Line bisection in simulated homonymous hemianopia

A systematic review on visual scanning behaviour in hemianopia considering task specificity, performance improvement, spontaneous and training-induced adaptations

PURPOSE

People with homonymous hemianopia (HH) benefit from applying compensatory scanning behaviour that limits the consequences of HH in a specific task. The aim of the study is to (i) review the current literature on task-specific scanning behaviour that improves performance and (ii) identify differences between this performance-enhancing scanning behaviour and scanning behaviour that is spontaneously adopted or acquired through training.

MATERIALS AND METHODS

The databases PsycInfo, Medline, and Web of Science were searched for articles on scanning behaviour in people with HH.

RESULTS

The final sample contained 60 articles, reporting on three main tasks, i.e., search (N = 17), reading (N = 16) and mobility (N = 14), and other tasks (N = 18). Five articles reported on two different tasks. Specific scanning behaviour related to task performance in search, reading, and mobility tasks. In search and reading tasks, spontaneous adaptations differed from this performance-enhancing scanning behaviour. Training could induce adaptations in scanning behaviour, enhancing performance in these two tasks. For mobility tasks, limited to no information was found on spontaneous and training-induced adaptations to scanning behaviour.

CONCLUSIONS

Performance-enhancing scanning behaviour is mainly task-specific. Spontaneous development of such scanning behaviour is rare. Luckily, current compensatory scanning training programs can induce such scanning behaviour, which confirms that providing scanning training is important.IMPLICATIONS FOR REHABILITATIONScanning behaviour that improves performance in people with homonymous hemianopia (HH) is task-specific.Most people with HH do not spontaneously adopt scanning behaviour that improves performance.Compensatory scanning training can induce performance-enhancing scanning behaviour.

Simulation of visual impairment in persons with normal vision for scientific research

Simulation of visual impairment in healthy eyes has multiple applications in students' training, research and product development. However, due to the absence of an existing standard protocol, the method of simulation was left to the discretion of the researcher. This review aimed to outline the various methods of simulating visual impairment and categorising them. A scoping review of the relevant publications was conducted. Of the 1593 articles originally retrieved from the databases, 103 were included in the review. The characteristics of the participants, the method for simulation of the visual impairment in persons with normal vision and the level or type of visual impairment that was simulated were extracted from the papers. None of the methods of simulation can be judged as being superior to the others. However, electronic displays produced the most consistent form of visual impairment simulation.

Visual Field Loss: Integrating Overlayed Information to Increase the Effective Field of View

Visual field loss is a debilitating impairment that can impact normal daily activities. The advancement of augmented and virtual realities brings opportunities for potential substitutive technologies for visual field loss. Here we outline a conceptual approach to increasing the amount of useful information by overlaying the blind field into the sighted field. In this proof-of-concept experiment, 33 observers were allocated to either a left or right blind condition (with a simulated scotoma). All observers completed a line bisection task in all three conditions (baseline, scotoma, manipulation), with the baseline condition always completed first. The scotoma condition (baseline with the addition of a simulated scotoma) and the manipulated condition (baseline with the addition of a simulated scotoma, and a "minified window overlay") were randomised in order of presentation. Predictably, our results show that a simulated scotoma impaired performance on the task. However, observers were able to make use the overlay to improve their estimation of the line's midpoint. Our results show that a substitutive augmentation of this type improved accuracy in estimating the midpoint of a line with a (simulated) scotoma.

The impact of simulated hemianopia on visual search for faces, words, and cars

Tests of visual search can index the effects of perceptual load and compare the processing efficiency for different object types, particularly when one examines the set-size effect, the increase in search time for each additional stimulus in an array. Previous studies have shown that the set-size effect is increased by manoeuvres that impede object processing, and in patients with object processing impairments. In this study, we examine how the low-level visual impairment of hemianopia affects visual search for complex objects, using a virtual paradigm. Forty-two healthy subjects performed visual search for faces, words, or cars with full-viewing as well as gaze-contingent simulations of complete left or right hemianopia. Simulated hemianopia lowered accuracy and discriminative power and increased response times and set-size effects, similarly for faces, words and cars. A comparison of set-size effects between target absent and target present trials did not show a difference between full-view and simulated hemianopic conditions, and a model of decision-making suggested that simulated hemianopia reduced the rate of accumulation of perceptual data, but did not change decision thresholds. We conclude that simulated hemianopia reduces the efficiency of visual search for complex objects, and that such impairment should be considered when interpreting results from high-level object processing deficits.

Impairments of Visuospatial Attention in Children with Unilateral Spastic Cerebral Palsy

Aim

This observational study aimed at assessing the prevalence of visuospatial attention deficits in children with unilateral spastic cerebral palsy (USCP), taking into consideration the affected hemibody and the localization of the brain lesion.

Method

Seventy-five children with USCP were assessed with four visuospatial attention tests: star cancellation, Ogden figure copy, line bisection, and proprioceptive pointing.

Results

A majority (64%) of children with USCP presented a deficit in at least one test compared to the reference values. The alterations observed in children with left or right USCP were related to egocentric or allocentric neglect, respectively. Children with cortico/subcortical lesion presented more often visuospatial attention deficits than children with periventricular lesion. Visuospatial attention deficits were not associated with brain lesion locations.

Interpretation

Visuospatial attention deficits are prevalent in children with USCP and should be taken into account during their rehabilitation process. The present results shed new light on the interpretation of motor impairments in children with USCP as they may be influenced by the frequent presence of visuospatial deficits.

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.

Word and text processing in acquired prosopagnosia

OBJECTIVE

A novel hypothesis of object recognition asserts that multiple regions are engaged in processing an object type, and that cerebral regions participate in processing multiple types of objects. In particular, for high-level expert processing, it proposes shared rather than dedicated resources for word and face perception, and predicts that prosopagnosic subjects would have minor deficits in visual word processing, and alexic subjects would have subtle impairments in face perception. In this study, we evaluated whether prosopagnosic subjects had deficits in processing either the word content or the style of visual text.

METHODS

Eleven prosopagnosic subjects, 6 with unilateral right lesions and 5 with bilateral lesions, participated. In the first study, we evaluated their word length effect in reading single words. In the second study, we assessed their time and accuracy for sorting text by word content independent of style, and for sorting text by handwriting or font style independent of word content.

RESULTS

Only subjects with bilateral lesions showed mildly elevated word length effects. Subjects were not slowed in sorting text by word content, but were nearly uniformly impaired in accuracy for sorting text by style.

INTERPRETATION

Our results show that prosopagnosic subjects are impaired not only in face recognition but also in perceiving stylistic aspects of text. This supports a modified version of the many-to-many hypothesis that incorporates hemispheric specialization for processing different aspects of visual text.

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.

The blindside: impact of monocular occlusion on spatial attention

Monocular occlusion has been posited to reduce activation of the contralateral hemisphere ("Sprague effect"), thus inducing a contralateral spatial bias (toward the viewing eye). Healthy right-handed participants bisected horizontal lines during monocular eye viewing. Although subjects tended to deviate away from the viewing eye, only left-eye viewing deviated significantly right of midline. These results suggest that eye patching may induce an attentional compensation similar to that in hemianopic patients. Alternatively, increased activation of higher cortical regions mediating spatial attention in contralateral hemispace may be an adaptive response to decreased activation of its ipsilateral superior colliculus induced by contralateral eye patching.

Visual search and line bisection in hemianopia: computational modelling of cortical compensatory mechanisms and comparison with hemineglect

Hemianopia patients have lost vision from the contralateral hemifield, but make behavioural adjustments to compensate for this field loss. As a result, their visual performance and behaviour contrast with those of hemineglect patients who fail to attend to objects contralateral to their lesion. These conditions differ in their ocular fixations and perceptual judgments. During visual search, hemianopic patients make more fixations in contralesional space while hemineglect patients make fewer. During line bisection, hemianopic patients fixate the contralesional line segment more and make a small contralesional bisection error, while hemineglect patients make few contralesional fixations and a larger ipsilesional bisection error. Hence, there is an attentional failure for contralesional space in hemineglect but a compensatory adaptation to attend more to the blind side in hemianopia. A challenge for models of visual attentional processes is to show how compensation is achieved in hemianopia, and why such processes are hindered or inaccessible in hemineglect. We used a neurophysiology-derived computational model to examine possible cortical compensatory processes in simulated hemianopia from a V1 lesion and compared results with those obtained with the same processes under conditions of simulated hemineglect from a parietal lesion. A spatial compensatory bias to increase attention contralesionally replicated hemianopic scanning patterns during visual search but not during line bisection. To reproduce the latter required a second process, an extrastriate lateral connectivity facilitating form completion into the blind field: this allowed accurate placement of fixations on contralesional stimuli and reproduced fixation patterns and the contralesional bisection error of hemianopia. Neither of these two cortical compensatory processes was effective in ameliorating the ipsilesional bias in the hemineglect model. Our results replicate normal and pathological patterns of visual scanning, line bisection, and differences between hemianopia and hemineglect, and may explain why compensatory processes that counter the effects of hemianopia are ineffective in hemineglect.

Leftward biases in picture scanning and line bisection: a gaze-contingent window study

A bias for humans to attend to the left side of space has been reported in a variety of experiments. While patients with hemispatial neglect mistakenly bisect horizontal lines to the right of centre, neurologically healthy individuals show a mean leftward error. Here, two experiments demonstrated a robust tendency for participants to saccade to the left when viewing photographs. We were able to manipulate this bias by using an asymmetrical gaze-contingent window, which revealed more of the scene on one side of fixation-causing participants to saccade more often in that direction. A second experiment demonstrated the same change in eye movements occurring rapidly from trial to trial, and investigated whether it would carry over and effect attention during a line bisection task. There was some carry-over from gaze-contingent scene viewing to the eye movements during line bisection. However, despite frequent initial eye movements and many errors to the left, manual responses were not affected by this change in orienting. We conclude that the mechanisms underlying asymmetrical attention in picture scanning and line bisection are flexible and can be separated, with saccades in scene perception driven more by a skewed perceptual span.

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.

Ipsilesional 'where' with contralesional 'what' neglect

Whereas contralesional spatial neglect is usually caused by right temporo-parietal lesions, ipsilesional spatial neglect is induced primarily by right frontal lesions. This report describes a 73-year-old woman with a right inferior parietal lesion who on 'where' tasks (line bisection and midline pointing) demonstrated ipsilesional neglect, but on 'what' tests (gap vs. no-gap detection cancellation and clothing tape removal) demonstrated contralesional neglect. This 'what' and 'where' directional dissociation provides evidence for independent 'what' and 'where' attentional networks; however, the reason this parietal lesion causes this contralesional vs. ipsilesional spatial attentional 'what' and 'where' dichotomy remains to be determined.

Visual neglect following stroke: current concepts and future focus

Visual neglect is a common, yet frequently overlooked, neurological disorder following stroke characterized by a deficit in attention and appreciation of stimuli on the contralesional side of the body. It has a profound functional impact on affected individuals. A assessment and management of this condition are hindered, however, by the lack of professional awareness and clinical guidelines. Recent evidence suggests that the underlying deficit in visual attention is due to a disrupted internalized representation of the outer world rather than a disorder of sensory inputs. Dysfunction of the cortical domains and white-matter tracts, as well as inter-hemispheric imbalance, have been implicated in the various manifestations of visual neglect. Optimal diagnosis requires careful history-taking from the patient, family, and friends, in addition to clinical assessment with the line bisection test, the star cancellation test, and the Catherine Bergego Scale. Early recognition and prompt rehabilitation employing a multidisciplinary approach is desirable. Although no treatment has been definitively shown to be of benefit, those with promise include prism adaptation, visual scanning therapy, and virtual reality-based techniques. Further high quality research to seek optimum short- and long-term rehabilitative strategies for visual neglect is required.