Neglect and hemianopia superimposed

In patients with posterior-parietal brain damage it is often difficult to decide whether left-sided omissions in perimetry are due to primary visual loss or due to visual neglect. We investigated 11 patients with combined neglect/hemianopia and 11 patients with pure hemianopia using a visual search task with single or double stimulation conditions. The second stimulus was either the fixation point itself (like in perimetry) or a distractor appearing in the hemifield opposite to the target. The fixation point did not worsen left-sided perception, but its disappearance led to a bias of exploration towards the right side in neglect patients but not in pure hemianopics. A distractor in the intact hemifield worsened the performance to left-sided stimuli, that is, neglect patients behaved as if they were completely hemianopic, even in intact parts of the visual field (VF). Three of the neglect patients showed unconscious processing of the distractor in the left VF, suggesting that the visual field defect was produced by neglect mechanisms rather than primary visual loss. This visual search paradigm appears to be helpful in understanding of the nature of hemianopia versus neglect deficits in individual patients.     

Neglect and Hemianopia Superimposed

In patients with posterior-parietal brain damage it is often difficult to decide whether left-sided omissions in perimetry are due to primary visual loss or due to visual neglect. We investigated 11 patients with combined neglect/hemianopia and 11 patients with pure hemianopia using a visual search task with single or double stimulation conditions. The second stimulus was either the fixation point itself (like in perimetry) or a distractor appearing in the hemifield opposite to the target. The fixation point did not worsen left-sided perception, but its disappearance led to a bias of exploration towards the right side in neglect patients but not in pure hemianopics. A distractor in the intact hemifield worsened the performance to left-sided stimuli, that is, neglect patients behaved as if they were completely hemianopic, even in intact parts of the visual field (VF). Three of the neglect patients showed unconscious processing of the distractor in the left VF, suggesting that the visual field defect was produced by neglect mechanisms rather than primary visual loss. This visual search paradigm appears to be helpful in understanding of the nature of hemianopia versus neglect deficits in individual patients.     

A psychophysical study of visual extinction: ipsilesional distractor interference with contralesional orientation thresholds in visual hemineglect patients

Visual extinction was investigated in left (n=15) and right (n=25) brain-damaged patients with or without visual neglect, and in normal control subjects (n=14), using a psychophysical paradigm. Orientation discrimination thresholds were determined for both left and right hemifield gratings presented either in isolation or simultaneously with a contralateral distractor grating. To minimize the influence of possible sensory-perceptual deficits, the luminances of both target and distractor gratings were chosen to be 20 times the luminances necessary to discriminate between horizontal and vertical grating orientations. The location of the target grating was always cued, making the distractor grating task irrelevant. Even after equalizing the visibility of left and right hemifield stimuli, neglect patients still displayed an increased interference effect from an ipsilesional distractor (and no interference from a contralesional distractor). Left or right brain-damaged controls did not show this asymmetric interference of irrelevant distractors, even the patients who demonstrated extinction on standard extinction testing. This suggests that visual extinction is a critical component of the visual neglect syndrome and that it involves an attentional deficit.     

P300 in neglect

ObjectiveAttentional deficits have been thought to underlie hemispatial neglect. The present study investigates this using the P300 component of the event-related potential, which is assumed to reflect the allocation of attention.MethodsWe recorded the P300 and oddball detection performance from patients with neglect. Infrequent targets appeared either in the affected contralesional or the unaffected ipsilesional visual hemifield.ResultsContralesional targets produced weaker average P300 responses than ipsilesional ones, but stronger responses than when only distractors were presented. In the affected hemifield, the difference in P300 amplitude between identified and missed targets was correlated with the miss rate. The number of missed contralesional targets for a given participant was positively correlated with the time required for completion of standard neglect tests.ConclusionsThe P300 not only reflects a general attentional impairment in neglect, but also varies as a function of the participants’ awareness of individual stimuli.SignificanceThe findings reflect an important link between attentional dysfunction and impaired stimulus processing in neglect and extinction.     

Implicit semantic evaluation of object symmetry and contralesional visual denial in a case of left unilateral neglect with damage of the dorsal paraventricular white matter

It is well documented that right brain damaged patients with left unilateral neglect may show unconscious processing of contralesional stimuli. However, the anatomical correlates of this phenomenon are still not well defined. Here we report the case of a neglect patient with a circumscribed lesion of the dorsal parietal-occipital white matter showing clear implicit processing of the contralesional side of line drawings and object based neglect. The patient was able to perform correct semantic analysis of the symmetry (or asymmetry) of the drawings. Additional testing disclosed denial of the existence of the contralesional side of asymmetrical geometrical shapes with correct perception of symmetrical ones and of shapes without the left side. These findings provide direct evidence supporting a critical role of the dorsal visual system in the activation of conscious representations of visual stimuli.     

Eye-movement training-induced changes of visual field representation in patients with post-stroke hemianopia

Changes in neuronal activity have been described in patients with hemianopia following ischemic lesions of the visual cortex. This reorganization may facilitate compensation of lost visual function that is rarely fully restituted. Improving exploratory eye movements with appropriate training has been shown to partially compensate for the visuoperceptive impairment during daily life activities. The changes in cortical processing of visual stimuli that may be induced by these training strategies, however, are less well described. We used fMRI to study the training effects of eye-movement training on cortical representation of visual hemifields. Brain activation during hemifield stimulation was measured in eight patients with an occipital cortical lesion of the striate cortex causing homonymous hemianopia. Starting 8 weeks after the stroke, patients received 4 weeks of eye movement training. fMRI measurements were performed at baseline and after training. In five patients, follow-up fMRI was performed 4 weeks after the end of training. Differences in activation between rest and hemifield stimulation as well as before and after training were assessed with statistical parametric mapping. Twelve healthy subjects were scanned twice at a 4-week interval. During stimulation of the affected hemifield, significant activation at baseline was found bilaterally in extrastriate cortical areas, with the strongest increases in the contralesional hemisphere. This activation pattern was maintained after training. Four weeks after the end of training, there was an additional activation of the extrastriate cortex in the contralesional hemisphere compared to baseline. No changes in the size of visual field defects were found. In this group of patients, eye-movement training induced altered brain activation in the unaffected extrastriate cortex.     

Explicit and implicit perception of illusory contours in unilateral spatial neglect: behavioural and anatomical correlates of preattentive grouping mechanisms

Studies of hemispatial neglect suggest that some perceptual processes still operate on contralesional stimuli independent from spatial attention or awareness. Here we examined whether preattentive processing in extrastriate areas may group unconnected elements inducing illusory contours despite neglect. While it has been debated whether illusory contours arise from preattentive grouping or higher cognitive processes, neurophysiological studies show that neurones in secondary visual cortex (V2) can code for illusory contours. Twelve patients with right hemisphere damage and left neglect were tested for implicit and explicit detection of illusory contours using, respectively: (1) a bisection task where patients were not explicitly required to attend to lateral elements and judged the midpoint of Kanizsa illusory stimuli, as well as other physically connected or unconnected stimuli of the same length; (2) a matching task where patients had to overtly attend to lateral elements and made same/different judgements on pairs of illusory stimuli with identical or different inducers on the right or left side. In some patients, bisection judgements were consistently similar on Kanizsa stimuli with illusory contours and connected stimuli with real contours but different on unconnected gap figures, regardless of their length, suggesting implicit grouping of inducing elements prior to processing stages where a spatial attentional bias arose. Their lesions centred on the inferior parietal cortex or thalamus. Other patients did not show a systematic bisection pattern and had lesions extending posteriorly in the lateral occipital cortex. However, both groups of patients failed to detect left-side inducers in explicit matching judgements, even though errors often revealed unconscious processing, and they showed similar neglect severity on other standard tests. These findings suggest that grouping by illusory contours can occur preattentively and influence bisection independently from the ability to detect contralateral inducers explicitly, severity of inattention, and other forms of unconscious processing. Implicit grouping may depend on the sparing of lateral occipital areas involved in figure-ground segmentation at early stages of visual processing.     

A temporal/nasal asymmetry for blindsight in a localisation task: evidence for extrageniculate mediation

Some patients with hemianopia due to striate cortex lesions show above chance ability in reporting visual stimuli presented in the blind visual field, a phenomenon commonly known as blindsight. Here we report a patient with a dense right hemianopia whose blindsight shows a temporal/nasal asymmetry. MP was tested in a two-alternative forced-choice localisation task, with either the right eye or the left eye patched in separate blocks. When targets appeared in the contralesional temporal hemifield, MP's localisation performance was extremely accurate, whilst she performed at chance with targets in the contralesional nasal hemifield. This is the first demonstration of a temporal/nasal asymmetry for blindsight in a forced-choice paradigm, and is consistent with blindsight in MP's hemianopic field being mediated by a subcortical, extrageniculate route.     

Emotionally negative stimuli can overcome attentional deficits in patients with visuo-spatial hemineglect

Left unilateral spatial neglect resulting from right brain damage is characterized by loss of awareness for stimuli in the contralesional side of space, despite intact visual pathways. We examined using fMRI whether patients with neglect are more likely to consciously detect in the neglected hemifield, emotionally negative complex scenes rather than visually similar neutral pictures and if so, what neural mechanisms mediate this effect. Photographs of emotional and neutral scenes taken from the IAPS were presented in a divided visual field paradigm. As expected, the detection rate for emotional stimuli presented in the neglected field was higher than for neutral ones. Successful detection of emotional scenes as opposed to neutral stimuli in the left visual field (LVF) produced activations in the parahippocampal and anterior cingulate areas in the right hemisphere. Detection of emotional stimuli presented in the intact right visual field (RVF) activated a distributed network of structures in the left hemisphere, including anterior and posterior cingulate cortex, insula, as well as visual striate and extrastriate areas. LVF-RVF contrasts for emotional stimuli revealed activations in right and left attention related prefrontal areas whereas RVF-LVF comparison showed activations in the posterior cingulate and extrastriate visual cortex in the left hemisphere. An additional analysis contrasting detected vs. undetected emotional LVF stimuli showed involvement of left anterior cingulate, right frontal and extrastriate areas. We hypothesize that beneficial role of emotion in overcoming neglect is achieved by activation of frontal and limbic lobe networks, which provide a privileged access of emotional stimuli to attention by top-down modulation of processing in the higher-order extrastriate visual areas. Our results point to the importance of top-down regulatory role of the frontal attentional systems, which might enhance visual activations and lead to greater salience of emotional stimuli for perceptual awareness.     

Parallel interhemispheric processing in aging and alcoholism: relation to corpus callosum size

We tested parallel processing of visual information using the redundant targets effect (RTE) in 12 alcoholics and 13 matched controls. The paradigm was a simple reaction time (RT) task with targets presented in the same (uncrossed), opposite (crossed), or both (redundant) visual-fields. In older alcoholics (>50 years) the RT gain invoked by redundant targets did not exceed probability measures, suggesting compromised interhemispheric processing of parallel information in this subgroup compared with controls or younger alcoholics. The difference between crossed and uncrossed reaction times (CUD), an index of interhemispheric transfer time (ITT), was greater in older than younger subjects. Moreover, the CUD was negatively correlated with the corpus callosum (CC) total area and body in controls, supporting the concept of a structure-function relationship of interhemispheric transfer. This relationship was not found in alcoholics, although the midsagittal area of the CC, genu, and body but not intracranial volume (ICV), was significantly smaller in alcoholics than controls. These results suggest that chronic alcohol abuse together with advancing age exert subtle disruption on parallel interhemispheric processing reliant on callosal connections.     

Restorative and compensatory therapy approaches in cerebral blindness - a review

In the first part of this review, different types of homonymous Visual Field Disorders (VFDs) and their resulting visual disabilities are analyzed in 313 patients with VFDs and 141 patients without VFDs from a neurorehabilitation centre for adults. Homonymous hemianopia was the most frequent visual field loss (54.7 %), followed by hemiamblyopia (23.3 %), quadrantanopia (15.3 %) and paracentral scotomata (6.7 %). About 70 % of all VFD patients had a visual field sparing of 5 degrees or less (macula or foveal sparing). Patients with VFDs frequently showed two types of disabilities: hemianopic alexia was subjectively reported and objectively found in 50-90 % of all patients, and visual exploration deficits in the scotoma were complained and found in 17-70 % of the VFD patients. While hemianopic alexia was related to parafoveal visual field sparing, and additionally to visual acuity in patients with bilateral VFDs, visual exploration deficits were correlated to the size of the area in the scotoma in which the patient searched for a stimulus with saccadic eye movements (search field). The size of the search field in the intact hemifield was not related to visual exploration deficits in unilateral VFDs. As a third disability, visual-spatial deficits in VFD patients are summarized. In the second part, restorative and compensatory treatment approaches for postchiasmatic VFDs are reviewed. Partial restitution of blind regions in the visual field is achieved in the majority of patients treated with purely restorative methods including saccadic localization or light detection in the scotoma. However, the amount of the visual field recovered is limited to 5-12 degrees (mean) in 90 % of these patients. Compensatory treatments seek to improve the substitution of the lost field region by large-scale saccadic eye movements to the scotoma, spatially organized search strategies in both visual hemifields, and by training small-scale eye movements required for reading. Significant improvements in these areas are achieved in 95 % of all VFD patients with these treatment techniques, with documented transfer to visually related activities of daily living. Furthermore, a significant though limited visual field increase of 5-7 degrees is achieved in 30-50 % of patients treated in this way. In the final section, promising approaches that might lead to new treatment techniques for VFDs are reviewed. Among these are the gaze-dependant modulation of scotomata, training-dependant enlargement of the useful field of view, a nd t he uncovering of residual visuomotor capacities to visual stimuli in a scotoma.     

Differential processing of word and color in unilateral spatial neglect

The aim of the present study was to investigate mechanisms underlying processing of contralesional visual stimuli in brain-damaged patients with unilateral spatial neglect (USN). Nine right-hemisphere-damaged stroke patients with left-sided neglect and nine controls performed a reaction-time task involving manual response to a central color patch (target stimulus) flanked to the left or right by a Stroop stimulus they had to ignore. While the word dimension of the flanker affected patients' responses considerably and equally when presented to either side, the color dimension of the flanker had no effect when presented to the left, but had a large effect when presented to the right. Four of the patients performed a control task requiring same/different judgments between either of the two flanker dimensions (color and word) and the central target. Their performance indicated that they were able to process color information from the contralesional field, despite their results in the first experiment. These findings demonstrate a dissociation between how the patients processed different dimensions of the same stimuli and imply that the extent of processing in the contralesional hemifield depends both on task requirements and on the exact features of the stimuli. The implications of these results on normal attentional mechanisms is also discussed.     

Task-sensitivity of unconscious word processing in spatial neglect

Neuropsychological studies of spatial neglect have shown that ignored visual stimuli can produce measurable behavioral changes without eliciting subjective perceptual experience. However, such non-conscious, implicit cognitive processing may not be fully automatic but rather could be influenced by the patients' voluntary behavioral control. Using a hemifield priming paradigm with two different task instructions, we studied spatial neglect patients to assess whether non-conscious processing of ignored words is modulated by behavioral task requirements. In each trial, participants named or categorized a centrally presented target following a masked prime flashed to the left or right hemifield. By delivering equally invisible stimuli to both hemifields, this design allowed rigorous testing of the impact of task instructions on non-conscious processing in neglect patients and control participants. We observed that neglect patients showed slightly different patterns of masked priming from those obtained in healthy and right-hemisphere control patients. Importantly, however, all these three groups showed strong sensitivity to task contexts during the unconscious processing of masked words. The present results provide neuropsychological evidence that robust task-sensitive neural pathways are covertly operating on weak and normally imperceptible visual stimuli even when visuospatial attention is severely compromised.     

Unlike in Clinical Blindsight Patients,Unconscious Processing of Chromatic Information Depends on Early Visual Cortex in Healthy Humans

BackgroundLarge amount of data concerning the neural mechanisms that mediate unconscious vision come from cortically blind patients who can process stimuli presented in their blind visual field. How well the findings generalize to neurologically healthy humans remains open.ObjectiveUsing transcranial magnetic stimulation (TMS), we studied whether chromatic processing depends on the early visual cortex in healthy participants.MethodWe employed a phenomenon called the redundant target effect (RTE): simple reaction times to two stimuli are faster than to one stimulus, even when one of the stimuli is presented outside consciousness.ResultsThe RTE produced by chromatically defined stimuli disappeared when the contralateral stimulus of two bilateral stimuli was suppressed from consciousness.ConclusionsIn contrast to studies on blindsight patients, the results imply that the early visual cortex is necessary for the processing of chromatic information in neurologically healthy humans.     

Crossing the midline: reducing attentional deficits via interhemispheric interactions

Patients with unilateral neglect and extinction show a profound lack of awareness of stimuli presented contralateral to their lesion. However, many processes of perception are intact and contralesional stimuli seem to reach a high level of representation, perceptual and semantic. Some of these processes can work to decrease the magnitude of the attentional deficit. Here, we examine two of these intact processes, feature detection and perceptual grouping. First, we demonstrate that feature detection occurs in parallel in the contralesional visual fields of neglect and extinction patients. Second, we attempt to dissociate the influence of perceptual contours across the vertical meridian from the presence of an object or higher-level perceptual unit (or group) that may be created by these contours. We find that connections across the midline affect attentional deficits independently of the objects they may create. This suggests that several effects of grouping on neglect and extinction may be mediated by long-range cortical interactions that arise from connections across the vertical meridian.     

Influence of response modality on awareness of contralesional tactile stimuli in right brain damaged patients

Thirty right brain-damaged (RBD) patients with left tactile extinction (19 of whom also showed neglect) were given a sequence of 240 tactile stimuli – 80 right, 80 left, 80 bilateral – across 4 different response conditions: (a) verbal report of stimulated side/s, (b) motor report of stimulated side/s, (c) verbal report of unstimulated side/s, (d) motor report of unstimulated side/s. Earlier experiments based on similar tasks but involving RBD patients with visual extinction and/or neglect have shown that visual awareness of contralesional stimuli can be influenced by manipulation of response conditions.

Since neglect and extinction can be double-dissociated both anatomically and behaviourally, the question arises of whether the underlying neuronal mechanisms are different. To answer this question the present work investigated the role of perceptual and premotor factors in generating tactile extinction in response to Double Simultaneous Stimulation (DSS). The hypothesis was that a directional response bias would result in an overall higher frequency of errors for verbal or motor responses indicating the ipsilesional side (right); a perceptual bias would instead result in errors distributed with similar frequency on the ipsilesional and the contralesional (left) side.

Results showed that, in RBD patients, contralesional extinction was not influenced by response conditions (verbal/motor; report of stimulated/unstimulated side) and presence/absence of neglect. This suggests that: (1) among RBD patients, directional response biases are unlikely to play a role in extinction of tactile stimuli on DSS; (2) the mechanisms underlying extinction are, at least to some extent, different from those underlying unilateral neglect.     

Implicit perception in patients with visual neglect: lexical specificity in repetition priming

Twelve patients with left-sided neglect, five patients with left-sided hemianopia, and 16 matched controls performed lexical decisions for foveally presented target words or pseudowords. Target words or pseudowords could be preceded by the same stimuli, presented for 150 ms as primes in either the left (LVF) or the right (RVF) visual hemifield. Primes in the RVF caused similar levels of priming across the three groups. In contrast, whereas primes in the affected LVF did not cause priming in hemianopic patients, patients with neglect showed highly significant priming from LVF primes, and the level of priming from these neglected primes was increased relative to normal control levels. This priming effect was attributed to lexical access, because no repetition priming was observed for pronounceable pseudowords. In striking contrast to the priming results, neglect patients' explicit recognition performance for these prime stimuli was at chance for the left hemifield, although it was near perfect for the right hemifield. This demonstrates that the observed priming effects in the left hemifield were indeed caused by stimuli that were not explicitly perceived.     

Beware and be aware: capture of spatial attention by fear-related stimuli in neglect

Stimuli with threat significance may be privileged in summoning attention, allowing fast detection even outside the field of attention. We studied patients with unilateral neglect and visual extinction, who usually remain unaware of contralesional stimuli presented together with concurrent ipsilesional stimuli, to learn whether emotional stimuli might differentially be affected by contralesional extinction. Pictures of spiders or flowers with similar features were presented in right, left, or both fields. On bilateral trials, the patients detected emotional stimuli (spiders) on the left side much more often than neutral pictures (flowers). While mechanisms of spatial attention are impaired after parietal damage in neglect patients, intact visual pathways to the ventral temporal lobe and amygdala might still mediate distinct mechanisms of emotional attention.     

Audiovisual integration in patients with visual deficit

In the present study, we investigated the possibility that bimodal audiovisual stimulation of the affected hemifield can improve perception of the visual events in the blind hemifield of hemianopic patients, as it was previously demonstrated in neglect patients. Moreover, it has been shown that "hetero-modal" and "sensory-specific" cortices are involved in cross-modal integration. Thus, the second aim of the present study was to examine whether audiovisual integration influences visual detection in patients with different cortical lesions responsible of different kinds of visual disorders. More specifically, we investigated cross-modal, audiovisual integration in patients with visual impairment due to a visual field deficit (e.g., hemianopia) or visuospatial attentional deficit (e.g., neglect) and patients with both hemianopia and neglect. Patients were asked to detect visual stimuli presented alone or in combination with auditory stimuli that could be spatially aligned or not with the visual ones. The results showed an enhancement of visual detection in cross-modal condition (spatially aligned condition) comparing to unimodal visual condition only in patients with hemianopia or neglect; by contrast, the multisensory integration did not occur when patients presented both deficits. These data suggest that patients with visual disorders can enormously benefit the multisensory integration. Moreover, they showed a different influence of cortical lesion on multisensory integration. Thus, the present results show the important adaptive meaning of multisensory integration and are very promising with respect to the possibility of recovery from visual and spatial impairments.     

Early visual processing in neglect patients: A study with steady-state VEPs

Reliable steady-state visual evoked potentials (VEPs) were recorded in a group of 19 right brain-damaged patients with visuospatial hemineglect (Neglect), and two control groups: 15 left brain-damaged (LBD) patients and 12 right brain-damaged (RBD) patients without neglect. Moreover, VEPs were recorded in two rare cases of left brain damage and right visuospatial hemineglect. Stimuli were gratings phase-reversed at various temporal frequencies presented in the left and right visual field. In the Neglect group, VEPs to stimuli displayed in the left visual field (contralesional stimuli) had longer latencies. The delay was not present for the two control groups. As regards the VEP amplitudes, the Neglect group data showed a less distinctive pattern than in the case of latency. VEPs to stimuli contralateral to the lesion were smaller than those recorded for stimuli ipsilateral to the lesion in both Neglect and RBD groups. On the contrary, the VEP amplitudes for the two hemifields were comparable in the LBD group. In the case of left brain damage and neglect, VEPs to right visual field stimuli had longer latencies and lower amplitudes compared to the ipsilesional responses in both patients. Overall, the data support the view that, in most cases, early visual processing is not intact in the neglected hemifield.