Ipsilesional Hemifield Research Articles

Impaired pre-saccadic shifts of attention in neglect patients

Every saccade is generally preceded by a mandatory shift of attention to the saccade endpoint, allowing us to process visual information more effectively. Whether this ‘pre-saccadic shift of attention’ is still intact in hemispatial neglect is unknown. Whereas neglect patients exhibit lateralized impairments of attention and often show impaired saccadic behaviour, it is not yet clear how the pre-saccadic shift of attention is affected during accurately executed eye movements. In this study, we used a gaze contingent visual discrimination task, in which neglect patients had to discriminate a probe presented before saccade onset. Results revealed an imbalance in discrimination performance between the two hemifields with poor performance to probes in the contralesional compared to the ipsilesional hemifield when accounting for saccadic impairments. These results suggest that attention and eye movements are both unique impairments of neglect patients. We hypothesize that the impaired pre-saccadic shift of attention could be one of the key problems of neglect and might underlie other spatial and non-spatial deficits often reported in neglect patients.

Ipsilesional perceptual deficits in hemispatial neglect: Case reports

Hemispatial neglect, usually after right hemisphere lesions, is characterized by contralesional deficits in attention and perception. However, little is known about impairments of perceptual processing in the ipsilesional region of visual space (the right visual field for right hemisphere lesions). In two right hemisphere neglect patients, we used a metacontrast masking paradigm to characterize systematic spatial and temporal visual processing deficits in the ipsilesional right visual field. The presence of a visual mask caused the neglect patients to miss targets in ipsilesional space, even when a mask was presented as long as 1.5 sec after the target and in a different spatial position. These prolonged and spatially extended masking effects were not measured in age-matched healthy controls or in two control patients with hemianopsia but without neglect. The results show that perceptual processing is distorted and delayed in a region of the visual field that has been thought to be unaffected – the ipsilesional hemifield in patients with neglect.

Hemifield coding in ventral object-sensitive areas – Evidence from visual hemiagnosia

Electrophysiological monkey and human neuroimaging studies have reported a lateralization of signal processing in object perception. However, it is unclear whether these results point to a unique topographically organized signal processing in either hemisphere, or if these results represent a rather negligible spatial organization of otherwise redundant object perception systems in both hemispheres. We tested a group of 10 patients with lesions to ventral object processing regions and spared primary visual functions with lateral presentations of different categories of object stimuli. Object perception in the contralesional visual field was impaired while object perception on the ipsilesional hemifield was intact. These results demonstrate that the object perception system needs two intact ventral pathways for unimpaired object perception across the whole visual field; the loss of one system cannot be fully compensated by its contralateral homolog or spared parts of the lesioned ventral stream.

P.060 Altered oculomotor learning in thalamic stroke patients

Background: Visuomotor learning can be elicited experimentally by displacing the target of a saccade during the ongoing eye movement. In healthy subjects, the resulting mismatch between expected and experienced visual error after saccade completion elicits a gradual adaptation of saccade amplitude. The goal of this project was to explore the role of cerebro-thalamo-cerebellar circuits in the dynamics of visuomotor learning. Methods: Patient RK is a 38-year-old right hand dominant male who suffered a focal thalamic stroke of the right thalamus, confined to ventral lateral posterior and ventral medial nuclei. We employed a standard saccadic adaptation paradigm and assessed dynamics of visuomotor learning by fitting a simple state-equation to saccade amplitudes towards the ipsi- and contralesional hemifield. Results: While RK was able to adapt saccade amplitudes in both directions, adaptation dynamics were different for leftward versus rightward saccades. Rightward, ipsilesional saccades exhibited a lower learning rate but similar retention of altered saccade metrics, compared to leftward, contralesional saccades. Conclusions: The present study assessed a patient with a focal lesion to the right cerebellar thalamus on a saccade adaptation paradigm. Results demonstrated slower visuomotor learning for saccades into the ipsilesional hemifield, suggesting an important contribution of cerebello-cortical projections mediated by thalamic relays for visuomotor learning.

Neglect following stroke: The role of sensory sensitivity in visuo-spatial performance

Both proprioceptive and visual manipulations have led to some improvement of the spatial neglect syndrome. Until now, their effects on visuo-spatial behaviour have never been compared simultaneously. The objective of this study was to determine their influence, as a function of the presence of neglect and the side of the brain damage. 19 stroke patients with right and 14 with left brain damage, without or with neglect; realized the Bells test in 5 conditions: a reference condition and 4 sensory conditions, defined according to the side of application (contralesional vs ipsilesional) and the type of perturbation (visual vs proprioceptive). The visuo-spatial behaviour was analyzed for global and spatial aspects and for individual extreme performances. For the neglect group, the restriction of the visual field to the ipsilesional hemi-field significantly diverted the centre of exploration towards the ipsilesional side compared to all other conditions. The weighting of visual cues from the ipsilesional hemi-field seems to be increased in sensory-motor integration processes in neglect patients. In all the groups, although some improvements in performance did occur with sensory manipulation, they were dependent on the individual, particularly for neglect patients. A same performance can be achieved through the use of different sensory-motor strategies, which are individual-related. It is thus important to consider the sensory sensitivity and the responsiveness of each patient before beginning any sensory therapies.

Characteristics of contralesional and ipsilesional saccades in hemianopic patients

In order to further our understanding of action-blindsight, four hemianopic patients suffering from visual field loss contralateral to a unilateral occipital lesion were compared to six healthy controls during a double task of verbally reported target detection and saccadic responses toward the target. Three oculomotor tasks were used: a fixation task (i.e., without saccade) and two saccade tasks (eliciting reflexive and voluntary saccades, using step and overlap 600 ms paradigms, respectively), in separate sessions. The visual target was briefly presented at two different eccentricities (5° and 8°), in the right or left visual hemifield. Blank trials were interleaved with target trials, and signal detection theory was applied. Despite their hemifield defect, hemianopic patients retained the ability to direct a saccade toward their contralesional hemifield, whereas verbal detection reports were at chance level. However, saccade parameters (latency and amplitude) were altered by the defect. Saccades to the contralesional hemifield exhibited longer latencies and shorter amplitudes compared to those of the healthy group, whereas only the latencies of reflexive saccades to the ipsilesional hemifield were altered. Furthermore, healthy participants showed the expected latency difference between reflexive and voluntary saccades, with the latter longer than the former. This difference was not found in three out of four patients in either hemifield. Our results show action-blindsight for saccades, but also show that unilateral occipital lesions have effects on saccade generation in both visual hemifields.

Effects of Pulvinar Inactivation on Spatial Decision-making between Equal and Asymmetric Reward Options

The ability to selectively process visual inputs and to decide between multiple movement options in an adaptive manner is critical for survival. Such decisions are known to be influenced by factors such as reward expectation and visual saliency. The dorsal pulvinar connects to a multitude of cortical areas that are involved in visuospatial memory and integrate information about upcoming eye movements with expected reward values. However, it is unclear whether the dorsal pulvinar is critically involved in spatial memory and reward-based oculomotor decision behavior. To examine this, we reversibly inactivated the dorsal portion of the pulvinar while monkeys performed a delayed memory saccade task that included choices between equally or unequally rewarded options. Pulvinar inactivation resulted in a delay of saccade initiation toward memorized contralesional targets but did not affect spatial memory. Furthermore, pulvinar inactivation caused a pronounced choice bias toward the ipsilesional hemifield when the reward value in the two hemifields was equal. However, this choice bias could be alleviated by placing a high reward target into the contralesional hemifield. The bias was less affected by the manipulation of relative visual saliency between the two competing targets. These results suggest that the dorsal pulvinar is involved in determining the behavioral desirability of movement goals while being less critical for spatial memory and reward processing.

Extinguishing Extinction: Hemispheric Differences in the Modulation of TMS-induced Visual Extinction by Directing Covert Spatial Attention

The topic of spatial attention is of great relevance for researchers in various fields, including neuropsychology, cognitive neuroscience, and cognitive psychology, as well as for clinical practice. Deficits of spatial attentional arising from parietal brain damage remain largely confined to the left visual field. The mechanisms underlying this hemispheric asymmetry are still elusive. We mimicked the neuropsychological syndrome of contralesional extinction by temporarily inducing a spatial attentional bias in healthy volunteers with TMS. We investigated whether directing covert spatial attention could enhance or, more importantly, counteract the resulting behavioral deficits. Although both the left and right parietal TMS induced contralateral extinction, only left hemifield extinction following right parietal TMS was severely aggravated by a competing stimulus in the ipsilesional (right) hemifield. We put forward the hypothesis that an asymmetry with respect to the ability of detaching attention from a distractor is contributing to the right hemispheric lateralization with regard to extinction. On a broader level, we suggest that "virtual patients" might be used for evaluating neuropsychological treatment in an early stage of development, reducing the burden on actual patients.

Impaired visual sensitivity within the ipsilesional hemifield following parietal lobe damage

The parietal cortex is considered to be part of a network of brain areas that modulates competitive interactions between targets and irrelevant distracters in early visual cortex, however there is currently little causal evidence to support this in human observers. It is also unclear as to whether parietal influences on visual perception in humans are limited to the contralesional hemispace or whether a unilateral lesion affects visual sensitivity bilaterally. Here we examined visual sensitivity in two patients with spatial neglect and extinction arising primarily from left-parietal damage. We used a sensitive psychophysical task based on those previously used to demonstrate loss of stimulus selection after lesions to extrastriate cortex. Observers discriminated the orientation of a lateralized suprathreshold target grating that appeared alone or in the context of nearby salient disc distracters. For parietal patients, target sensitivity within both the contralesional and ipsilesional fields was compromised by the presence of distracters. Conversely, healthy matched controls were unaffected by distracters. These results indicate that parietal cortex damage can influence visual perception within both the ipsi- as well as the contralesional field.

Saccades to the seeing visual hemifield in hemidecorticate patients exhibit task-dependent reaction times and hypometria

In three patients who had one cortical hemisphere removed surgically (hemidecortication), we studied visually-triggered saccades directed contralateral to the intact cortical hemisphere (i.e., ipsilesional saccades). Both saccade reaction times (SRTs) and accuracy of these saccades have been reported as abnormal in hemidecorticate patients, but not monkeys. One explanation for this difference is that deficits in hemidecorticate patients may not have been directly caused by removal of cortical oculomotor structures themselves, but may have been a manifestation of compensatory strategies used to cope with contralesional hemianopia. We hypothesized that deficits in saccade performance to the ipsilesional (seeing) visual hemifield would be directly linked to how easily patients could localize targets in their blind hemifield with searching saccades. To test this hypothesis, we examined how deficits in our patients varied when targets were: (1) randomly presented to either the seeing or blind hemifield for long durations thereby permitting searching saccades in the blind hemifield; (2) presented as in Experiment 1, but briefly flashed thereby removing visual feedback prior to saccade onset thereby rendering searching saccades useless; (3) briefly flashed as in Experiment 2, but at random locations in only the seeing hemifield (blind hemifield irrelevant). Mean SRTs to the seeing hemifield were 165 ms longer than normal in Experiment 2, but only about 40 ms longer in Experiments 1 and 3. Saccade accuracy was characterized by task-dependent hypometria in all three experiments with a mean undershoot of about twice the amplitude variance. The largest undershoots were in Experiments 2 and 3. Our data suggest that deficits resulted from the direct effects of the lesions themselves coupled with context-dependent strategies used to cope with contralesional hemianopia.

A role for the inferior colliculus in multisensory speech integration

Multisensory integration can occur at relatively low levels within the central nervous system. Recent evidence suggests that multisensory audio-visual integration for speech may have a subcortical component, as acoustic processing in the human brainstem is influenced by lipreading during speech perception. Here, stimuli depicting the McGurk illusion (a demonstration of auditory-visual integration using speech stimuli) were presented to a 12-year-old child (FX) with a circumscribed unilateral lesion of the right inferior colliculus. When McGurk-type stimuli were presented in the contralesional hemifield, illusory perception reflecting bimodal integration was significantly reduced compared with the ipsilesional hemifield and a group of age-matched controls. These data suggest a functional role for the inferior colliculus in the audio-visual integration of speech stimuli.

Goal-driven selective attention in patients with right hemisphere lesions: how intact is the ipsilesional field?

Patients with right hemisphere (RH) lesions often display a spatial bias in attention towards the ipsilesional hemifield. The behavioural manifestations of this spatial bias are typically interpreted as reflecting increased or enhanced attention for stimuli within the 'intact' ipsilesional field, and impaired attentional functioning within the contralesional field. In the healthy brain, goal-driven and stimulus-driven attentional processes interact to determine which stimuli should be prioritized for selection. Although unilateral brain damage increases the relative attentional salience of stimuli within the ipsilesional field, it might also cause problems in filtering or attenuating task-irrelevant information. We examined whether goal-driven attention modulates the processing of ipsilesional and contralesional information in 6 patients with unilateral brain damage following RH stroke (5 male, 1 female; mean age 60.8 years) and a group of age and sex-matched controls. We used a flanker task in which participants made speeded judgements on a central target item (a coloured letter). On each trial the target was flanked by a coloured letter in the left and right hemifields. In separate blocks, participants were instructed to judge either the identity or the colour of the central target and to ignore the flankers. The flanker on one side could be congruent, incongruent or neutral with respect to the target, on either the letter or the colour dimension, whereas the flanker on the other side was always neutral on both dimensions. Healthy controls showed significant interference from incongruent flankers on either side. Crucially, however, this effect only occurred for the task-relevant dimension [F(2,10) = 24.60; P < 0.001]. For patients, however, both the task-relevant and task-irrelevant dimensions of ipsilesional flankers interfered with response times [task-relevant: F(2,10) = 7.50, P < 0.05; task-irrelevant: F(1,5) = 6.20, P < 0.05]. Conversely, contralesional flankers influenced response times only when the target and distractor were incongruent on the task-relevant dimension [F(2,10) = 4.85; P < 0.05]. Our findings demonstrate that following RH damage, goal-driven biases cannot constrain the processing of task-irrelevant features of ipsilesional stimuli. We speculate that a lateralized bias in spatial attention leads to unselective prioritization of all feature-based attributes of stimuli appearing within the ipsilesional hemifield, whether or not they are relevant to performance. Attentional selection for ipsilesional stimuli in disorders such as spatial neglect and extinction may not therefore be entirely normal, as previously assumed.

What kind of visual spatial attention is impaired in neglect?

The distribution of spatial attention across the horizontal meridian of the visual field, as assessed by a simple reaction time (RT) paradigm, is dramatically abnormal in neglect patients. In the contralesional hemifield, RT increases sharply from centre to periphery, while in the ipsilesional hemifield, it decreases paradoxically from centre to mid-periphery. In the present study, we firstly asked whether this abnormal distribution of spatial attention is still present when patients know in advance the location of the impending stimulus, and second whether and to which extent it may be influenced by the concomitant presence of hemianopia. In Experiment 1, the stimuli were presented either predictably (blocks of same–point presentations) or unpredictably (blocks of randomised presentations) to one of several contralesional and ipsilesional field locations. As was the case for control subjects, neglect patients showed an overall RT decrease with same-point presentations. However, their abnormal contralesional RT lengthening and ipsilesional speeding were still present. In Experiment 2, the trials were blocked to same-hemifield presentations. In the ipsilesional field condition, neglect patients with and without hemianopia showed the same distorted distribution of attention favouring mid-periphery over central field locations. Two conclusions can be drawn from these experiments: first, the bulk of the abnormal deployment of spatial attention in neglect patients is related to an impairment of exogenous attention which cannot be compensated for by a spared endogenous control. Second, hemianopia does not affect the paradoxical speeding up of RT typically found in the mid-periphery of the ipsilesional field of neglect patients.

The thalamus interrupts top-down attentional control for permitting exploratory shiftings to sensory signals.

When attention is involuntarily drawn in a direction different to that of the target, slower motor response times are observed (i.e. the meridian effect). Previous data suggested that the thalamus might participate in the generation of visual salience. What may be the role of the thalamus in the capture by luminance transients when attentional control is in action? A single experiment was administrated in a group of ten healthy volunteers as well as in a group of three patients with unilateral thalamic infarcts. Subjects participated in a task where attentional control was interrupted by a distractor. The meridian effect was present only in the performance of the healthy volunteers and when distractors occurred in the ipsilesional (intact) hemifield of the thalamic patients. These results suggest that when an important signal appears during attentional focalization, the thalamus interrupts current focalization and permits the compilation of an attentional program in the midbrain aiming at generating an orienting response towards the source of this signal.

Hyperattention in Neglect Patients: Perceptual or Pre-Motor Phenomenon?

Hemineglect consists in a profound unawareness of the contralesional spacewhich occurs in patients with damage often centred, but not necessarilyrestricted to, the right parietal lobe (Vallar, 1993; Vallar et al., 1994). Mosttheories of neglect concur that there is an attentional imbalance favouring theipsilesional space (De Renzi et al., 1989; Ladavas et al., 1990). In a previousstudy (Smania et al., 1998) we found that neglect patients tested on a simplereaction time (RT) paradigm with lateralized visual stimuli show opposite resultsdepending on the side of hemifield presentation. In the contralesional hemifield,accuracy and RT speed decreased with eccentricity. In contrast, in theipsilesional hemifield, both accuracy and RT showed a paradoxical improvementas one goes from central to more peripheral visual field locations. We namedthis effect “hyperattention” by broad analogy with the clinical phenomenon of“magnetic attraction” toward the side of the lesion shown by neglect patients(De Renzi, 1982). Two hypotheses can be advanced to explain this phenomenon.One is that hyperattention affects RTs by speeding up stimulus detection. Theother is that it influences the response stage of RT. We carried out a study usingthe stop signal paradigm to disentangle the relative contribution of these twofactors. This paradigm involves the presentation of a go signal, to which thesubject has to respond as quickly as possible, followed by the presentation of astop signal requiring the subject to refrain from responding (Cavina-Pratesi etal., 2001). According to a horse-race account of the stop signal paradigm(Logan, 1994), the probability of inhibiting P(i) a response depends on a racebetween the go signal and the stop signal. These two controlled processes raceto reach a point of no return after which the response is emitted in a ballisticfashion. If the go signal wins, a response is produced, if the stop signal wins, noresponse takes place. In normal subjects RT increases for stimuli presented atincreasing eccentricities along the horizontal meridian of the visual field(Chelazzi et al., 1988; Marzi and Di Stefano, 1981). If this phenomenon isrelated to the controlled stage of RT then, according to Logan’s account, P(i)should increase with eccentricity since the response to the go signal becomesprogressively slower and is more likely to lose the race with the stop signal. Incontrast to controls, in the ipsilesional hemifield of neglect patients RTparadoxically decreases with eccentricity. If this decrease is the result of an

The spatial distribution of visual attention in hemineglect and extinction patients.

We studied the visual field distribution of speed and accuracy of manual responses to small brief light flashes, in patients with left hemineglect or extinction resulting from right hemisphere vascular lesions and in brain-damaged and healthy control subjects. All patients with right hemisphere lesions showed a greater impairment in both the speed of response and the detection rate in the contralesional than in the ipsilesional hemifield. This interfield difference increased with the eccentricity of stimulus presentation and was especially pronounced in neglect patients who showed a paradoxical increase in speed of response and detection rate at increasingly larger eccentricities in the ipsilesional hemifield. We hypothesize that both the contralesional slowing down and the ipsilesional speeding up of the response depends upon an exaggerated gradient of attention towards the ipsilesional hemifield. To assess whether these abnormalities concern automatic or controlled attentional processes, in a second experiment, we manipulated the predictability of the side of the stimulus presentation by using blocked rather than randomized stimulus presentations. This resulted in a speeding up of responses in both hemifields thus showing that the patients were able to focus attention to the side of stimulus presentation voluntarily. However, there was no modification of the contra-ipsilesional differences which, therefore, are likely to be related to abnormal automatic processes rather than controlled attention.

Response Channel Activation and the Temporoparietal Junction

When we learn to make one motor response to one visual stimulus and a different motor response to another, representations of these stimulus–response associations must be maintained to efficiently transduce perception into action. When an irrelevant distractor is presented adjacent to a target stimulus, interference is observed when the two stimuli are associated with conflicting responses, presumably due to response channel activation by the incompatible information. We have explored the neural bases of these interference effects. In a previous study, patients with hemispatial neglect showed normal interference from contralesional flankers. In another study, patients with lesions of the lateral prefrontal cortex were found not to show interference from distractors presented in the contralesional hemifield. The current study provided a more anatomically detailed investigation of the effects of posterior association cortex lesions on flanker interference. Patients with chronic, unilateral lesions involving the temporoparietal junction (TPJ), two of whom had hemispatial neglect, were compared with patients with lesions of the posterior association cortex not involving the TPJ. All patients performed a color discrimination task at fixation while a congruent or incongruent colored flanker was briefly presented (16.7 ms) in the adjacent contralesional or ipsilesional hemifield. Patients with TPJ lesions showed no interference effects from the contralesional flankers. These results suggest that the TPJ, in combination with the dorsolateral prefrontal cortex, is involved in transducing perception into action.

Contrast Sensitivity Loss in The Neglected Hemifield

Contrast sensitivity to gratings of various spatial frequencies displayed in the left and right visual hemifield was measured in a group of ten right brain-damaged patients with unilateral visuospatial neglect. Two groups of ten left brain-damaged (LBD) and ten right brain-damaged (RBD) patients without neglect served as controls. All patients had normal visual fields according to standard clinical procedure. Stimuli were patches of sinusoidal gratings of 1, 2, 4 and 8 c/deg spatial frequency. The patches subtended 6 deg and were displayed at 3 deg of eccentricity. A two-alternative forced-choice technique was employed. Results showed a reduction in contrast sensitivity for stimuli presented to the contralesional hemifield with respect to the ipsilesional hemifield in patients with neglect. No difference in contrast sensitivity between hemifields was found for LBD and RBD groups. These findings indicate a basic visual impairment in the contralesional hemifield in patients with neglect.

Input and Response Determinants of Visual Extinction: A Case Study

We have studied a patient, CZ, with contralateral visual extinction due to a large ischaemic frontal-parietal-temporal lesion in the right hemisphere. We found that manipulation of intensity of the visual stimulus had little effect while an increase in eccentricity substantially increased extinction rate. An important factor was represented by the hemifield of stimulus presentation: when double stimuli were presented to the contralesional (left) hemifield, the leftmost stimulus was consistently extinguished while when stimuli were presented to the ipsilesional (right) hemifield, extinction was absent. Such effect was specific to hemifield rather than to head- and trunk-defined hemispace. Manipulation of response-related variables affected extinction to a large extent: In particular, the use of nonverbal responses diminished extinction considerably. This suggests that an important component of extinction may be represented by an impaired access of visual information to the left hemisphere. Finally, the RT results confirmed previous evidence of an ipsilesional attentional bias favouring, the rightmost stimuli both in the contralesional and in the ipsilesional hemifield.

Saccadic eye movement programming in unilateral neglect

The present study examined the eye movements made by patients with unilateral neglect under fixation gap and overlap conditions. The prior offset of fixation in a + 100 msec gap condition did not produce an increase in the numbers of contralesional saccades made by 3 out of 4 patients. This finding is incompatible with the view that the deficit in producing contralesional saccades reflects an inability to disengage visual attention from fixation. A significant reduction in saccade latency was, however, obtained in the gap condition (‘gap effect’). The latency reduction in the gap condition is consistent with models which attribute the gap effect to warning signal effects and the release of an ocular rather than an attentional disengagement mechanism. Saccade latency was not increased when two targets were presented bilaterally and simultaneously in both hemifields (in contrast to the increase in latency shown by normal subjects). The lack of a normal ‘bilateral target effect’ in neglect is attributed to an imbalance in the level of activity in the saccadic system. Three patients showed visual ‘extinction’ and did not make saccades to the contralesional bilateral targets. In contrast R.R. who shows object-based neglect did not show extinction and made saccades to the contralesional bilateral targets. This suggests that visual extinction may be influenced by the form of neglect shown by the patient. The effects on saccade amplitude of presenting two targets in the same hemifield were also examined in a global effect task. One patient showed a much greater global effect than normal when pairs of targets were presented in his ipsilesional hemifield. In contrast R.R. showed a normal magnitude global effect. It appears that the form of neglect shown by a patient is a factor that influences their eye movement behaviour on simple saccade tasks and these eye movement abnormalities cannot be accounted for by a deficit of attentional disengagement.