Triangular backgrounds shift line bisection performance in hemispatial neglect: the critical point.

Isosceles triangular backgrounds influence line bisection performance in normal control participants and patients with hemispatial neglect. When the triangles are oriented asymmetrically with the vertex in I visual field, and the base in the other, the perceived midpoint of horizontal lines within the triangle is shifted towards the base, and away from the vertex. The current study examines this illusion further by systematically varying the extent of the triangle presented. With only fragments of the triangle in the background of the line, the vertex is the critical component driving the illusory shift in perceived midpoint. Patients with neglect and controls are equally sensitive to the illusion. Similar geometric illusions that are also intact in neglect, along with these results, suggest that preattentive, implicit visual processing is preserved in neglect and drives these illusions.     

Does spatial cueing affect line bisection in chronic hemianopia?

Patients with homonymous hemianopia often show a contralesional shift towards their blind field when bisecting horizontal lines ("hemianopic line bisection error", HLBE). The reasons for this spatial bias are not well understood and debated. Cueing of spatial attention modulates line bisection significantly in patients with visuospatial neglect. Moreover, recent evidence showed that attention training significantly improves deficits of visual search in hemianopia. Here, we tested in 20 patients with chronic homonymous hemianopia (10 left-sided, 10 right-sided) without visual neglect, 10 healthy control subjects, 10 neurological control patients, and 3 patients with left visuospatial neglect and leftsided hemianopia whether spatial cueing influences the HLBE. Subjects indicated verbally the midpoint of horizontal lines in a computerized line bisection task under four experimental cue positions (cue far left, mid-left, mid-right or far-right within the horizontal line). All 20 hemianopic patients showed the typical HLBE towards their blind field, while the two control samples showed only a small but significant leftward shift (pseudoneglect). None of the 4 cueing manipulations had a significant effect on the HLBE in the hemianopic patients. Moreover, no differential effects of cueing on line bisection results were obtained when analyzed in lesion subgroups of hemianopic patients with circumscribed occipital lesions (N=8) as contrasted with patients having more extended (occipito-temporal or temporal) lesions (N=12). This null-effect contrasts with marked cueing effects observed in 3 neglect patients with left hemianopia in the same tasks, showing the principal efficacy of our cueing manipulation. These results argue against attentional explanations of the HLBE.     

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.     

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.     

Figural modulation of visuo-spatial neglect: a case study

We report a case of severe left visuo-spatial neglect consequent upon right-hemisphere stroke. In Experiment 1, horizontal and vertical (radial) line bisection is performed; the patient displays left neglect and "altitudinal" (radial) neglect, placing his transections too far to the right and to the top, respectively. In Experiment 2, the patient is required to place a dot at the centre of squares and circles, the horizontal and vertical extents of which are identical to the length of the lines employed in Experiment 1. Performance is now extremely accurate. In Experiment 3, the height of the rectangular or elliptical figure whose midpoint is to be estimated is held constant whilst length varies. In Experiment 4, the length of the figure is held constant whilst height is varied. Both manipulations exert a profound, lawful influence on the patient's estimate of each figure's midpoint. We provide some preliminary hypotheses concerning how configurational constraints may affect perceptual and attentional processes in visual neglect.     

Contralesional cross-over in chronic neglect: Visual search patterns reveal neglect of the ipsilesional hemispace

Introduction: With neuropsychological rehabilitation approaches neglect patients can learn to compensate for the reduced awareness of the contralesional hemispace that is often observed after right brain damage. Here, we report contralesional "cross-over" deviations in line bisection that are hypothesized to be a result of focusing on the contralesional hemispace while the intact hemispace is "neglected". We investigate whether this unexpected pattern of deviation is related to defects in the visual field, motor intention/hypokinesia deficits or deficits in working memory.Methods: Neglect patients with and without homonymous field defects were screened for contralesional cross-over deviations in line bisection of long lines. During line bisection eye movements were recorded in two conditions with and without requiring hand movements in order to search for directional hypokinesia. Visual fields were tested with near-threshold perimetry and with supra-threshold campimetry.Results: Of 53 chronic neglect patients only 8 showed cross-over in line bisection. Evidence for directional hypokinesia was found in only one patient. Patients with cross-over focused more often to the left than to the right of the objective line midpoint. Patients with and without visual field defects did not differ in the extent of cross-over deviations. Cross-over deviation and inconsistent stimulus detection in left hemispace were correlated irrespective of the presence of a visual field deficit. Larger cross-over deviations were associated with poorer verbal working memory span, and disorganized patterns of eye movement were related to reduced visuo-spatial working memory capacity.Conclusion: Increasing awareness of the disorder and the use of compensatory strategies may have led to a cross-over shift of visual search dominance towards the neglected side resulting in an exploration deficit of the ipsilesional side.     

Attention biases the perceived midpoint of horizontal lines

In patients with right brain damage and left visual neglect, attention tends to be captured by right-sided objects and cannot easily disengage from them. While these phenomena can account for several clinical and experimental patterns of performance such as biased visual search, its role is more controversial for other neglect-related signs, such as the typical rightward shifts in horizontal line bisection. It is thus important to see whether and how attentional orienting can bias line bisection in normal participants using standard clinical bisection stimuli. In 3 experiments, we explored the Attentional Repulsion Effect (ARE, Suzuki & Cavanagh, 1997) on pre-bisected lines. Normal observers saw horizontal lines with a vertical bisection mark near the center, preceded by a cue to the left or right of the line, or by no cue. On each trial, observers indicated whether they saw the bisection mark to the left or at the right of the midpoint. We plotted the proportion of ‘seen-at-right’ responses as a function of the mark's actual position. For uncued lines, the point of subjective equality was slightly at the left of the true center, consistent with the pseudoneglect phenomenon. Right-sided cues shifted the apparent bisection point to the left (and vice versa), as predicted by the ARE. Similar results occurred with different task instructions (compare the length of the left-sided line segment to the right-sided segment) and in the presence or absence of central fixation marks. These results obtained in normal participants support attentional accounts of biased line bisection in neglect patients.     

Line bisection under an attentional gradient induced by simulated neglect in healthy subjects

Whether an attentional gradient favouring the ipsilesional side is responsible for the line bisection errors in visual neglect is uncertain. We explored this by using a conjunction-search task on the right side of a computer screen to bias attention while healthy subjects performed line bisection. The first experiment used a probe detection task to confirm that the conjunction-search task created a rightward attentional gradient, as manifest in response times, detection rates, and fixation patterns. In the second experiment subjects performed line bisection with or without a simultaneous conjunction-search task. Fixation patterns in the latter condition were biased rightwards as in visual neglect, and bisection also showed a rightward bias, though modest. A third experiment using the probe detection task again showed that the attentional gradient induced by the conjunction-search task was reduced when subjects also performed line bisection, perhaps explaining the modest effects on bisection bias. Finally, an experiment with briefly viewed pre-bisected lines produced similar results, showing that the small size of the bisection bias was not due to an unlimited view allowing deployment of attentional resources to counteract the conjunction-search task's attentional gradient. These results show that an attentional gradient induced in healthy subjects can produce visual neglect-like visual scanning and a rightward shift of perceived line midpoint, but the modest size of this shift points to limitations of this physiological model in simulating the pathologic effects of visual neglect.     

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.     

Terminal, but not concurrent prism exposure produces perceptual aftereffects in healthy young adults

A short period of prism adaptation (PA) has been shown to reduce spatial neglect symptoms. Recent evidence suggests that the positive effects of PA might be restricted to visually guided actions, with PA having little effect on perception. However, the majority of studies have adopted a concurrent exposure technique that fosters the development of a change in felt arm position (proprioceptive straight ahead, PSA). Few studies have used terminal exposure that promotes a change in the perceived visual direction (visual straight ahead, VSA). The positive effects of PA might appear to be primarily action based because studies have adopted an exposure technique that promotes a change in proprioception. Here, we compare the effects of the two exposure types on a perceptual and a manual line bisection task in healthy young adults. Before and after seven minutes of exposure to leftward displacing prisms we measured performance on two line bisection tasks (manual and perceptual) and perceived straight ahead (PSA and VSA). During the exposure period participants made pointing movements while the view of their pointing arm was either (i) restricted to the second half of the pointing movement (concurrent exposure) or (ii) restricted to the final part of the pointing movement (terminal exposure). In line with the previous research, concurrent exposure produced a large shift in PSA and a shift on the manual line bisection task. Interestingly, terminal exposure produced a large shift in VSA and a shift in performance on the perceptual line bisection task. Our results shed light on the underlying mechanisms of prism-induced neglect recovery and help to address an apparent discrepancy within the literature.     

Processing of illusion of length in spatial hemineglect: a study of line bisection

Bisection of horizontal lines and of the Brentano form of the Müller-Lyer illusion was investigated in six right brain-damaged patients with left spatial hemineglect, and in six control subjects. Patients bisected the lines to the right of the objective mid-point. Comparable illusory effects on line bisection were however found in both patients and control subjects. Relative to the baseline condition, in both groups the subjective midpoint was displaced towards the side expanded by the illusion, both leftwards and rightwards. By contrast, line length and spatial position of the stimulus had differential effects. In neglect patients, the rightward bisection error increased disproportionately with line length, and when the stimulus was located in the left, neglected, side of egocentric space. Control subjects showed no such effects. The suggestion is made that the visual, non-egocentric, processes underlying these illusory effects of length may be spared in patients with left spatial neglect. The possible neural basis of this dissociation is discussed.     

Visual,proprioceptive and tactile performance in left neglect patients

Patients with unilateral spatial neglect due to right-hemisphere lesions typically fail to attend to and explore left-sided stimulus objects. It has been postulated that in right-brain damaged (RBD) patients an ipsilesional displacement of the egocentric frame of reference (ER), whether visual or tactile, may be responsible for a contralesional supramodal spatial bias causing their left neglect behavior. However, this hypothesis had been proposed without testing, in the same patients, the position of the ER or their performance in the visual and tactile modalities. Thus, the aim of the present study was to test the hypothesis that an ipsilateral shift of the ER is responsible for a supramodal spatial bias in neglect.For this purpose, a within-subject design is required. Consequently, 12 left neglect patients and 12 control subjects were asked to perform a proprioceptive straight-ahead pointing task while blindfolded, as well as visual and tactile bisection tasks.In the left neglect patients, we found:no systematic deviation of the ER on the ipsilesional right side;a significant rightward bias in visual bisection, and normal performance in tactile bisection;no correlation among the three tasks;that only visual bisection correlated with the severity of neglect.These results are discussed with regard to the egocentric and attentional hypothesis of neglect.     

Positive correlation in the bisection of long and short horizontal Oppel-Kundt illusory gradients: Implications for the interpretation of the "cross-over" effect in spatial neglect

Right brain damaged patients with left spatial neglect typically bisect long horizontal lines to the right of their midpoint. However, bisections of very short lines can favour the emergence of a paradoxical “cross-over” effect in which lines are bisected to the left of the true midpoint. It has been suggested that in healthy participants similar variations in the position of the subjective line midpoint can be observed in the bisections of long and short Oppel–Kundt (O-K) illusory gradients (Savazzi et al., 2007). This analogy was taken as proof that patients with neglect suffer a distorted representation of horizontal space that is equivalent to illusory distortions that O-K gradients induce in the intact brain (Savazzi et al., 2007). In contrast to this proposal, however, it has been noted that reversal of O-K illusion with short gradients was never described in literature (Doricchi et al., 2008). To resolve this incongruence, it was argued that such a reversal can be observed in healthy participants showing strong conventional illusory effects with long gradients (Savazzi, 2008). This proposal suggests that the greater the shift in the conventional direction of the illusion for long gradients, the greater the shift in the opposite direction with equivalent short gradients (i.e., negative correlation). Here we tested this hypothesis in a sample of 100 healthy participants who bisected horizontal O-K illusory gradients of different lengths (2, 4, 8 and 16 cm). We found no reversal of O-K illusion with short gradients and a positive, rather than negative, correlation between bisection of long and short gradients. Participants showing strong illusory effects in the bisection of long gradients showed analogous effects in the bisection of very short ones. These findings do not support the space anisometry interpretation of line bisection performance and the cross-over effect in patients with neglect.     

How do patients with neglect see a horizontal line?

Abstract. Twelve patients with left unilateral spatial neglect were examined with a newly devised coloured line bisection task. They were presented with a horizontal line printed in blue on one side and in red on the other side; the proportions of the blue and red segments were varied. Immediately after placement of the subjective midpoint, the line was concealed and the patients were asked to name the colours of the right and left ends. Five patients who identified the left-end colour almost correctly had no visual field defect, while the other seven whose colour naming was impaired on the left side had left visual field defect. The rightward bisection errors were similarly distributed in the fair and poor colour-naming patients except for two patients from the latter group. The lesions of the fair colour-naming patients spared the lingual and fusiform gyri, which are known to be engaged in colour processing. Patients with neglect whose visual field is preserved may neglect the leftward extension of a line but not the colour in the neglected space. The poor colournaming patients frequently failed to name the left-end colour that appeared to the left of their subjective midpoint, which indicates that they hardly searched leftward beyond that point. In such trials, they reported that the left end had the same colour as the right end. The results suggest that in patients with neglect and left visual field defect, both the leftward extent and the colour of a line may be represented on the basis of the information from the attended right segment.     

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.     

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.     

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.     

Spatial representation in the normal visual field: a study of hemifield line bisection

We examined bisection of lines viewed in only one hemifield by normal subjects. Subjects first performed a traditional version of line bisection, by indicating the perceived midpoint of a line on paper with a penmark. Bisection was accurate when they were allowed to shift their gaze over the stimulus, but it was biased towards the central visual field (centripetally) when gaze was fixed so that the line was seen in only one hemifield. In a second experiment, lines with transectors at various locations were presented briefly on a screen and subjects had to indicate on which side of the perceived midpoint the transector was located. A centripetal bias was still found, indicating that it has a perceptual origin. The interaction between bias and effects of tangent line presentation suggested that subjects were performing an angle bisection rather than a line bisection. Also, there was bias in not only right and left hemifields but also upper and lower hemifields. In a third experiment, increasing the width of the stimulus bars peripherally did not eliminate this bias. Bias was size-invariant along the horizontal meridian. This spatial version of Weber's law was modeled by a magnification function using an exponential equation. The slope of this function is much shallower than those currently known for V1, V4 and V5. We conclude that a centripetal bias exists for hemifield line bisection and that this bias likely contributes to the contralateral bias of line bisection by hemianopic patients found in other studies.     

Preserved obstacle avoidance during reaching in patients with left visual neglect

We asked 12 patients with left visual neglect to bisect the gap between two cylinders or to reach rapidly between them to a more distal target zone. Both tasks demanded a motor response but these responses were quite different in nature. The bisection response was a communicative act whereby the patient indicated the perceived midpoint. The reaching task carried no imperative to bisect the gap, only to maintain a safe distance from either cylinder while steering to the target zone. Optimal performance on either task could only be achieved by reference to the location of both cylinders. Our analysis focused upon the relative influence of the left and right cylinders on the lateral location of the response. In the bisection task, all neglect patients showed qualitatively the same asymmetry, with the left cylinder exerting less influence than the right. In the reaching task, the neglect group behaved like normal subjects, being influenced approximately equally by the two cylinders. This was true for all bar two of the patients, who showed clear neglect in both tasks. We conclude that the visuomotor processing underlying obstacle avoidance during reaching is preserved in most patients with left visual neglect.     

Components of neglect from right-hemisphere damage: an analysis of line bisection

Two versions of a line bisection task were given to patients with posterior right-hemisphere damage and normal control subjects. One, which we refer to as the directed-manual task, was the traditional bisection task in which lines were transected with a pen held in the right hand. In the other task, referred to as the directed-visual task, subjects observed the experimenter move a pen along a line from right-to-left (the left-scan task) or from left-to-right (the right-scan task) and they verbally indicated the subjective midpoint. Patients showed significant left neglect in the manual and the left-scan tasks only. Controls showed no consistent biases and no influence of scanning direction. Right and left cues biased bisection for both groups. The results indicate that when the directed manual response is eliminated, scan direction determined the presence or absence of neglect on bisection. The findings are discussed in terms of the efficiency of visual orienting.