentional bias toward right hemispace.     

The effect of acute ethanol challenge on global visuospatial attention: Exaggeration of leftward bias in line bisection

Acute alcohol challenge has been associated with a selective impairment of right hemisphere function. A hallmark of visuospatial neglect syndrome is that patients with right hemisphere lesions misbisect horizontal lines far rightward of veridical centre. Neurologically intact participants misbisect lines with a systematic leftward bias (pseudoneglect). Neuroimaging studies in neurologically intact participants reveal predominant right hemisphere activation during performance of line bisection tasks. The current study assessed whether acute alcohol challenge alters global visuospatial attention. Participants (N=18; 10 male; strongly right-handed; mean age 23 years) engaged in a forced-choice tachistoscopic line bisection task in both ethanol challenge (mean BAC=.077) and no ethanol control conditions. Mean leftward bisection error in the control condition was ?0.238 degrees visual angle (1.05% line length), and leftward bisection error significantly increased (p=.001) under ethanol challenge (?0.333 degrees visual angle, 1.47% line length). Mean bisection precision in the control condition was 0.358 degrees visual angle (1.58% line length); bisection precision significantly deteriorated (p=.008) under ethanol challenge (0.489 degrees, 2.17% line length). Decreased bisection precision indicates that ethanol disrupts the fidelity of visuospatial performance. The exaggerated leftward bisection error implies that ethanol may exert a differential effect on left versus right hemispheric function with respect to the control of global visuospatial attention.     

Ineffective leftward search in line bisection and mechanisms of left unilateral spatial neglect

We examined the eye-fixation pattern of a patient with severe left unilateral spatial neglect who showed leftward searches of various extent in more than half of line bisection trials. Because of complete left homonymous hemianopia, he perceived only the segment of the line between its right endpoint and the point of the leftmost fixation. In the trials with leftward searches, he frequently placed the subjective midpoint on the right part of the perceived segment. In the trials without leftward searches, he placed it near the left extreme point of the perceived segment. For all these bisections, the subjective midpoint was constantly placed far to the right of the true midpoint of the line irrespective of the length perceived. We consider that in severe left unilateral spatial neglect, rightward attentional bias is the predominant factor that determines where to place the subjective midpoint. Transient attentional shift to the left may produce leftward searches, but it does not induce effective processing of line bisection.     

Image of a line is not shrunk but neglected. Absence of crossover in unilateral spatial neglect

Patients with left unilateral spatial neglect following right hemisphere lesions usually err rightward when bisecting a horizontal line. For very short lines (e.g. 25 mm), however, leftward errors or seemingly 'right' neglect is often observed. To explain this paradox of crossover in the direction of errors, rather complicated models have been introduced as to the distribution of attention. Neglect may be hypothesized to occur in representational process of a line or estimation of the midpoint on the formed image, or both. We devised a line image task using a computer display with a touch panel and approached the representational image of a line to be bisected. Three patients with typical left neglect were presented with a line and forced to see its whole extent with cueing to the left endpoint. After disappearance of the line, they pointed to the right endpoint, the left endpoint, or the subjective midpoint according to their representational image. The line image between the reproduced right and left endpoints was appropriately formed for the 200 mm lines. However, the images for the shorter 25 and 100 mm lines were longer than the physical lengths with overextension to the left side. These results proved the context effect that short lines may be perceived longer when they are presented in combination with longer lines. One of our patients had an extensive lesion that involved the frontal, temporal, and parietal lobes, and the other two had a lesion restricted to the posterior right hemisphere. The image for a fully perceived line may be represented far enough into left space even when left neglect occurs after a lesion that involves the right parietal lobe. The patients with neglect placed the subjective midpoint rightward from the centre of the stimulus line for the 100 and 200 mm lines and leftward for the 25 mm lines. This crossover of bisection errors disappeared when the displacement of the subjective midpoint was measured from the centre of the representational line image. Left neglect may occur consistently in estimation of the subjective midpoint on the representational image, which may be explained by a simple rightward bias of attentional distribution.     

The influence of stimulus properties on visual neglect.

Twenty five patients with right cerebral hemisphere damage and neglect participated in a series of bisection experiments. As expected, long lines were bisected to the right of true midpoint. By contrast, large circles and long white paper strips were bisected accurately, or with leftward errors. Small objects were less sensitive to stimulus properties: short lines and paper strips, and small circles, were bisected to the left of true midpoint, and these leftward errors were equally common as rightward errors with long lines. When asked to draw a perpendicular line of the same length as the presented horizontal line, patients overestimated the length of short lines but underestimated that of long lines. Presenting lines in near and far extrapersonal space selectively affected bisection of short lines. The results suggest that two opposing, independent mechanisms determine bisection performance in left neglect.     

The greyscales task: a perceptual measure of attentional bias following unilateral hemispheric damage

The two cerebral hemispheres in humans have been suggested to control contralaterally opposed attentional biases. These biases may be revealed by unilateral hemispheric damage, which often causes contralesional spatial neglect, particularly when the right hemisphere (RH) is affected. Subtle attentional biases have also been observed in normal observers in tasks requiring judgements of horizontal spatial extent, brightness, numerosity and size. Here, we examined attentional biases for judging the darker of two left-right mirror-reversed brightness gradients under conditions of free viewing (the greyscales task). We compared performances of patients with damage to the RH (n=78) and left hemisphere (LH; n=20) with those of normal controls (n=20). Controls showed a small but significant leftward bias, implying a subtle asymmetry favouring the RH. In contrast, RH and LH patients showed extreme rightward and leftward biases, respectively, both of which differed significantly from that of controls. For the patient groups, performance on clinical tests of neglect (cancellation and line bisection) did not predict their greyscales scores. Pathological biases were present in patients without clinical neglect or visual field defects, suggesting that the attentional bias measured by the greyscales task can be dissociated from clinical neglect and visual sensory loss. The greyscales task offers an efficient means of quantifying pathological attentional biases in unilateral lesion patients; it is easy to administer and score, and may be particularly useful for clinical trials of recovery and rehabilitation following stroke.     

The centre is not in the middle: Evidence from line and word bisection

English and German readers have been shown to mark a position to the left of the true centre as the subjective midpoint in word bisection. This effect resembles a well-known phenomenon observed with the bisection of solid lines (pseudoneglect), although this behavioural similarity does not imply a common origin. The purpose of the present study was twofold: on the one hand, to investigate the perceptual and lexical features that influence the bisection of Italian orthographic strings and, on the other hand, to investigate whether identical or partially independent processing mediate bisection of line and orthographic stimuli. Five experiments were carried out to explore to what extent stimulus type (lines, words, pseudowords, consonant strings, symbols), stimulus length (from 3 to 13 characters), list context (pure and mixed), and written word frequency (high and low) affected the bisection performance. The results showed that list context modulated the processing similarities across different materials and that word frequency failed to influence the magnitude of the bisection bias.More critically, across all five experiments, the results showed different effects for solid lines versus orthographic material. Lines were always bisected to the left, independent of length and list context. By contrast, a crossover effect emerged with orthographic material; for long stimuli (above five letters) the bias was consistently to the left, while short stimuli showed a consistent rightward bias. The results indicate that manual bisection involved partly different cognitive mechanisms during word and line perception and that this may depend on the characteristics of the stimuli (words/discrete vs. lines/continuous).     

Approaches to subjective midpoint of horizontal lines in unilateral spatial neglect

Patients with unilateral spatial neglect usually bisect longer lines with greater rightward errors, while they sometimes err leftward for very short lines (e.g., 25 mm). We analysed movements of eye fixation from the time before line presentation to elucidate whether patients with neglect approach the subjective midpoint differently for lines of various lengths. Four patients with left neglect bisected 200 mm, 100 mm, and 25 mm lines that appeared across the centre of a liquid crystal display (LCD) monitor. The fixation immediately before line presentation was located on average near the centre of the lines. Three of the patients approached the subjective midpoint point directly from the left side in more than 70% of the 200 mm and 100 mm trials. The subjective midpoint frequently deviated leftward on the "attended" segment between the leftmost point of fixation and the right endpoint, while it was displaced rightward on the total extent. The three patients initially explored the 25 mm lines searching for the left endpoint. They thereafter bisected the same lines with leftward errors approaching the subjective midpoint from the left side. The remaining patient searched beyond the right endpoint and in turn approached the subjective midpoint from the right side in about half of the trials and independently of line length. In the 200 mm and 100 mm trials, the subjective midpoint divided the attended right segment nearer to the right endpoint. On the attended right extent of a line, patients with neglect may place the subjective midpoint toward the side from which they approached that point. In the bisection of very short lines, approaches from the left endpoint may cause leftward errors of the subjective midpoint. For longer lines, however, approaches from the left side may result in rightward error of bisection for the total length, as the leftward extent from the fixation immediately before line presentation is hardly explored.     

The influence of stimulus proximity on judgments of spatial relationships in patients with chronic unilateral right or left hemisphere stroke

Objective: This was to learn how chronic right hemispheric damage (RHD) versus left hemispheric damage (LHD) may influence attentional biases in proximal and distal space. Background: Prior research has suggested that the left hemisphere primarily attends to proximal space and the right hemisphere to distal space. The purpose of this study was to contrast line bisection performed in proximal versus distal space in patients with chronic LHD versus RHD. Design/method: Participants were 32 LHD and 26 RHD patients who sustained a stroke a mean of 3.4 years prior to testing, along with 9 healthy controls. Subjects attempted to bisect 30 lines in proximal space and 30 lines in distal space. Results: Patients with both RHD and LHD had a greater contralesional bias in proximal than distal space (rightward bias for patients with LHD and leftward bias for patients with RHD). Compared to controls, patients with LHD were most different in proximal space, and patients with RHD were most different in distal space. Conclusions: Proximity appears to influence spatial judgments of patients with RHD and LHD in an opposing manner. Relatively, both patient groups bisect lines contralesionally in proximal space and ipsilesionally (relative to proximal) in distal space. Patients with RHD have the biggest difference between their proximal and distal judgments. The reason for these differences is unknown. However, these biases may be related to an attentional or action-intentional grasp or a learned compensation strategy, and proximity may increase the allocation of attention or intention and thereby enhance this grasp or use of this compensation strategy. Another contributing factor may be dominance of the left and right hemisphere for information presented in proximal and distal space, respectively.     

Things that go bump in the right: the effect of unimanual activity on rightward collisions

Patients with right parietal damage and spatial neglect ignore the leftward features of their environment - causing them to bump into the left-side of doorways. In contrast, the normal population shows a mild attentional bias towards the left. Self-report measures show more collisions to the right in everyday settings. We sought to obtain a quantitative measure of lateralised bumping in a laboratory setting. Participants (n=276) walked through a narrow doorway and the experimenter recorded collisions. To investigate the association between bumping and paper-and-pencil tests of pseudoneglect, a line bisection task was administered. Unilateral activation of the hemispheres has been found to ameliorate the effects of spatial neglect. We investigated the effect of activation by asking participants to move their left-, right- or both-hands as they walked. In the both hands condition, which acted as a baseline, there were more right bumps than left bumps. The rightward bias was exasperated when the left hand moved, presumably because this movement activated the right hemisphere. In contrast, there were more left bumps when the right hand moved. The results demonstrate that bumping is not random and that we collide with the right side more often. Biases in bumping, however, were not related to biases in line bisection. The effect of hand-movement demonstrates that bumping is brought about by an imbalance of activation between the hemispheres.     

利用情绪面孔嵌合图形研究精神分裂症的注意偏向

目的探讨精神分裂症患者的注意偏向情况。方法利用情绪面孔嵌合图形试验和线段等分试验对年龄、文化程度、简易精神状态量表(mini-mental state examination,MMSE)评分等相匹配的32例精神分裂症患者和32名正常对照进行测试。结果精神分裂症患者在本顿面孔识别试验中和对照组差异无统计学意义(t=-0.85,P>0.05)。情绪(喜)面孔嵌合试验和20 cm线段等分试验,患者组的偏移率均比对照组高[-18.75(-31.25,-6.25)vs.-12.50(-23.44,4.69);-1.40(-2.65,-0.93)vs.-0.45(-1.50,0.30)],差异有统计学意义。患者组的情绪(喜)面孔嵌合和线段等分之间的偏移率相关(r=0.68,P<0.01)。结论精神分裂症患者在情绪(喜)面孔嵌合试验中注意左偏,这与线段等分试验结果相同,提示患者右侧空间注意受损,可能与左侧脑半球功能障碍有关。     

The influence of distractors and numerical direction on mental number line bisection

Spatial relationships are used to mentally represent numerical information. We visualize small numbers on the left and large numbers on the right, forming a mental number line. When determining the midpoint between two numbers, healthy participants show a bias to the left, similar to the bias on line bisection. Upper space distractors increase leftward line bisection biases and the current study examined the influence of distractors on mental number line bisection. Participants viewed sequential number triplets, presented concurrently with distractors. In Experiment 1, participants performed more accurately when numbers were presented in ascending order and showed stronger left biases for descending trials. Further, a responding bias was observed, where participants pressed the response key on the same side as the distractors. In Experiment 2, an orthogonal response mapping was used, which confirmed the occurrence of a responding bias. Interestingly, distractors weakened pseudoneglect, demonstrating a dissociation between line bisection and mental number line bisection in terms of distractibility. Numerical direction influenced asymmetries, such that leftward biases were stronger on descending trials. It is suggested that a right-to-left orientation of attention on descending trials might lead participants to overestimate how far along the number line they have moved, which increases leftward biases.     

Context and crossover in unilateral neglect

Patients with left-neglect bisect horizontal lines to the right of true center. Their bisection bias is affected by line length, so that longer lines are bisected further to the right. Patients often crossover and bisect very short lines to the left of true center. We tested the hypothesis that the context in which lines are apprehended accounts for the crossover phenomenon. We replicated previous findings that a line is bisected further leftward when it is preceded by a longer line and further rightward when it is preceded by a shorter line. These contextual effects occur with relatively short and relatively long target lines. Bisection patterns in two different series of lines, one ranging from 10 to 150 mm, and the other from 110 to 250 mm, were investigated. If crossover bisections were simply due to contextual effects then left-sided errors would be observed on bisections of the shorter lines of both series. Our findings did not support this hypothesis. Crossover bisections occurred only with objectively short lines, those shorter than 40 mm. Even though we found significant contextual effects on line bisection biases, these effects per se do not account for the crossover phenomenon. Rather, our data suggest that the absolute length of the line is associated specifically with the crossover phenomenon.     

Hemispheric asymmetry and callosal integration of visuospatial attention in schizophrenia: a tachistoscopic line bisection study

BACKGROUND: A hallmark of visuospatial neglect syndrome is that patients with lesions to right parietal cortex misbisect horizontal lines far rightward of veridical center. Neurologically normal subjects misbisect lines with a systematic leftward bias (pseudoneglect). Both phenomena, as well as neuroimaging studies, disclose a predominant right-hemisphere control of spatial attention. Numerous studies of patients with schizophrenia have implicated global deficits of either right or left hemisphere function, as well as compromised integrity of the corpus callosum. METHODS: To better understand the functional implications of schizophrenia we utilized a forced-choice tachistoscopic line bisection task to probe the status of right-hemisphere control of spatial attention, and compared left- versus right-hand unimanual responses to index the degree of callosal transfer of visuospatial information in both patient and control groups. RESULTS: In contrast to the significant leftward bisection errors of control subjects, patients exhibit no significant leftward error. Whereas control subjects evince a significant correlation between left- and right-hand bisection errors, patients lack a significant intermanual correlation. CONCLUSIONS: The lack of significant leftward bisection error of patients implies a deficit of right-hemisphere function. The lack of a significant correlation between left- and right-hand bisection errors in patients implies a loss of callosal integrity.     

Space representation in children with dyslexia and children without dyslexia: Contribution of line bisection and circle centering tasks

Line bisection tasks (different space locations and different line lengths) and circle centering tasks (visuo-proprioceptive and proprioceptive explorations, with left or right starting positions) were used to investigate space representation in children with dyslexia and children without dyslexia. In line bisection, children with dyslexia showed a significant rightward bias for central and right-sided locations and a leftward bias for left-sided location. Furthermore, the spatial context processing was asymmetrically more efficient in the left space. In children without dyslexia, no significant bias was observed in central lines but the spatial context processing was symmetrical in both spaces. When the line length varied, no main effect was shown. These results strengthen the ‘inverse pseudoneglect’ hypothesis in dyslexia. In the lateral dimension of the circle centering tasks, children showed a response bias in the direction of the starting hand location for proprioceptive condition. For radial dimension, the children showed a forward bias in visuo-proprioceptive condition and more backward error in proprioceptive condition. Children with dyslexia showed a forward bias in clockwise exploration and more accurate performance in counterclockwise exploration for left starting position which may be in accordance with leftward asymmetrical spatial context processing in line bisection. These results underline the necessity to use the line bisection task with different locations as an appropriate experimental paradigm to study lateral representational bias in dyslexia. The contribution of the present results in the understanding of space representation in children with dyslexia and children without dyslexia is discussed in terms of attentional processes and neuroanatomical substrate.     

Line bisection in visuo-spatial neglect: disproof of a conjecture

We describe two patients with left neglect who show a linear relationship between the magnitude of their line bisection displacements and the veridical length of the line presented. For lines smaller than those typically employed in previous investigations, the patients' mean performance is fairly accurate; with even smaller lines, the direction of displacement crosses over true centre to give a leftward displacement ('right neglect'). In both cases, the slope of the regression line falsifies the conjecture that (intact) left hemisphere mechanisms (correctly) bisect the (subtotal or supertotal) line length that is available in right hemispace.     

Horizontal visual motion modulates focal attention in left unilateral spatial neglect.

Patients with unilateral spatial neglect are impaired in directing focal attention toward the contralesional side of space. Provision of static spatial cues on the neglected side has previously been shown to help overcome this deficit. Common movement of visual stimuli may also guide the allocation of spatial attention, although such effects have not been examined in patients with unilateral spatial neglect. Eleven patients with right hemisphere damage and clinical evidence of left unilateral spatial neglect, and 11 matched, healthy controls were tested on a task of horizontal line bisection. Lines were presented on a computer display, with a neutral, static, or slowly drifting, random dot background. Under conditions of motion, background stimuli drifted either leftward or rightward, across the full width of the display, at speeds that did not elicit optokinetic nystagmus or perceptual aftereffects. Controls were accurate in all conditions, and showed minimal effects of background conditions. By contrast, patients with left unilateral spatial neglect were sensitive to leftward background motion, showing a significant leftward shift in bisection error, relative to neutral, static, and rightward moving backgrounds. There was no significant effect of rightward motion in comparison with the neutral and static conditions. The extent to which patients were susceptible to the effects of background motion was not related to severity of unilateral spatial neglect, as measured by clinical tests. The benefits of leftward motion may reflect activity of preserved motion processing mechanisms, which provide input to an otherwise dysfunctional attentional network. The use of visual motion to assist in contralesionally guiding focal attention may be useful in the rehabilitation of unilateral spatial neglect.