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

The attentional repulsion effect and relative size judgments

Rapid shifts of involuntary attention have been shown to induce mislocalizations of nearby objects. One pattern of mislocalization, termed the Attentional Repulsion Effect (ARE), occurs when the onset of peripheral pre-cues lead to perceived shifts of subsequently presented stimuli away from the cued location. While the standard ARE configuration utilizes vernier lines, to date, all previous ARE studies have only assessed distortions along one direction and tested one spatial dimension (i.e., position or shape). The present study assessed the magnitude of the ARE using a novel stimulus configuration. Across three experiments participants judged which of two rectangles on the left or right side of the display appeared wider or taller. Pre-cues were used in Experiments 1 and 2. Results show equivalent perceived expansions in the width and height of the pre-cued rectangle in addition to baseline asymmetries in left/right relative size under no-cue conditions. Altering cue locations led to shifts in the perceived location of the same rectangles, demonstrating distortions in perceived shape and location using the same stimuli and cues. Experiment 3 demonstrates that rectangles are perceived as larger in the periphery compared to fixation, suggesting that eye movements cannot account for results from Experiments 1 and 2. The results support the hypothesis that the ARE reflects a localized, symmetrical warping of visual space that impacts multiple aspects of spatial and object perception.

Equidistant Intervals in Perspective Photographs and Paintings

Human vision is extremely sensitive to equidistance of spatial intervals in the frontal plane. Thresholds for spatial equidistance have been extensively measured in bisecting tasks. Despite the vast number of studies, the informational basis for equidistance perception is unknown. There are three possible sources of information for spatial equidistance in pictures, namely, distances in the picture plane, in physical space, and visual space. For each source, equidistant intervals were computed for perspective photographs of walls and canals. Intervals appear equidistant if equidistance is defined in visual space. Equidistance was further investigated in paintings of perspective scenes. In appraisals of the perspective skill of painters, emphasis has been on accurate use of vanishing points. The current study investigated the skill of painters to depict equidistant intervals. Depicted rows of equidistant columns, tiles, tapestries, or trees were analyzed in 30 paintings and engravings. Computational analysis shows that from the middle ages until now, artists either represented equidistance in physical space or in a visual space of very limited depth. Among the painters and engravers who depict equidistance in a highly nonveridical visual space are renowned experts of linear perspective.

Line Bisection and Rebisection: The Crossover Effect of Space Location

Objective. To investigate the relationship between the bisection test and the severity of behavioral hemineglect and to verify if this test can predict the behavioral hemineglect. Methods. Thirty stroke patients with left hemiparesis were divided into 4 groups according to the Catherine Bergego Scale, which assessed the behavioral hemineglect: severe unilateral neglect (UN), moderate UN, mild UN, and lack of UN. Eleven healthy subjects served as age-matched control subjects. In the bisection test, 18 lines were presented on the left, middle, and right of an A4 paper, respectively. The subjects were asked to place a short cross mark in the exact middle point of each line on the paper using their right hand. The middle 6 lines in the above bisection test were extracted on another sheet of A4 paper for the rebisection test. The subjects were asked to divide a line into 4 segments by successive bisections. The proportion of the right part to the length of line for bisecting was calculated. Results. In the bisection test, the main effect of space was significant in every group except the mild neglect group. The crossover effect of space location was found in the severe UN group, the group without UN, and the controls. In the severe UN group, the patients bisected the left and middle lines with rightward bias (<50%) but bisected the right lines with leftward bias (>50%). In the group without UN and the controls, the subjects bisected the left lines with leftward bias (>50%) but bisected the middle and right lines with rightward bias (<50%). Almost the same results were seen in the rebisection test. Conclusions. This study showed that if the spatial crossover effect occurred in the right space condition, it was strongly supported that this patient had moderate to severe behavioral hemineglect. The crossover effect of the space location was explained by a new model.

The Effect of Gaze Position on Reaching Movements in an Obstacle Avoidance Task

Numerous studies have addressed the issue of where people look when they perform hand movements. Yet, very little is known about how visuomotor performance is affected by fixation location. Previous studies investigating the accuracy of actions performed in visual periphery have revealed inconsistent results. While movements performed under full visual-feedback (closed-loop) seem to remain surprisingly accurate, open-loop as well as memory-guided movements usually show a distinct bias (i.e. overestimation of target eccentricity) when executed in periphery. In this study, we aimed to investigate whether gaze position affects movements that are performed under full-vision but cannot be corrected based on a direct comparison between the hand and target position. To do so, we employed a classical visuomotor reaching task in which participants were required to move their hand through a gap between two obstacles into a target area. Participants performed the task in four gaze conditions: free-viewing (no restrictions on gaze), central fixation, or fixation on one of the two obstacles. Our findings show that obstacle avoidance behaviour is moderated by fixation position. Specifically, participants tended to select movement paths that veered away from the obstacle fixated indicating that perceptual errors persist in closed-loop vision conditions if they cannot be corrected effectively based on visual feedback. Moreover, measuring the eye-movement in a free-viewing task (Experiment 2), we confirmed that naturally participants’ prefer to move their eyes and hand to the same spatial location.

Spatial distortions in localization and midline estimation in hemianopia and normal vision

Studies have shown that individuals with hemianopia tend to bisect a line toward their blind, contralesional visual field, termed the hemianopic line bisection error (HLBE). One theory proposes that the HLBE is a perceptual distortion resulting from expansion of the central region of visual space. If true, perceptual expansions of the central regions in the intact hemifield should also be present and observable across different tasks. We tested this hypothesis using a peripheral localization task to assess localization and midpoint estimation along the horizontal axis of the visual field. In this task, participants judged the location of a target dot presented inside a Goldmann perimeter relative to their perceived visual field boundary. In Experiment 1, we tested neurologically healthy participants on the peripheral localization task as well as a novel midpoint assessment task in which participants reported their perceived midpoint along the horizontal axis of their left and right visual fields. The results revealed consistency in individual biases across the two tasks. We then used the peripheral localization task to test whether two patients with hemianopia showed a selective expansion of central visual space. For these patients, three axes were tested: the spared temporal horizontal axis and the upper and lower vertical axes. The results support the notion that the HLBE is due to expansion of perceived space along the spared temporal axis. Together, the results of both experiments validate the use of these novel paradigms for exploring perceptual asymmetries in both healthy individuals and patients with visual field loss.

Modelling the differential effects of prisms on perception and action in neglect

Damage to the right parietal cortex often leads to a syndrome known as unilateral neglect in which the patient fails to attend or respond to stimuli in left space. Recent work attempting to rehabilitate the disorder has made use of rightward-shifting prisms that displace visual input further rightward. After a brief period of adaptation to prisms, many of the symptoms of neglect show improvements that can last for hours or longer, depending on the adaptation procedure. Recent work has shown, however, that differential effects of prisms can be observed on actions (which are typically improved) and perceptual biases (which often remain unchanged). Here, we present a computational model capable of explaining some basic symptoms of neglect (line bisection behaviour), the effects of prism adaptation in both healthy controls and neglect patients and the observed dissociation between action and perception following prisms. The results of our simulations support recent contentions that prisms primarily influence behaviours normally thought to be controlled by the dorsal stream.

Pathways involved in human conscious vision contribute to obstacle-avoidance behaviour

Human patients with visual field defects following damage to their primary visual cortex (V1) will often misperceive the midpoint of a horizontal line. They tend to shift the midpoint away from the real position towards their blind field. In patients with unilateral neglect, where midpoint shifts can also be observed, these perceptual errors do not lead to errors in an obstacle-avoidance task, which also requires the ability to find the midpoint between two obstacles. This dissociation in neglect patients was taken as evidence that obstacle-avoidance performance is guided by visual information from the dorsal visual stream. Recently it was shown that a patient with hemianopia could avoid an obstacle presented in his blind field. This suggests that obstacle-avoidance behaviour can be guided by subconscious vision alone involving a direct route from extrageniculate structures in the brain to dorsal stream areas. To investigate whether obstacle avoidance relies only on this subconscious route or also uses information from pathways involved in conscious vision, we examined the effect of the hemianopic shift on obstacle-avoidance behaviour. This shift is found in tasks where a conscious visual judgement is required and presumably arises in pathways underlying conscious vision (V1 and ventral stream areas). We compared the performance of six patients with left hemianopia with the performance of six patients with right hemianopia. We found a clear bias in both groups, which also affected obstacle-avoidance performance. It is thus concluded that obstacle avoidance does not bypass the system for conscious vision completely.

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.

Is the origin of the hemianopic line bisection error purely visual? Evidence from eye movements in simulated hemianopia

It is still unclear whether the contralateral line bisection error in unilateral homonymous hemianopia is caused by the visual field defect, strategic oculomotor adaptation or by additional extrastriate brain injury. We therefore simulated hemianopia in healthy participants using a gaze-contingent display paradigm and investigated its effects on manual and ocular line bisection performance and eye-movements. Although simulated hemianopia impaired line bisection and induced the adaptive oculomotor eye-movement pattern of hemianopic patients, it did not induce the contralateral bisection error, suggesting that neither the visual field defect nor oculomotor adaptation to it are the primary causes of the hemianopic bisection error.

Line bisection: does introspection inform cognitive strategy?

Line bisection is widely used to diagnose and quantify hemispatial neglect, yet there is little consensus as to the cognitive mechanisms used to perform this simple task. Current cognitive accounts have been deduced solely from behavioural measures. The aim of this study was to discover if subject's own knowledge of the mental strategies used to perform the task actually informs behavioural performance. One hundred and forty healthy volunteers bisected a set of lines and were asked to describe the mental strategies used. Three distinct strategies were identified. These were (1) comparing two segments, (2) computing the centre of mass, and (3) externally centred strategies. Strategies 1 and 2 have previously been described but externally centred strategies have not been reported as a distinct strategy in bisection. Although none of the three strategies predicted performance this may have been due to the fact that 44% of subjects failed to describe any strategy. Men and women bisected lines equally well, however more men than women reported use of externally centred strategies.

Hemispatial asymmetries in judgment of stimulus size

Recent research has demonstrated a leftward bias in judgments of size. In the present experiments, hemispatial size bias was measured through simultaneous presentation of a circle and an ellipse varying in horizontal or vertical extent. A consistent leftward bias of horizontal size judgments (but not vertical) was obtained; at the point of subjective equality, the width of the objects that were presented in left hemispace was smaller than the width of the objects that were presented in right hemispace. These data suggest that the horizontal extent of stimuli appear larger in left hemispace than in right hemispace. Results also indicated that symmetrical stimulus presentation, with respect to the vertical meridian, is required for the bias to emerge. Furthermore, increasing or decreasing stimulus eccentricity weakened the effect. Attenuation of this bias upon the manipulation of parameters indicates that this phenomenon is context specific and is affected by similar parameters that are known to influence the magnitude of error in pseudoneglect.

A deficit in discriminating gaze direction in a case with right superior temporal gyrus lesion

The superior temporal sulcus (STS) region is well recognized as being heavily involved in detecting and discriminating gaze. Lesions confined to this area are quite rare in humans, and so the research has mainly depended on animal studies and functional neuroimaging in normal human subjects. We report one such rare case, a 54-year-old Japanese female with a possible congenital s anomaly who, after a cerebral hemorrhage, demonstrated a lesion almost completely confined to the entire right superior temporal gyrus (STG). In the subacute phase, the patient showed evidence of left hemispatial neglect, from which she gradually recovered. In the chronic phase, she showed a puzzling difficulty in obtaining eye-contact. We have conducted, in conjunction with conventional neuropsychological evaluations, experimental assessment of her ability in gaze cognition. Her performance on neuropsychological testing demonstrated no compromise in intellect, memory, or language skills, and a close-to-full recovery from neglect. On gaze cognition experiments, she was repeatedly shown to perceive left gaze as straight, and to a lesser degree, straight gaze as right. We suggest that the function of the STG in detecting gaze, together with the directional information it receives from earlier visual areas, may be associated, when damaged, with this deficit in detecting contra-directional gaze. We have demonstrated for the first time that a single circumscribed lesion to the STG results in both gaze processing deficit and concurrent aberrant gaze behavior of the victim herself, implicating a mechanism within the STG as an interface between gaze of others and gaze of self.

Spatial Coding of the Predicted Impact Location of a Looming Object

Avoiding or intercepting looming objects implies a precise estimate of both time until contact and impact location. In natural situations, extrapolating a movement trajectory relative to some egocentric landmark requires taking into account variations in retinal input associated with moment-to-moment changes in body posture. Here, human observers predicted the impact location on their face of an approaching stimulus mounted on a robotic arm, while we systematically manipulated the relation between eye, head, and trunk orientation. The projected impact point on the observer's face was estimated most accurately when the target originated from a location aligned with both the head and eye axes. Eccentric targets with respect to either axis resulted in a systematic perceptual bias ipsilateral to the trajectory's origin. We conclude that (1) predicting the impact point of a looming target requires combining retinal information with eye position information, (2) that this computation is accomplished accurately for some, but not all, possible combinations of these cues, (3) that the representation of looming trajectories is not formed in a single, canonical reference frame, and (4) that the observed perceptual biases could reflect an automatic adaptation for interceptive/defensive actions within near peripersonal space.

Perceptual asymmetries in normal children and children with attention deficit/hyperactivity disorder

Perceptual asymmetries in normal right-handed children (7-12 years of age) and children with attention deficit/hyperactivity disorder (ADHD), combined type, were investigated using various chimeric stimuli in free-viewing conditions. In the face-matching task, participants indicated which of two symmetrical face chimeras more closely resembled the original; in the chimeric faces task, participants indicated which of a pair of faces appeared happier; and in the grey scales task participants indicated which of two shaded rectangles appeared to be darker overall. Leftward biases were found for normal children with no effects of age. Contrary to expectations, normal leftward biases were also found for ADHD children in the face-matching and the chimeric faces tasks; however, a significant leftward bias was not observed in the grey scales task. The absence of anomalous perceptual bias in ADHD children on these purely perceptual tasks, suggests that anomalous perceptual asymmetries observed in other tasks (line bisection and cancellation tasks) may have been confounded by the motor response, and/or the explicit spatial components of those tasks.

Simulating a lesion in a basis function model of spatial representations: comparison with hemineglect

The basis function theory of spatial representations explains how neurons in the parietal cortex can perform nonlinear transformations from sensory to motor coordinates. The authors present computer simulations showing that unilateral parietal lesions leading to a neuronal gradient in basis function maps can account for the behavior of patients with hemineglect, including (a) neglect in line cancellation and line bisection experiments; (b) neglect in multiple frames of reference simultaneously; (c) relative neglect, a form of what is sometime called object-centered neglect; and (d) neglect without optic ataxia. Contralateral neglect arises in the model because the lesion produces an imbalance in the salience of stimuli that is modulated by the orientation of the body in space. These results strongly support the basis function theory for spatial representations in humans and provide a computational model of hemineglect at the single-cell level.

The influence of unimanual response on pseudoneglect magnitude

Various factors influence the degree of leftward error (pseudoneglect) (Bowers & Heilman, 1980) that typifies the performance of normal subjects in line bisection tasks (Jewell & McCourt, 2000). The results of this experiment show that unimanual responding also exerts a subtle but significant modulating influence on spatial attention, as indexed by the differential magnitude of pseudoneglect. Using a forced-choice tachistoscopic line bisection protocol, 184 subjects (92 male and 92 female) bisected horizontally oriented lines (22.3 degrees wide x 0.39 degrees height) presented to central vision in two conditions, in which bisection responses were executed via button presses using the first two fingers of either the left (LH) or right (RH) hand. Perceived line midpoint deviated significantly leftward of veridical (p <.05) in both conditions. There was no significant influence of subject sex (p >.05). A significant influence of unimanual response was revealed (p <.05) where pseudoneglect magnitude was greater in the LH than the RH condition. The results are interpreted within the framework of the activation-orientation theory of attentional asymmetry.

Line versus representational bisections in unilateral spatial neglect

OBJECTIVES

To clarify the mechanisms of left unilateral spatial neglect found in the bisection of lines after cueing to the left end point and to determine whether neglect occurs for the mental representation of a line.

METHODS

A new representational bisection task was developed to eliminate the influence of the right segment of the physical line that would attract attention. Eight patients with typical left unilateral spatial neglect underwent line and representational bisection tasks on a computer display with a touch panel. In the line bisection with cueing, they bisected a line after touching the left end point. In the representational bisection, the patients were presented with a line until they touched the left end point. On the blank display, they pointed to the subjective midpoint of the erased line. The performances of the two bisection tasks were compared when the length and position of stimulus lines were varied.

RESULTS

The rightward errors in the representational bisection were greater than or equivalent to those in the line bisection with cueing. The effect of line length in which the errors became greater for the longer lines was equally found in the line bisection with cueing and the representational bisection. This was confirmed in the condition where the right end point was placed at a fixed position and the line length was varied.

CONCLUSIONS

After cueing to the left end point, rightward bisection errors of patients with neglect are not caused by overattention to the right segment of the physical line. Left neglect occurs mainly for the mental representation formed at the time of cueing or seeing the whole extent of a line.

Asymmetries of visuospatial attention are modulated by viewing distance and visual field elevation: pseudoneglect in peripersonal and extrapersonal space

Many factors influence the degree of leftward error (pseudoneglect) that typifies the line bisection performance of normal subjects. We find that viewing distance also exerts a modulating influence on spatial attention in normal subjects, as it appears to do in neglect syndrome. Using forced-choice tachistoscopic line bisection, 38 right-handed subjects (15 male, 23 female) bisected horizontal lines (13.7 degrees w x 0.24 degrees h) presented in the midsagittal plane as a function of line elevation (- 3.6 degrees, 0 degrees, and 3.6 degrees relative to horizontal midline) and viewing distance (45 and 90 cm). We find a significant main effect of viewing distance, F (1, 37) = 10.04, p = .003, where pseudoneglect is larger in peripersonal (45 cm) than in extrapersonal (90 cm) space. We replicate an effect of line elevation, F (2, 74) = 4.40, p = .016, where pseudoneglect is greatest in the superior visual field (McCourt and Jewell, 1999). The interaction was not significant, p > .05. Thus, we find evidence for independent spatiotopic (viewing distance) and retinotopic (line elevation) effects on line bisection performance in normal observers, suggesting that the allocation of visuospatial attention is modulated within multiple frameworks.

Stimulus modulation of pseudoneglect: influence of line geometry

A variety of stimulus factors have recently been shown to influence the performance of normal subjects on line bisection tasks (i.e., pseudoneglect), independent of motoric factors such as scanning or hand use [McCourt & Jewell (1999) Neuropsychologia 35, 843-55]. An experiment is described which further examined the modulating influence of line geometry in determining the magnitude of pseudoneglect. Subjects bisected horizontally oriented trapezoidal lines presented in central vision whose narrow end pointed either left or right. A highly significant influence of line geometry was found which modulated a tonic leftward error (i.e., pseudoneglect). The results are interpreted in the context of a "center-of-mass" effect [Shuren, Jacobs & Heilman (1997) Brain and Cognition, 34, 293-300]. Further studies designed to tease apart the potentially independent effects of perceptual and attentional asymmetry on bisection performance are suggested.

Pseudoneglect: a review and meta-analysis of performance factors in line bisection tasks

An exhaustive qualitative (vote-counting) review is conducted of the literature concerning visual and non-visual line bisection in neurologically normal subject populations. Although most of these studies report a leftward bisection error (i.e., pseudoneglect), considerable between-study variability and inconsistency characterize this literature. A meta-analysis of this same literature is performed in which the total quantitative data set, comprising 73 studies (or sub-studies) and 2191 subjects, is analyzed with respect to 26 performance factors. The meta-analytic results indicate a significant leftward bisection error in neurologically normal subjects, with an overall effect size of between -0.37 and -0.44 (depending on integration method), which is significantly modulated to varying degrees by a number of additional task or subject variables. For example, visual bisection tasks, midsagittal-pointing tasks and tactile bisection tasks all lead to leftward errors, while kinesthetic tasks result in rightward errors. Tachistoscopic forced-choice testing methods reveal much greater estimates of bisection error (effect size = -1.32) than do manual method-of-adjustment procedures (effect size= -0.40). Subject age significantly modulates line bisection performance such that older subjects err significantly rightward compared to younger subjects, and to veridical line midpoint. Male subjects make slightly larger leftward errors than do female subjects. Handedness has a small effect on bisection errors, with dextrals erring slightly further to the left than sinistral subjects. The hand used to perform manual bisection tasks modulated performance, where use of the left hand lead to greater leftward errors than those obtained using the right hand. One of the most significant factors modulating bisection error is the direction in which subjects initiate motor scanning (with either eye or hand), where a left-to-right scan pattern leads to large leftward errors while a right-to-left scan pattern leads to rightward errors.