Blind Hemifield Research Articles

Role of corpus callosum in unconscious vision

The existence of unconscious visually triggered behavior in patients with cortical blindness (e.g., homonymous hemianopia) has been amply demonstrated and the neural bases of this phenomenon have been thoroughly studied. However, a crosstalk between the two hemispheres as a possible mechanism of unconscious or partially conscious vision has not been so far considered. Thus, the aim of this study was to assess the relationship between structural and functional properties of the corpus callosum (CC), as shown by probabilistic tractography (PT), behavioral detection/discrimination performance and level of perceptual awareness in the blind field of patients with hemianopia. Twelve patients were tested in two tasks with black-and-white visual square-wave gratings, one task of movement and the other of orientation. The stimuli were lateralized to one hemifield either intact or blind. A PT analysis was carried out on MRI data to extract fiber properties along the CC (genu, body, and splenium). Compared with a control group of participants without brain damage, patients showed lower FA values in all three CC sections studied. For the intact hemifield we found a significant correlation between PT values and visual detection/discrimination accuracy. For the blind hemifield the level of perceptual awareness correlated with PT values for all three CC sections in the movement task. Importantly, significant differences in all three CC sections were found also between patients with above-vs. chance detection/discrimination performance while differences in the genu were found between patients with and without perceptual awareness. Overall, our study provides evidence that the properties of CC fibers are related to the presence of unconscious stimulus detection/discrimination and to hints of perceptual awareness for stimulus presentation to the blind hemifield. These results underline the importance of information exchange between the damaged and the healthy hemisphere for possible partial or full recovery from hemianopia.

Poster Session: Evidence for preserved conscious orientation discrimination in perimetrically-blind fields early after V1-damage.

Cortically-blind (CB) patients with stroke damage to the primary visual cortex (V1) lose conscious vision but many exhibit blindsight - the ability to unconsciously detect or discriminate moving or flickering targets inside their blind-fields. However, the prevalence of conscious visual abilities in CB is less clear. Having developed a new method to assess vision inside perimetrically-defined blind fields, we found that >50% of subacute CB patients (<6 months post-stroke) can consciously discriminate global motion inside their blind field. Here, we asked if they can also discriminate orientation of static targets, which do not typically elicit blindsight. In 10 subacute patients, we mapped their intact and blind hemifields using static, non-flickering, 1cpd Gabors across a wide range of luminance contrasts. Blind-field locations were labeled "preserved" if performance was >72.5% correct. Considering overall performance, only 1 participant had preserved static orientation perception in the blind-field. However, this increased to 4 participants when only considering performance at high contrasts (>50%), all of whom reported awareness of stimuli. Thus, early after V1 damage, conscious percepts for oriented, high-contrast, static targets can remain inside CB fields, similar in incidence to global motion discriminations. We are now testing additional patients to assess if these abilities persist into the chronic period and to detail their underlying neural substrates.

Innovative vision rehabilitation method for hemianopsia: Comparing pre- and post audio-luminous biofeedback training for ocular motility improving visual functions and quality of life.

Homonymous hemianopsia (HH) corresponds to vision loss in one hemi-field secondary to retro-chiasmal injury. Patients with HH experience difficulties in scanning and orientation in their environment. Near vision daily activities such as reading can also be impaired. There is an unmet need for standardized vision rehabilitation protocols for HH. We investigated the effectiveness of biofeedback training (BT), used for vision rehabilitation in patients with central vision loss, in individuals with HH. In this prospective pilot pre/post study, 12 participants, with HH consecutive to brain injury, performed 5 weekly BT sessions for 20 min each under supervision using the Macular Integrity Assessment microperimeter. BT consisted of relocation of the retinal locus 1-4° toward the blind hemi-field. Outcomes measured post-BT were paracentral retinal sensitivity, visual acuity (near vision), fixation stability, contrast sensitivity, reading speed, and visual functioning questionnaire. Statistical analysis was performed using Bayesian paired t-tests. Paracentral retinal sensitivity significantly increased by 2.7 ± 0.9 dB in the treated eye in 9/11 of the participants. Significant improvements with medium-to-large effect size were observed for fixation stability (8/12 participants), contrast sensitivity (6/12 participants) and near vision visual acuity (10/12 participants). Reading speed increased by 32.5 ± 32.4 words per minute in 10/11 participants. Quality of vision scores improved significantly with large effect size for visual ability, visual information and mobility. BT led to encouraging improvements in visual functions and functional vision in individuals with HH. Further confirmation with larger trials is required.

A Scientific Approach to Conscious Experience, Introspection, and Unconscious Processing: Vision and Blindsight.

Although subjective conscious experience and introspection have long been considered unscientific and banned from psychology, they are indispensable in scientific practice. These terms are used in scientific contexts today; however, their meaning remains vague, and earlier objections to the distinction between conscious experience and unconscious processing, remain valid. This also applies to the distinction between conscious visual perception and unconscious visual processing. Damage to the geniculo-striate pathway or the visual cortex results in a perimetrically blind visual hemifield contralateral to the damaged hemisphere. In some cases, cerebral blindness is not absolute. Patients may still be able to guess the presence, location, shape or direction of movement of a stimulus even though they report no conscious visual experience. This “unconscious” ability was termed “blindsight”. The present paper demonstrates how the term conscious visual experience can be introduced in a logically precise and methodologically correct way and becomes amenable to scientific examination. The distinction between conscious experience and unconscious processing is demonstrated in the cases of conscious vision and blindsight. The literature on “blindsight” and its neurobiological basis is reviewed. It is shown that blindsight can be caused by residual functions of neural networks of the visual cortex that have survived cerebral damage, and may also be due to an extrastriate pathway via the midbrain to cortical areas such as areas V4 and MT/V5.

The Mechanism of Macular Sparing.

Patients with homonymous hemianopia sometimes show preservation of the central visual fields, ranging up to 10°. This phenomenon, known as macular sparing, has sparked perpetual controversy. Two main theories have been offered to explain it. The first theory proposes a dual representation of the macula in each hemisphere. After loss of one occipital lobe, the back-up representation in the remaining occipital lobe is postulated to sustain ipsilateral central vision in the blind hemifield. This theory is supported by studies showing that some midline retinal ganglion cells project to the wrong hemisphere, presumably driving neurons in striate cortex that have ipsilateral receptive fields. However, more recent electrophysiological recordings and neuroimaging studies have cast doubt on this theory by showing only a minuscule ipsilateral field representation in early visual cortical areas. The second theory holds that macular sparing arises because the occipital pole, where the macula is represented, remains perfused after occlusion of the posterior cerebral artery because it receives collateral flow from the middle cerebral artery. An objection to this theory is that it cannot account for reports of macular sparing in patients after loss of an entire occipital lobe. On close scrutiny, such reports turn out to be erroneous, arising from inadequate control of fixation during visual field testing. Patients seem able to detect test stimuli on their blind side within the macula or along the vertical meridian because they make surveillance saccades. A purported treatment for hemianopia, called vision restoration therapy, is based on this error. The dual perfusion theory is supported by anatomical studies showing that the middle cerebral artery perfuses the occipital pole in many individuals.In patients with hemianopia from stroke, neuroimaging shows preservation of the occipital pole when macular sparing is present. The frontier dividing the infarcted territory of the posterior cerebral artery and the preserved territory of the middle cerebral artery is variable, but always falls within the representation of the macula, because the macula is so highly magnified. For physicians, macular sparing was an important neurological sign in acute hemianopia because it signified a posterior cerebral artery occlusion. Modern neuroimaging has supplanted the importance of that clinical sign but at the same time confirmed its validity. For patients, macular sparing remains important because it mitigates the impact of hemianopia and preserves the ability to read fluently.

The presence of placeholders modulates the naso-temporal asymmetry in the remote distractor effect

The remote distractor effect (RDE) is a well-known and robust phenomenon whereby latencies of saccades are increased when a distractor is presented simultaneously along with the saccade target. Studies of the RDE in patients with a loss of vision in one visual field (hemianopia) following damage to primary visual cortex have provided conflicting results. Rafal, Smith, Krantz, Cohen, and Brennan (1990) reported a naso-temporal asymmetry in the RDE in patients with hemianopias, with a greater influence of distractors presented in their blind temporal visual field. This asymmetry was not observed in typically sighted controls. By contrast, Walker, Mannan, Maurer, Pambakian, and Kennard (2000) observed no effect of distractors presented to either the blind nasal or blind temporal hemifield of hemianopes, but the naso-temporal asymmetry was observed in typically sighted controls. The present study addressed one potential methodological differences between the two studies by investigating the inhibitory effect of a distractor on saccade latency in neurotypical participants. Here participants were tested monocularly and the effect of a nasal/temporal hemifield distractor on saccade latency observed in the presence or absence of peripheral placeholders. Our results showed a naso-temporal asymmetry in the magnitude of the RDE in the no placeholder condition, with a greater RDE when the distractor was presented in the temporal visual field. However, in the placeholder condition the opposite asymmetry was observed, that is an increased RDE when the distractor was presented in the nasal visual field. Our results suggest that the presence/absence of a placeholder might be the critical factor explaining the discrepancy between Rafal et al. (1990) and Walker et al. (2000) in participants without visual field loss. The current results can be interpreted in terms of additional inhibitory or attentional processes that bias selection towards stimuli in the nasal hemifield in the presence of placeholders, still, the mechanisms underlying these effects remain unclear.

Visuo-spatial attention to the blind hemifield of hemianopic patients: Can it survive the impairment of visual awareness?

The general aim of this study was to assess the effect produced by visuo-spatial attention on both behavioural performance and brain activation in hemianopic patients following visual stimulus presentation to the blind hemifield. To do that, we tested five hemianopic patients and six age-matched healthy controls in an MRI scanner during the execution of a Posner-like paradigm using a predictive central cue. Participants were instructed to covertly orient attention toward the blind or sighted hemifield in different blocks while discriminating the orientation of a visual grating.In patients, we found significantly faster reaction times (RT) in valid and neutral than invalid trials not only in the sighted but also in the blind hemifield, despite the impairment of consciousness and performance at chance. As to the fMRI signal, in valid trials we observed the activation of ipsilesional visual areas (mainly lingual gyrus – area 19) during the orientation of attention toward the blind hemifield. Importantly, this activation was similar in patients and controls. In order to assess the related functional network, we performed a psychophysiological interactions (PPI) analysis that revealed an increased functional connectivity (FC) in patients with respect to controls between the ipsilesional lingual gyrus and ipsilateral fronto-parietal as well as contralesional parietal regions. Moreover, the shift of attention from the blind to the sighted hemifield revealed stronger FC between the contralesional visual areas V3/V4 and ipsilateral parietal regions in patients than controls. These results indicate a higher cognitive effort in patients when paying attention to the blind hemifiled or when shifting attention from the blind to the sighted hemfield, possibly as an attempt to compensate for the visual loss.Taken together, these results show that hemianopic patients can covertly orient attention toward the blind hemifield with a top-down mechanism by activating a functional network mainly including fronto-parietal regions belonging to the dorsal attentional network.

Sacrificing one visual hemifield during pediatric epilepsy surgery: Effects on visual search.

To investigate early and late effects of planned surgically acquired homonymous hemianopias on visual search in children and adolescents. This prospective study included five patients (5y 5m-18y 0m; 2 girls) with pharmaco-refractory epilepsies in whom one visual hemifield was sacrificed as part of the surgical strategy, and, as controls, seven patients (5y 11m-18y 0m; 6 girls) undergoing epilepsy surgeries not affecting the visual fields. Visual search was studied using the "TableTest", which is an everyday life-like visual search test. General processing speed was studied using a standard IQ subtest. All five patients with newly acquired homonymous hemianopias showed a relative disadvantage of visual search times for objects in their newly blind hemifields immediately after the surgery. Six months later, this relative disadvantage had recovered completely in all patients. Nevertheless, compared with the preoperative situation, overall search times were still prolonged in the hemianopic patients, but this effect could be mitigated or even overcompensated by improvements in processing speed. Children with homonymous hemianopias inflicted by epilepsy surgery develop effective compensation strategies to minimize the relative disadvantage of visual search in their blind hemifields. For changes in overall visual search times between the preoperative and the six-month follow-up examination, we could demonstrate overlapping effects of (a) deterioration by hemianopia and (b) amelioration by improved processing speed as part of the cognitive improvements achieved by amelioration of the epilepsy.

What cortical areas are responsible for blindsight in hemianopic patients?

The presence of above-chance unconscious behavioral responses following stimulus presentation to the blind hemifield of hemianopic patients (blindsight) is a well-known phenomenon. What is still lacking is a systematic study of the neuroanatomical bases of two distinct aspects of blindsight: the unconscious above chance performance and the phenomenological aspects that may be associated. Here, we tested 17 hemianopic patients in two tasks i.e. movement and orientation discrimination of a visual grating presented to the sighted or blind hemifield. We classified patients in four groups on the basis of the presence of above chance unconscious discrimination without or with perceptual awareness reports for stimulus presentation to the blind hemifield. A fifth group was represented by patients with interruption of the Optic Radiation. In the various groups we carried out analyses of lesion extent of various cortical areas, probabilistic tractography as well as assessment of the cortical thickness of the intact hemisphere.All patients had lesions mainly, but not only, in the occipital lobe and the statistical comparison of their extent provided clues as to the critical anatomical substrate of unconscious above-chance performance and of perceptual awareness reports, respectively. In fact, the two areas that turned out to be critical for above-chance performance in the discrimination of moving versus non-moving visual stimuli were the Precuneus and the Posterior Cingulate Gyrus while for perceptual awareness reports the crucial areas were Intracalcarine, Supracalcarine, Cuneus, and the Posterior Cingulate Gyrus. Interestingly, the proportion of perceptual awareness reports was higher in patients with a spared right hemisphere. As to probabilistic tractography, all pathways examined yielded higher positive values for patients with perceptual awareness reports. Finally, the cortical thickness of the intact hemisphere was greater in patients showing above-chance performance than in those at chance. This effect is likely to be a result of neuroplastic compensatory mechanisms.

Dissociations between perception and awareness in hemianopia.

The most common visual defect to follow a lesion of the retrochiasmal pathways is homonymous hemianopia (HH), whereby patients are blind to the contralesional visual field of each eye. Homonymous hemianopia has been studied in terms of its deleterious consequences on perceptual, cognitive and motor tasks as well as because it represents an interesting model of vision loss after a unilateral lesion of the occipital lobe. From a behavioral perspective, in addition to exhibiting a severe deficit in their contralesional visual field, HH patients can also exhibit dissociations between perception and awareness. Firstly, HH patients suffering from anosognosia may be unaware of their visual field defect. Secondly, HH patients can present with unconscious visual abilities in the blind hemifield, a phenomenon referred to as blindsight. Thirdly, recent reports demonstrate that HH patients can suffer from a subtle deficit in their ipsilesional visual field that they are unaware of, a condition called sightblindness (i.e. the reverse case of 'blindsight'). Finally, HH patients may also exhibit visual hallucinations in their blind field; however, such patients are not systematically aware that their perceptions are unreal. In this review, we provide an overview of the visual-field losses in HH patients after a left or right unilateral occipital lesion. Furthermore, we explore the implications of these four phenomena for models of visual processing and rehabilitation of visual field defects in HH patients. Finally, in contrast to the traditional view that HH is solely a visual-field defect, we discuss why this deficit is an interesting model for studying the dissociation between perception and awareness.

Neural bases of unconscious orienting of attention in hemianopic patients: Hemispheric differences

The aim of this research was to study the behavioral and neurophysiological correlates of visual attention orientation to unseen stimuli presented to the blind hemifield of hemianopic patients, and the existence of hemispheric differences for this kind of unconscious attention. Behaviorally, by using a Posner paradigm, we found a significant attention effect in speed of response to unseen stimuli similar to that observed in the sighted hemifield and in healthy participants for visible stimuli. Moreover, event-related potential (ERP) and oscillatory attention-related activity were present following stimulus presentation to the blind hemifield. Importantly, in patients this pattern of activity was different as a function of the side of the brain lesion: Left damaged patients showed attention-related ERP and oscillatory activity broadly similar to that found in healthy participants. In contrast, right damaged patients showed a radically different pattern. These data confirm and extend to neurophysiological mechanisms the existence of unconscious visual orienting and are in keeping with a right hemisphere dominance for both unconscious and conscious attention.

Making sense of blindsense: a reply to Phillips

We recently published the results of a study on the occurrence of blindsight among eight, post-stroke homonymous hemianopic (HH) patients (Garric et al., 2019), in whom we measured blindsight through forced-choice tasks and assessed perceptual experiences by a new awareness scale, the Sensation Awareness Scale (SAS). Within the cohort, we found different profiles of dissociation between objective and subjective performance. Importantly, we were able to describe several cases of a dissociation phenomenon that we named blindsense, whereby patients exhibited marked subjective sensitivity in their blind hemifield despite being unable to discriminate the different stimuli. Following publication of our article (Garric et al., 2019), Prof. Ian Phillips (Phillips, 2019) wrote a Commentary in which he questioned the methodology we used to measure and analyze objective and subjective perception in our HH patients. As opposed to our original interpretation of our results to describe the new profile of blindsense, based on a non-visual experience hypothesis (Kentridge, 2015), Prof. Phillips re-evaluated the different blindsight profiles that we identified in our study through the lens of a degraded conscious vision hypothesis (Overgaard, Fehl, Mouridsen, Bergholt, & Cleeremans, 2008). In the present response, we explain that, although we agree that dichotomous visual scales lead to highly conservative responses and mask conscious perceptual experience of patients, we still support the notion that nuanced report protocols can enable more-sensitive measurements of perceptual experiences in the hemianopic, so-called blind visual field. Furthermore, we affirm that the additional awareness-scale phenomenal levels that such protocols enable are more consistent with patients' experiences and lead patients to provide more liberal responses when describing their subjective perceptions.

Neural bases of visual processing of moving and stationary stimuli presented to the blind hemifield of hemianopic patients

Unilateral damage to post-chiasmatic visual pathways or cortical areas results in the loss of vision in the contralateral hemifield, known as hemianopia. Some patients, however, may retain the ability to perform an above chance unconscious detection or discrimination of visual stimuli presented to the blind hemifield, known as “blindsight”. An important finding in blindsight research is that it can often be elicited by moving stimuli. Therefore, in the present study, we wanted to test whether moving stimuli might yield blindsight phenomena in patients with cortical lesions resulting in hemianopia, in a discrimination task where stimulus movement is orthogonal to the feature of interest. This could represent an important strategy for rehabilitation because it might improve discrimination ability of stimulus features different but related to movement, e.g. line orientation.We tested eight hemianopic patients and eight age-matched healthy controls in an orientation discrimination task with moving or static visual stimuli. During performance of the task we carried out fMRI scanning and tractography. Behaviourally, we did not find a reliable main effect of motion on orientation discrimination; however, an important result was that in different patients blindsight could occur only with moving or stationary stimuli or with both. As to brain imaging results, following presentation of moving stimuli to the blind hemifield, a widespread fronto-parietal bilateral network was recruited including areas of the dorsal stream and in particular bilateral motion area hMT + whose activation positively correlated with behavioural performance. This bilateral network was not activated in controls suggesting that it represents a compensatory functional change following brain damage. Moreover, there was a higher activation of ipsilesional area hMT+ in patients who performed above chance in the moving condition. By contrast, in patients who performed above chance in the static condition, we found a higher activation of contralesional area V1 and extrastriate visual areas. Finally, we found a linear relationship between structural integrity of the ipsilesional pathway connecting lateral geniculate nucleus (LGN) with motion area hMT+ and both behavioural performance and ipsilesional hMT + activation. These results support the role of LGN in modulating performance as well as BOLD amplitude in the absence of visual awareness in ipsilesional area hMT+ during an orientation discrimination task with moving stimuli.

Using superior colliculus principles of multisensory integration to reverse hemianopia

The diversity of our senses conveys many advantages; it enables them to compensate for one another when needed, and the information they provide about a common event can be integrated to facilitate its processing and, ultimately, adaptive responses. These cooperative interactions are produced by multisensory neurons. A well-studied model in this context is the multisensory neuron in the output layers of the superior colliculus (SC). These neurons integrate and amplify their cross-modal (e.g., visual-auditory) inputs, thereby enhancing the physiological salience of the initiating event and the probability that it will elicit SC-mediated detection, localization, and orientation behavior. Repeated experience with the same visual-auditory stimulus can also increase the neuron's sensitivity to these individual inputs. This observation raised the possibility that such plasticity could be engaged to restore visual responsiveness when compromised. For example, unilateral lesions of visual cortex compromise the visual responsiveness of neurons in the multisensory output layers of the ipsilesional SC and produces profound contralesional blindness (hemianopia). The possibility that multisensory plasticity could restore the visual responses of these neurons, and reverse blindness, was tested in the cat model of hemianopia. Hemianopic subjects were repeatedly presented with spatiotemporally congruent visual-auditory stimulus pairs in the blinded hemifield on a daily or weekly basis. After several weeks of this multisensory exposure paradigm, visual responsiveness was restored in SC neurons and behavioral responses were elicited by visual stimuli in the previously blind hemifield. The constraints on the effectiveness of this procedure proved to be the same as those constraining SC multisensory plasticity: whereas repetitions of a congruent visual-auditory stimulus was highly effective, neither exposure to its individual component stimuli, nor to these stimuli in non-congruent configurations was effective. The restored visual responsiveness proved to be robust, highly competitive with that in the intact hemifield, and sufficient to support visual discrimination.

Interhemispheric visual competition after multisensory reversal of hemianopia

Unilateral lesions of visual cortex have the secondary consequence of suppressing visual circuits in the midbrain superior colliculus (SC), collectively producing blindness in contralesional space (“hemianopia”). Recent studies have demonstrated that SC visual responses and contralesional vision can be reinstated by a non‐invasive multisensory training procedure in which spatiotemporally concordant visual‐auditory pairs are repeatedly presented within the blind hemifield. Despite this recovery of visual responsiveness, the loss of visual cortex was expected to result in permanent deficits in that hemifield, especially when visual events in both hemifields compete for attention and access to the brain's visuomotor circuitry. This was evaluated in the present study in a visual choice paradigm in which the two visual hemifields of recovered cats were simultaneously stimulated with equally valent visual targets. Surprisingly, the expected disparity was not found, and some animals even preferred stimuli presented in the previously blind hemifield. This preference persisted across multiple stimulus intensity levels and there was no indication that animals were less aware of cues in the previously blind hemifield than in its spared counterpart. Furthermore, when auditory cues were combined with visual cues, the enhanced performance they produced on a visual task was no greater in the normal than in the previously blind hemifield. These observations suggest that the multisensory rehabilitation paradigm revealed greater inherent visual information processing potential in the previously blind hemifield than was believed possible given the loss of visual cortex.

Neuronal mechanisms of motion detection underlying blindsight assessed by functional magnetic resonance imaging (fMRI)

Brain imaging offers a valuable tool to observe functional brain plasticity by showing how sensory inputs reshape cortical activations after a visual impairment. Following a unilateral post-chiasmatic lesion affecting the visual cortex, patients may suffer a contralateral visual loss referred to homonymous hemianopia. Nevertheless, these patients preserve the ability to unconsciously detect, localize and discriminate visual stimuli presented in their impaired visual field. To investigate this paradox, known as blindsight, we conducted a study using functional magnetic resonance imaging (fMRI) to evaluate the structural and functional impact of such lesion in a 33-year old patient (ML), who suffers a complete right hemianopia without macular sparing and showing strong evidences of blindsight. We thus performed whole brain and sliced thalamic fMRI scan sequences during an event-related motion detection task. We provided evidence of the neuronal fingerprint of blindsight by acquiring and associating neural correlates, specific structures and functional networks of the midbrain during blindsight performances which may help to better understand this condition. Accurate performance demonstrated the presence of residual vision and the ability to unconsciously perceive motion presented in the blind hemifield, although her reaction time was significantly higher in her blind-field. When the normal hemifield was stimulated, we observed significant contralateral activations in primary and secondary visual areas as well as motion specific areas, such as the supramarginal gyrus and middle temporal area. We also demonstrated sub-thalamic activations within the superior colliculi (SC) and the pulvinar. These results suggest a role of secondary subcortical structures in normal spontaneous motion detection. In a similar way, when the lesioned hemifield was stimulated, we observed contralateral activity in extrastriate areas with no activation of the primary lesioned visual cortex. Moreover, we observed activations within the SC when the blind hemifield was stimulated. However, we observed unexpected ipsilateral activations within the same motion specific areas, as well as bilateral frontal activations. These results highlight the importance of abnormal secondary pathways bypassing the primary visual area (V1) in residual vision. This reorganization in the structure and function of the visual pathways correlates with behavioral changes, thus offering a plausible explanation for the blindsight phenomenon. Our results may potentially impact the development of rehabilitation strategies to target subcortical pathways.

Neural Correlates of Visuospatial Attention to Unseen Stimuli in Hemianopic Patients. A Steady-State Visual Evoked Potential Study.

The relationship between attention and awareness is a topic of great interest in cognitive neuroscience. Some studies in healthy participants and hemianopic patients have shown dissociation between these two processes. In contrast, others confirmed the classic notion that the two processes are mutually exclusive. To try and cast further light on this fascinating dilemma, in the present study we have investigated the neural mechanisms of visual spatial attention when perceptual awareness is totally lacking. To do that, we monitored with steady-state visual evoked potentials (SSVEPs) the neurophysiological correlates of endogenous spatial attention to unseen stimuli presented to the blind field of hemianopic patients. Behaviourally, stimulus detection (a brief change in the orientation of a gabor grating) was absent in the blind hemifield while in the sighted field there was a lower, but non-significant, performance in hit rate with respect to a healthy control group. Importantly, however, in both blind and sighted hemifield of hemianopics (as well as in healthy participants) SSVEP recordings showed an attentional effect with higher frequency power in the attended than unattended condition. The scalp distribution of this effect was broadly in keeping with the location of the dorsal system of endogenous spatial attention. In conclusion, the present results provide evidence that the neural correlates of spatial attention are present regardless of visual awareness and this is in accord with the general hypothesis of a possible dissociation between attention and awareness.

Effect of Eye Movement Training on Balance in Post Stroke Patients with Unilateral Spatial Neglect. First Preliminary report

Background: Unilateral Spatial Neglect (USN) is a complex disorder associated with stroke and is more severe and persistent following right hemisphere damage. Stroke patients have mobility limitations, muscle weakness, and behaviors disorders. The problem with USN is lack of orientation and information from affected side that may cause balance problems specially in elderly survivors. Training eye movements are aimed at improving visual function by training patients to navigate the blind hemifield and increase their overall sensitivity. Accurate reliable knowledge of balance in stroke survivors is important for prognosis and to predict falls. Aim of the study: This study was designed to find out the effect of training eye movements on balance in elderly post stroke survivors with USN. Subjects and Methods: The study included 15 of both sexes with post stroke USN (11 males and 4 females) their mean age was 69.1 ± 1.9 years. Patients received sequentially in same session training for eye movement program and a traditional gait training program 4 days/week for 6 weeks. Balance assessment was measured by Berg Balance Scale (BBS), Time Up and go test (TUG), 10 meters Talking Test (10mWT), and 6 Minutes Walking Test (6MWT). Results: there were significant changes in between pre and post-study as the analysis of mean values of BBS, TUG, 10mW, and 6MWT.Conclusion: eye movement training had a significant effect on improving balance in post stroke survivors with USN. Eye movement training improved patients with USN ability to explorer the blind hemispace and to improve balance.

Oculomotor control after hemidecortication: One hemisphere encodes normal ipsilateral oblique anti-saccades

A critical question in neurology is how the brain reorganizes its structure and function following injury. Here, we consider oculomotor control following a massive brain lesion, a hemispherectomy. We used the oblique anti-saccade task which requires the suppression of a saccade towards a visual cue, flashed anywhere in a patient's seeing hemifield, and the generation, in the dark, of an anti-saccade to a task-dependent location in the opposite blind hemifield; inverting either the horizontal or both horizontal and vertical components. Anti-saccades require a visuo-motor vector inversion that normally involves bilateral interactions between frontal, parietal and subcortical structures across both hemispheres. Here, oblique anti-saccades presented a major challenge to the patient's single hemisphere, requiring one site in visual cortex to communicate with an instruction-dependent site in oculomotor cortex. Patients with discrete frontal lobe damage can be strongly impaired in anti-saccades. By contrast, hemispherectomy patients performed oblique anti-saccades normally, contrasting with their permanent contralesional hemianopia and severe hemiparesis.

Development and Implementation of a New Telerehabilitation System for Audiovisual Stimulation Training in Hemianopia.

Telerehabilitation, defined as the method by which communication technologies are used to provide remote rehabilitation, although still underused, could be as efficient and effective as the conventional clinical rehabilitation practices. In the literature, there are descriptions of the use of telerehabilitation in adult patients with various diseases, whereas it is seldom used in clinical practice with child and adolescent patients. We have developed a new audiovisual telerehabilitation (AVT) system, based on the multisensory capabilities of the human brain, to provide a new tool for adults and children with visual field defects in order to improve ocular movements toward the blind hemifield. The apparatus consists of a semicircular structure in which visual and acoustic stimuli are positioned. A camera is integrated into the mechanical structure in the center of the panel to control eye and head movements. Patients can use this training system with a customized software on a tablet. From hospital, the therapist has complete control over the training process, and the results of the training sessions are automatically available within a few minutes on the hospital website. In this paper, we report the AVT system protocol and the preliminary results on its use by three adult patients. All three showed improvements in visual detection abilities with long-term effects. In the future, we will test this apparatus with children and their families. Since interventions for impairments in the visual field have a substantial cost for individuals and for the welfare system, we expect that our research could have a profound socio-economic impact avoiding prolonged and intensive hospital stays.