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.     

Lucky numbers: Spatial neglect affects physical, but not representational, choices in a Lotto task

Spatial neglect can be characterized by a “magnetic attraction” towards the right side of a visual stimulus array and a selection of stimuli from that hemispace. This study examined whether these distinctive characteristics in visuo-motor space are also evident in representational number space. Given that numbers are thought to be represented along a left-to-right oriented mental number line, an affinity for the spontaneous selection of larger numbers was anticipated for neglect patients. Contrary to this expectation, neglect patients (n = 20) picked a similar range of numbers compared to controls (n = 17) when generating a number between 1000 and 10?000 and when playing an imaginary lottery game. There was, however, a positive correlation between the biases for the imaginary lottery, number generation and a number bisection task - demonstrating that exploration asymmetries along the mental number line are consistent within individuals across tasks. Some of the patients selected smaller numbers in all of these tasks, confirming reports of dissociations between physical and numerical-representational forms of neglect. Conversely, only four (20%) of the patients could reliably be classified as demonstrating a neglect in number space. When filling out a physical lottery ticket, the neglect patients showed the expected bias towards picking numbers placed on the right-hand side of the ticket. These results demonstrate that the magnetic attraction towards the right side of mental representations is rather weak and that representational forms of neglect only occasionally co-exist with neglect in physical space.     

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.     

Perseveration in left spatial neglect: drawing and cancellation tasks

Perseveration in target cancellation tasks and in drawing by copy and from memory was investigated in 21 right-brain-damaged patients, seven with no evidence of left visuo-spatial neglect, and 14 with neglect. Eight such neglect patients showed perseveration in both cancellation and drawing tasks, although no correlation was found with the severity of neglect. Patients with perseveration were not disproportionately impaired in tasks assessing executive (fluency, Stroop colour-word interference, and Weigl's sorting test), and visuo-spatial short-term memory function. In the context of a two-component hypothesis, graphic perseveration (the first component) is a specific disorder that manifests in a variety of tasks, particularly those requiring serial graphic production. Unilateral spatial neglect (the second component) may trigger and facilitate the production of perseveration errors, with a contra-ipsilateral gradient of increasing severity.     

When right goes left: an investigation of line bisection in a case of visual neglect

We report a case of severe "left" visual neglect consequent upon lesion of the right temporo-parietal region. The patient's performance on line bisection is studied in three conditions: (a) standard line bisection; (b) evaluating and correcting the patient's previous erroneous bisections; and (c) evaluating and "correcting" correctly bisected lines. In all conditions, the magnitude of absolute displacement increases as a function of stimulus length; in all conditions, the direction of displacement changes from rightwards to leftwards at smaller lengths. We propose a new theory of line bisection in which both normal and pathological performance is interpreted in terms of two constructs: the magnitude of the Weber fraction, and the direction of approach to an "indifference zone" whose size is determined by the Weber fraction.     

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.     

Simulating unilateral neglect in normals using prism adaptation: implications for theory

Rightward deviation on line bisection is considered one of the most classic clinical signs of unilateral visual neglect--a cognitive disorder of spatial processing that commonly follows right brain damage. Recently, short-term adaptation to wedge prisms has been shown to significantly reduce neglect on this and other conventional diagnostic tasks. Our previous study has shown that visuomotor adaptation in normals produces a similar pattern of directional bias on a line bisection task. Based on the good working knowledge of how neglect patients perform on different versions of the standard diagnostic task, we showed here that using leftward-deviating prisms in normals, it is possible to produce: (1) a reliable bias on line bisection, (2) a rightward specific deviation, (3) a modulation of rightward deviation, which depends on the relative spatial location of the target lines and (4) a line length effect. A final experiment confirmed that these after-effects are specific to prism adaptation rather than passive prism exposure. Collectively, these findings confirm that adaptation to left-deviating prisms in normals produces a reliable right-sided bias and as shown by a previous visuospatial judgement task, these findings cannot be adequately explained by the symmetric sensori-motor effects of prism adaptation. Taken together with the improvement of spatial neglect shown by right-deviating prisms only, the present study suggests that low level sensori-motor adaptations play a greater role in right hemisphere organisation for spatial cognition than previously thought.     

Increased attentional load moves the left to the right

Introduction: Unilateral brain damage can heterogeneously alter spatial processing. Very often brain-lesioned patients fail to report (neglect) items appearing within the contralesional space. Much less often patients mislocalize items’ spatial position. We investigated whether a top-down attentional load manipulation (dual-tasking), known to result in contralesional omissions even in apparently unimpaired cases, might also induce spatial mislocalizations. Method: Nine right-hemisphere-damaged patients performed three computer-based tasks encompassing different levels of attentional load. The side of appearance of visual targets had to be reported either in isolation or while processing additional information (visual or auditory dual task). Spatial mislocalizations (from the contralesional hemispace towards the ipsilesional unaffected one) were then contrasted with omissions both within and across tasks, at individual as well as at group level. Results: The representation of ipsilesional targets was accurate and not affected by dual-tasking requirements. Contralesional targets were instead often omitted and, under dual-task conditions, also mislocalized by four patients. Three cases reported a significant number of left targets as appearing on the right (alloesthesia). Two of these patients perceived more targets (albeit to a wrong spatial location) under dual- than under single-task load. In a fourth patient, increased visual load resulted in synchiria, the (mis)perception of single, contralesional targets as being two (one on each side). Conclusions: When the neural circuitry subtending spatial processing is damaged, an increase in task load can lead to either a disregard or a bias in the processing of contralesional hemispace. The spatial bias subtending mislocalizations seems to index a more severe deficit than neglect, as if contralesional space would be completely erased rather than merely ignored.     

Classification images of two right hemisphere patients: a window into the attentional mechanisms of spatial neglect

While spatial neglect most commonly occurs after right hemisphere lesions, damage to diverse areas within the right hemisphere may lead to neglect, possibly through different mechanisms. To identify potentially different causes of neglect, the visual information used (the 'perceptual template') in a cueing task was estimated with a novel technique known as 'classification images' for five normal observers and two male patients with right-hemisphere lesions and previous histories of spatial neglect (CM, age 85; HL, age 69). Observers made a yes/no decision on the presence of a 'White X' checkerboard signal (1.5 degrees ) at one of two locations, with trial-to-trial stimulus noise added to the 9 checkerboard squares. Prior to the stimulus, a peripheral precue (140 ms) indicated the signal location with 80% validity. The cueing effects and estimated perceptual templates for the normal observers showed no visual field differences. Consistent with previous studies of spatial neglect, both patients had difficulty with left (contralesional) signals when preceded by a right (ipsilesional) cue. Despite similar behavioral results, the patients' estimated perceptual templates in the left field suggested two different types of attentional deficits. For CM, the left template matched the signal with left-sided cues but was opposite in sign to the signal with right-sided cues, suggesting a severely disrupted selective attentional strategy. For HL, the left templates indicated a general uncertainty in localizing the signal regardless of the cue's field. In conclusion, the classification images suggested different underlying mechanisms of neglect for these two patients with similar behavioral results and hold promise in further elucidating the underlying attentional mechanisms of spatial 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.     

Looking left with left neglect: The role of spatial attention when active vision selects local image features for fixation

When we actively explore the visual environment, our gaze preferentially selects regions characterized by high contrast and high density of edges, suggesting that the guidance of eye movements during visual exploration is driven to a significant degree by perceptual characteristics of a scene. Converging findings suggest that the selection of the visual target for the upcoming saccade critically depends on a covert shift of spatial attention. However, it is unclear whether attention selects the location of the next fixation uniquely on the basis of global scene structure or additionally on local perceptual information. To investigate the role of spatial attention in scene processing, we examined eye fixation patterns of patients with spatial neglect during unconstrained exploration of natural images and compared these to healthy and brain-injured control participants. We computed luminance, colour, contrast, and edge information contained in image patches surrounding each fixation and evaluated whether they differed from randomly selected image patches. At the global level, neglect patients showed the characteristic ipsilesional shift of the distribution of their fixations. At the local level, patients with neglect and control participants fixated image regions in ipsilesional space that were closely similar with respect to their local feature content. In contrast, when directing their gaze to contralesional (impaired) space neglect patients fixated regions of significantly higher local luminance and lower edge content than controls. These results suggest that intact spatial attention is necessary for the active sampling of local feature content during scene perception.     

The spatial encoding of body parts in patients with neglect and neurologically unimpaired participants

Body parts are represented in the brain in a very specific fashion, as compared to other three-dimensional objects, with reference to their prototypic shape and multisensory coding. However, evidence is lacking about the spatial representation of body parts. To address this issue, in Experiment 1 we first compared the metric representation of body parts and of non-bodily objects in 14 right-brain-damaged patients with left unilateral spatial neglect (USN), and in 14 neurologically unimpaired control participants. Participants bisected, by manual pointing, a three-dimensional object, or their own left forearm. Patients showed the well-known ipsilesional rightward displacement of the subjective midpoint, in both forearm and solid bisection. Both USN patients and control participants were overall more accurate in the bisection of their own forearm, relative to the extracorporeal object. In four patients this advantage of the forearm was significant in a single-case analysis, while two patients showed the opposite dissociation, being more accurate with the solid object. In Experiment 2 neurologically unimpaired participants were more accurate in the bisection of a fake forearm, as well as of their own forearm, as compared to the extrapersonal object. Overall, the results indicate that the representation of the metric of the body is more reliable than that of extrapersonal objects, and also more resistant to the disruption of spatial representations brought about by USN, possibly due to the prototypical shape of body parts. Furthermore, the double dissociation found in USN patients suggests that the metrics of body parts and of extrapersonal objects are supported by independent spatial processes.     

The importance of time limits in detecting signs of left visual peripersonal neglect: a multiple single-case,pilot study

ABSTRACT

We developed a new visual search test to assess signs of left visual peripersonal neglect. Five right-hemisphere-damaged patients, 10 healthy controls, and 10 orthopedic controls were administered the test in four conditions: easy task (no distractors)/time-limited (45’’), easy task (no distractors)/time-unlimited, difficult task (distractors)/time-limited (45’’), difficult task (distractors)/time-unlimited. With respect to controls, most RHDP showed signs of left visual peripersonal neglect in the time-limited condition, but not in the time-unlimited condition, particularly on the difficult task. We suggest that the presence of appropriate time limits, in difficult visual search tasks, could considerably improve the diagnosis of left visual peripersonal neglect.     

Paradoxical extension into the contralesional hemispace in spatial neglect

To explore the idea of a perceptual distortion of space in spatial neglect, neglect patients, age-matched healthy controls and right hemisphere control patients judged the vanishing point of horizontally and vertically-moving stimuli. Hemifield of presentation and movement direction of the stimulus presentation was manipulated. The results suggest that neglect patients show a stronger response bias in the direction of the moving stimuli ("representational momentum") than healthy and right hemisphere controls. Furthermore, neglect patients, but not the control groups, showed a direction-specific response whereby the presence of neglect was associated with a larger representational momentum for leftward-moving stimuli. The one left-hemisphere patient with right-sided neglect showed the opposite effect. Thus, neglect patients showed a relative overextension into their neglected side of space. While these findings are in line with the idea of an extension in the representation of contralesional space, other explanations such as deficient spatial remapping, impairments in smooth pursuit and distortions in memorized visuo-motor movements are considered.     

Perceived gaze direction in faces and spatial attention: a study in patients with parietal damage and unilateral neglect

Perceived gaze in faces is an important social signal that may influence orienting of attention in normal observers. Would such effects of gaze still occur in patients with right parietal damage and left neglect who usually fail to attend to contralesional space? Two experiments tested for effects of perceived gaze on visual extinction. Face or shape stimuli were presented in the right, left, or both hemifields, with faces looking either straight ahead or toward the opposite field. On bilateral trials, patients extinguished a left shape much less often when a concurrent right face looked leftward rather than straight ahead. This occurred, even though gaze was not relevant to the task and processing of facial signals implied attention to a competing ipsilesional stimulus. By contrast, rightward gaze in faces presented on the left side had no effect on extinction, suggesting that gaze cues are not extracted without attention. Two other experiments examined effects of perceived gaze on the detection of peripheral targets. Targets appeared at one of four possible locations to the right or left of a central face looking either toward the target location, another location on the same side, the opposite side, or straight ahead. Face and gaze were not relevant to the task and not predictive of target location. Patients responded faster when the face looked toward the target on both the contralesional and ipsilesional sides. In contralesional space, gaze allowed shifting of attention in a specific quadrant direction, but only to the first target along the scan path when there were different possible locations on the same side. By contrast, in intact ipsilesional space, attention was selectively directed to one among different eccentric locations. Control experiments showed that symbolic arrow cues did not produce similar effects. These results indicate that even though parietal damage causes spatial neglect and impairs the representation of location on the contralesional side, perceived gaze in faces can still trigger automatic shifts of attention in the contralesional direction, suggesting the existence of specific and anatomically distinct attentional mechanisms.     

Task-related modulation of visual neglect in cancellation tasks

Unilateral neglect involves deficits of spatial exploration and awareness that do not always affect a fixed portion of extrapersonal space, but may vary with current stimulation and possibly with task demands. Here, we assessed any ‘top-down’, task-related influences on visual neglect, with novel experimental variants of the cancellation test. Many different versions of the cancellation test are used clinically, and can differ in the extent of neglect revealed, though the exact factors determining this are not fully understood. Few cancellation studies have isolated the influence of top-down factors, as typically the stimuli are changed also when comparing different tests. Within each of three cancellation studies here, we manipulated task factors, while keeping visual displays identical across conditions to equate purely bottom-up factors. Our results show that top-down task demands can significantly modulate neglect as revealed by cancellation on the same displays. Varying the target/non-target discrimination required for identical displays has a significant impact. Varying the judgement required can also have an impact on neglect even when all items are targets, so that non-targets no longer need filtering out. Requiring local versus global aspects of shape to be judged for the same displays also has a substantial impact, but the nature of discrimination required by the task still matters even when local/global level is held constant (e.g. for different colour discriminations on the same stimuli). Finally, an exploratory analysis of lesions among our neglect patients suggested that top-down task-related influences on neglect, as revealed by the new cancellation experiments here, might potentially depend on right superior temporal gyrus surviving the lesion.     

Orientation bias in unilateral neglect: representational contributions

Left-neglect patients bisect horizontal lines to the right of true center. Longer lines are bisected further to the right than shorter lines. This line-length effect might be explained by an increase in the rightward bias of attention because longer lines extend further ipsilesionally. Alternatively, neglect patients might be limited in their abilities to internally represent horizontal magnitudes. Patients might orient further rightward with longer lines because these lines have longer representations. If the line-length effect occurs on lines of identical objective length but they are represented differently, then central mechanisms must contribute to the orientation bias. We constructed two types of lines that were perceived by normal subjects as having different lengths, but were of identical extents. Three neglect patients bisected lines perceived as longer, further to the right than lines perceived as shorter. These results demonstrate that relative magnitudes of internal representations contribute to the degree of bias in neglect patients.     

Focused visual attention distorts distance perception away from the attentional locus

Several lines of evidence show that visual perception is altered at the locus of visual attention: detection is faster, performance better and spatial resolution increased. It is however not known whether attention can affect visual perception further away from its locus. In the present study, we specifically question whether and how visual attention influences spatial perception away from its locus, independently from any saccadic preparation. We use a landmark task in which subjects have to estimate the location of a bisection stimulus relative to two landmark stimuli 15° apart, while fixating one of them. This task is combined with a highly demanding discrimination task performed on one of the two landmarks. This allows us to test for the effect of spatial attention allocation on distance perception, as measured by the subject estimation of the landmarks midpoint. We show that the estimated midpoint is displaced towards the attentional locus, both when attention is instructed on the central landmark or on the peripheral landmark. These results suggest an overrepresentation of space around the attentional locus that can affect perception up to 8° away, and question the existence of an objective spatial representation. They are in line with reports of spatial distortion in hemineglect patients while they strikingly contrast with the spatial compression reported around the time of saccadic execution.     

Damage to the right superior longitudinal fasciculus in the inferior parietal lobe plays a role in spatial neglect

Functional neurological changes after surgery combined with data from diffusion tensor imaging (DTI) studies can provide direct evidence of anatomical localization of brain function. The goal of the present study was to characterize mechanisms of spatial neglect using these techniques by analyzing two patients with development or worsening of left neglect after surgery at our hospital in 2008. In both cases, the surgical approach was via the right inferior parietal lobes, and damage to the superior longitudinal fasciculus (SLF) was demonstrated after surgery by DTI tractography. By contrast, neither the inferior longitudinal fasciculus (ILF) nor the inferior fronto-occipital fasciculus (IFOF) was damaged. These results suggest that damage to the right SLF in the inferior parietal lobe plays a critical role in the development of spatial neglect.