Hemianopic Patients Research Articles

Saccadic adaptation for rehabilitation in hemianopic patients

Saccadic adaptation for rehabilitation in hemianopic patients

Increased Adenosine A1 Receptor Levels in Hemianopia Patients After Cerebral Injury

The aim of this study was to apply positron emission tomography (PET) with C-8-dicyclopropylmethyl-1-methyl-3-propylxanthine (MPDX), a radioligand for adenosine A1 receptor (A1R), to patients with hemianopia caused by brain injury to study neurorepair mechanisms in the brain. Four patients with homonymous hemianopia and 15 healthy subjects were examined using PET to measure cerebral glucose metabolism, C-flumazenil (FMZ) binding to the central benzodiazepine receptor, and MPDX binding to A1R. Left and right regions of interest (ROIs) were selected, and semiquantitative data on the 3 kinds of PET examinations were obtained. The ROIs were referenced using the data for homologous regions in the contralateral hemisphere [ipsilateral/contralateral (I/C) ratio]. The I/C ratios for cerebral glucose metabolism and FMZ binding were low in the primary visual cortex (PVC) and visual association cortex in all the patients, whereas MPDX binding increased in the PVC in patients 1 and 2. Patients 1 and 2 experienced improvement in their visual field after 1 year. However, the other 2 patients showed no changes. We observed an increase in MPDX binding to A1R in the injured portion of the PVC in the patients who recovered. Evaluation of A1R by MPDX-PET may be useful for predicting prognosis and understanding the compensatory and reorganization processes in hemianopia caused by organic brain damage.

An fMRI Investigation of the Cortical Network Underlying Detection and Categorization Abilities in Hemianopic Patients

The current study aims to investigate visual scene perception and its neuro-anatomical correlates for stimuli presented in the central visual field of patients with homonymous hemianopia, and thereby to assess the effect of a right or a left occipital lesion on brain reorganization. Fourteen healthy participants, three left brain damaged (LBD) patients with right homonymous hemianopia and five right brain damaged (RBD) patients with left homonymous hemianopia performed a visual detection task (i.e. "Is there an image on the screen?") and a categorization task (i.e. "Is it an image of a highway or a city?") during a block-designed functional magnetic resonance imaging recording session. Cerebral activity analyses of the posterior areas-the occipital lobe in particular-highlighted bi-hemispheric activation during the detection task but more lateralized, left occipital lobe activation during the categorization task in healthy participants. Conversely, in patients, the same network of activity was observed in both tasks. However, LBD patients showed a predominant activation in their right hemisphere (occipital lobe and posterior temporal areas) whereas RBD patients showed a more bilateral activation (in the occipital lobes). Overall, our preliminary findings suggest a specific pattern of cerebral activation depending on the task instruction in healthy participants and cerebral reorganization of the posterior areas following brain injury in hemianopic patients which could depend upon the side of the occipital lesion.

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.

Contralesional spatial bias in chronic hemianopia: the role of (ec)centric fixation, spatial cueing and visual search

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. Eccentric fixation and adaptive orienting of eye movements towards the blind field have been suggested as hypothetical explanations but were not tested experimentally yet. Moreover, the role of spatial attention and visual search in the blind field are unsettled issues. Here, we tested in 20 stroke patients with chronic homonymous hemianopia (10 left-sided, 10 right-sided) without visual neglect, 10 healthy control subjects and 10 neurological control patients without hemianopia whether the HLBE is related to (a) eccentric fixation and (b) is influenced by spatial-attentional cueing (left, right) and (c) related to the degree of oculomotor compensation in the blind field. Perimetric mapping of the blind spot in the ipsilesional eye was performed in 39/40 subjects. Both hemianopic patient groups showed the typical HLBE towards their blind field, while the two control samples showed only a small but significant leftward shift known as pseudoneglect. The horizontal and vertical position of the blind spot in the ipsilesional eye was within normal limits in 38 out of 40 subjects, and did not differ significantly between the four samples. Moreover, the HLBE was not significantly correlated to the horizontal or vertical position of the centre of the blind spot, thus excluding eccentric fixation as an explanation for this spatial error. Furthermore, spatial cueing by manipulating the starting position of the bisection cue (left, right) did not affect the HLBE, arguing against attentional cueing effects well known from the line bisection error in patients with spatial neglect. Finally, the size of the saccadic search field in the scotoma was not significantly correlated to the HLBE in hemianopia. We conclude that eccentric fixation, contralesional hyperattention or ipsilesional hypoattention, or good or poor oculomotor compensation of the field loss itself are not likely causes of the HLBE in chronic homonymous hemianopia. Implications of these findings and alternative explanations are discussed.

The neural correlates of impaired collision avoidance in hemianopic patients

The aim of this study was to assess the brain regions associated with impaired performance in a virtual, dynamic collision avoidance task, in a group of patients with homonymous visual field defects (HVFDs) because of unilateral vascular brain lesions. Overall task performance was quantitatively assessed as the number of collisions while crossing an intersection at two levels of traffic density. Twenty-six patients were divided into two subgroups using the median split method: patients with 'performance above average' (HVFD(A), i.e. lower number of collisions) and patients with 'performance below average' (HVFD(B), i.e. higher number of collisions). In order to identify the anatomical structures that might be specifically affected in HVFD(B) patients but spared in HVFD(A) patients, overlap, subtraction and voxel-based lesion-symptom mapping analyses were performed using the MRIcron software. No significant difference in collision avoidance between patients with left- and right-hemispheric lesions was revealed. Separate lesion analysis in 12 patients with right- and 14 patients with left-hemispheric lesions showed that the cortical structures associated with impaired collision avoidance were the parieto-occipital region and posterior cingulate gyrus in the right hemisphere and the inferior occipital cortex and parts of the fusiform (occipito-temporal) gyrus in the left hemisphere. In the present collision avoidance paradigm, impaired performance of patients with right-hemispheric lesions is associated with damage in the dorsal processing stream and potential impact on the visual spatial working memory (WM), while impaired performance of patients with left-hemispheric lesions is associated with damage in the ventral stream and potential impact on the visual object WM.

Assessment of Damaged and Intact Visual Fields with a New Kind of Perimeter

An inexpensive method to characterize visual field defects due to neural damages was developed. Stimuli are presented on a large computer screen at a number of pre-selected locations in a randomized order. The sensitivities at these locations are measured independently with a number of simultaneously running threshold estimation algorithms. The stimuli are M-scaled, which means their size is increased with increasing eccentricity in inverse proportion to the cortical magnification factor. The purpose of M-scaling was to make the stimulus representation homogenous on the primary visual cortex so that the sensitivity values measured at different visual field locations would be comparable. The task was either the detection of flickering or static disks or recognition of flickering or static letters. Two groups of subject participated in the experiments - normal sighted and hemianopic patients. There was a general tendency that sensitivities increased toward peripheral vision to some extent in the intact visual field. This effect was slightly more pronounced for letter recognition than disk detection. In disk detection flicker more than doubled the sensitivity in comparison to static stimulus detection. However, in letter recognition flicker only had a negligible effect. In the hemianopic visual field, the sensitivity was in general considerably lower than in the intact visual field. However, completely blind spots were rare, and all patients exhibited residual vision of the defected visual field. Hemianopia produced a reduction of relative sensitivity that was not significantly dependent of stimulus type. The results also showed that the new method is sensitive to cortical visual field defects and it is reasonably fast.

The effect of central (macula) sparing on contralateral line bisection bias: A study with virtual hemianopia

Hemianopic patients show a contralesional bisection bias, but it is unclear whether this is a consequence of their field loss or related to extrastriate damage. One observation cited against the former is that hemianopic bisection bias does not vary with the degree of central (macular) sparing; however, it is unclear to what extent central sparing should affect this bias. Our goal was to determine the effect of central sparing on line bisection biases from field loss alone, with two approaches. First, we studied 12 healthy subjects viewing lines under conditions of virtual hemianopia, created by a gaze-contingent technique. Second, we calculated the effect predicted by a visuospatial model of the effect of central magnification on line representations in the visual system. Our results first replicated the contralateral line bisection bias with hemianopia, confirming that this can be generated by visual hemifield loss in the absence of extrastriate damage. Central sparing had only a modest effect on hemianopic bisection bias, with only slightly less bias with 10° compared to 2° of central sparing. In accordance with these empiric data, computing the center of mass for line representations in our model showed only a shallow decline in bisection bias as central sparing increased from 0 to 10°. We conclude that contralateral bisection bias only decreases slightly with central sparing, and that the absence of a statistically significant effect of central sparing in patients cannot be taken as evidence against a visual origin of contralateral hemianopic line bisection bias.

Refractive surgery in anisometropic adult patients induce plastic changes in primary visual cortex

To prospectively study the effect of refractive surgery in the primary visual cortex of adult anisometropic and isometropic myopic patients. Two anisometropic and two isometropic myopic patients were examined with multifocal functional magnetic resonance imaging technique (mffMRI) before refractive surgery and at 3, 6, 9 and 12 months postoperatively. Two controls without refractive surgery were also examined with mffMRI in the beginning and in the end of the study. Anisometropic patients had only their more myopic eye operated to correct the anisometropia. The myopic isometropic patients had their both eyes operated. Operated anisometropic eyes showed 65% reduced amount of active voxels in foveal data at 12 months postoperatively compared with the preoperative situation. In unoperated anisometropic eyes, the corresponding value was 86% and in myopic patients and controls 31% and 1%, respectively. To confirm this finding, the number of activated voxels representing the innermost ring of the stimulus was also calculated, and an exactly similar phenomenon was encountered in the anisometropic patients. Both anisometropic patients improved the best-spectacle-corrected visual acuity in the operated eye after refractive surgery. Our results suggest that plastic changes may take place in the primary visual cortex of anisometropic adult patients after refractive surgery.

Imaging Reveals Optic Tract Degeneration in Hemianopia

To investigate whether there is transsynaptic degeneration in the human optic tract in hemianopia. To consider how the degeneration varies with duration of hemianopia and location of insult. Seven patients with damage to the primary visual cortex (V1), the lateral geniculate nucleus (LGN), or the optic tract were scanned with structural MRI. The volume and cross-sectional area of the left and right optic tracts were computed based on the intensity values of the T1-weighted image. High values correspond to voxels with high white matter content, and the values decrease as the white matter content drops (indicating degeneration). A laterality index to compare the tract size in the two hemispheres was calculated at different intensity values. The three hemianopic patients with longstanding damage to either V1 or LGN showed laterality indices greater than 0.5 at the highest intensity values, indicating significant optic tract degeneration. Those with recent damage to the optic tract had even higher laterality indices due to direct degeneration. Even 18 months after V1 lesion, there was a significant correlation between the cross-section and volume indices at different intensity thresholds, whereas no control subject showed any correlation. Transsynaptic degeneration had already begun 18 months after lesion. Although there was no visible decrease in volume at this stage, the white matter integrity was compromised. Significant decrease in volume could be visualized at longer durations of hemianopia. This method of objectively assessing structural images provides an effective, noninvasive approach to monitor the timescale of optic tract degeneration.

Oculomotor behavior of hemianopic chronic stroke patients in a driving simulator is modulated by vision training

Visual Restorative function training aims to decrease visual field defect size after acquired brain damage. Some chronic stroke patients regain permission to drive a car after training. This points to a concomitant change in oculomotor behavior, because visual field enlargement is hardly ever large enough for legal driving. This study investigated vRFT-induced changes in oculomotor behavior, using a driving simulator. Driving performance and oculomotor behavior were measured before and after training in 6 hemianopia patients who had trained 65 hours with vRFT on a PC at home. Two patients showed negligible visual field enlargement (VFE) and four showed moderate to substantial VFE. Because less visual cortex is devoted to the processing of peripheral than central visual field the same VFE corresponds to less functional restoration of cortex when the defect is at high eccentricity. When this is taken into account, then precisely the two patients that showed the largest cortical gains made significantly more eye movements in the direction of their visual field defect after training. vRFT with mandatory eye fixation can result in increased eye movement behavior towards the defect. Our study suggests that a threshold amount of cortical functional restoration is required for this effect.

Task-Irrelevant Blindsight and the Impact of Invisible Stimuli

Despite their subjective invisibility, stimuli presented within regions of absolute cortical blindness can both guide forced-choice behavior when they are task-relevant and modulate responses to visible targets when they are task-irrelevant. We here tested three hemianopic patients to learn whether their performance in an attention-demanding rapid serial visual presentation task would be affected by task-irrelevant stimuli. Per trial, nine black letters and one white target letter appeared briefly at fixation; the white letter was to be named at the end of each trial. On 50% of trials, a task-irrelevant disk (−0.6 log contrast) was presented to the blind field; in separate blocks, the same or a very low negative contrast distractor was presented to the sighted field. Mean error rates were high and independent of distractor condition, although the high-contrast sighted-field disk impaired performance significantly in one participant. However, when trials with and without distractors were considered separately, performance was most impaired by the high-contrast disk in the blind field, whereas the same disk in the sighted field had no effect. As this disk was least visible in the blind and most visible in the sighted field, attentional suppression was inversely related to visibility. We suggest that visual awareness, or the processes that generate it and are compromised in the blind hemisphere, enhances or enables effective attentional suppression.

Awareness of visual targets within the field defect of hemianopic patients

Awareness of visual targets within the field defect of hemianopic patients

Absence of behavioural recovery because of absence of cortical reorganization? An fMRI investigation of a left hemianopic patient

Absence of behavioural recovery because of absence of cortical reorganization? An fMRI investigation of a left hemianopic patient

Functional compensation of visual field deficits in hemianopic patients under the influence of different task demands

We investigated the task-specific role of eye and head movements as a compensatory strategy in patients with homonymous visual field deficits (HVFDs) and in age-matched normal controls. All participants were tested in two tasks, i.e. a dot counting (DC) task requiring mostly simple visual scanning and a cognitively more demanding comparative visual search (CVS) task. The CVS task involved recognition and memory of geometrical objects and their configuration in two test fields. Based on task performance, patients were assigned to one of two groups, “adequate” (HVFDA) and “inadequate” (HVFDI); the group definitions based on either task turned out to be identical. With respect to the gaze related parameters in the DC task we obtained results in agreement with previous studies: the gaze pattern of HVFDA patients and normal controls did not differ significantly, while HVFDI patients showed increased gaze movement activity. In contrast, for the more complex CVS task we identified a deviating pattern of compensatory strategy use. Adequately performing subjects, who had used the same gaze strategies as normals in the DC task, now changed to increased gaze movement activity that allowed coping with the increasing task demands. Inadequately performing patients switched to a novel pattern of compensatory behavior in the CVS task. Different compensatory strategies are discussed with respect to the task-specific demands (in particular working memory involvement), the specific behavioral deficits of the patients, and the corresponding brain lesions.

Line bisection in simulated homonymous hemianopia

Hemianopic patients make a systematic error in line bisection, showing a contra-lesional bias towards their blind side, which is the opposite of that in hemineglect patients. This error has been attributed variously to the visual field defect, to long-term strategic adaptation, or to independent effects of damage to extrastriate cortex. To determine if hemianopic bisection error can occur without the latter two factors, we studied line bisection in healthy subjects with simulated homonymous hemianopia using a gaze-contingent display, with different line-lengths, and with or without markers at both ends of the lines. Simulated homonymous hemianopia did induce a contra-lesional bisection error and this was associated with increased fixations towards the blind field. This error was found with end-marked lines and was greater with very long lines. In a second experiment we showed that eccentric fixation alone produces a similar bisection error and eliminates the effect of line-end markers. We conclude that a homonymous hemianopic field defect alone is sufficient to induce both a contra-lesional line bisection error and previously described alterations in fixation distribution, and does not require long-term adaptation or extrastriate damage.

Line bisection in unilateral homonymous visual field defects

The contralesional line bisection error in unilateral homonymous hemianopia is a frequent but neglected clinical phenomenon. Our knowledge about this bisection error is based on small samples of hemianopic patients. Moreover, horizontal line bisection has never been investigated in other unilateral visual field defects. The present study is the first to examine line bisection in a large, representative sample of patients with unilateral homonymous visual field defects. We investigated horizontal line bisection in 129 patients with left- or right-sided homonymous hemianopia (60.5%), upper and lower quadranopia (24.8%), and paracentral scotoma (14.7%), and determined the magnitude and direction of line bisection error. The contralesional horizontal line bisection error was present not only in patients with hemianopia but also in those with upper or lower quadranopia or paracentral scotoma. Neither the type nor the severity of the visual field defect was found to determine the bisection error. Only the side of the field defect seemed to determine the horizontal direction of the bisection error (left-/rightward). The contralesional bisection error is not a specifically “hemianopic” phenomenon. It is frequently associated with any unilateral homonymous visual field defect, i.e., hemianopia, upper/lower quadranopia, paracentral scotoma. Moreover, our results further support the recent finding that the contralesional bisection error is not a direct consequence of the visual field defect. Yet, they also suggest that, although the visual field defect does not seem to be the primary cause of the contralesional bisection error, it may nevertheless contribute to it.

The imbalance of oculomotor capture in unilateral visual neglect

Visual neglect has been associated with an imbalance in the level of activity in the saccadic system: activity in the contralesional field is suppressed, which makes target selection unlikely. We recorded eye movements of a patient with hemispatial neglect and a group of healthy participants during an oculomotor distractor paradigm. Results showed that the interfering effects of a distractor were very strong when presented in her ipsilesional visual field. However, when the distractor was presented in her contralesional field, there were no interfering effects when the target was presented in her ipsilesional field. These findings could not be explained by the presence of a visual field defect as revealed by the results of two hemianopic patients. Our results are in line with an imbalance in the level of activity in the saccadic system in visual neglect because visual elements presented in the contralesional field did not compete for saccadic selection.

Distortion of auditory space in hemianopia

Sound localization was investigated in patients with homonymous hemianopia, a visual field defect characterized by a loss of vision in one hemifield that is caused by unilateral brain lesions involving the visual cortex or its afferents. The primary aim was to clarify whether or not the known distortion of visual space in hemianopia results in processes of long-term cross-modal spatial adaptation, thus eventually inducing related alterations in auditory space perception. For this purpose, patients were tested by using tasks of either head pointing or manual pointing to acoustic targets in the azimuthal plane, under anechoic conditions in total darkness. The results obtained with both tasks consistently indicated slight, but significant, systematic errors compared with normal controls. In particular, the errors found can be interpreted by both rotation and compression of auditory space toward the anopic side. These findings can be explained by a visual miscalibration of the auditory space, as has been analogously demonstrated in studies on normal-sighted subjects after exposure to consistent auditory-visual disparity, for example by wearing prism lenses. The precision in sound localization of hemianopic patients was generally reduced across both hemispaces. Taken together, one may conclude that processes of cross-modal spatial adaptation, but not those of compensatory plasticity, occurred in patients with hemianopia.

Specific impairments in visual processing following lesion side in hemianopic patients

Introduction Following unilateral damage of the primary visual cortex one of the most common visual field defects observed is Homonymous Hemianopia (HH), a loss of vision of the contralesional hemifield in each eye. The ipsilesional (“intact”) part of the central visual field is often used to compensate for difficulties encountered in the peripheral hemianopic visual field. However, the quality of vision within the central visual field is not well-known. Methods To better describe and understand visual processing in hemianopia, two tasks were conducted with 25 healthy controls, six left hemianopes, and five right hemianopes. Filtered (in high, above 6 cycles/degree, or low, below 4 cycles/degree, spatial frequencies – HSF and LSF, respectively) and unfiltered natural scene images (5° of visual angle) were briefly presented (100 msec) centrally on a computer screen. Participants were required either to respond when a natural scene was presented (yes/no detection task) or to indicate if the stimulus was a city or a highway (categorization task). Results The three groups showed similar accuracy levels but significant differences were observed in response times. More precisely, left hemianopes were impaired both in the detection and in the categorization tasks whereas right hemianopes were only impaired in the categorization task. However, the three groups had similar responses to spatial frequencies: HSF were processed more slowly than LSF. Conclusions Overall these results suggest that central vision is not intact in hemianopia. Lesion side selectively affects reaction times (RTs) in the detection and the categorization tasks, but does not seem to determine a specific deficit in spatial frequency processing.