Reliability in reporting perceptual experience: Behaviour and electrophysiology in hemianopic patients

Patients with hemianopia can present with the so called blindsight phenomenon: the ability to perform above chance in the absence of acknowledged awareness. Proper awareness reports are, thus, crucial to distinguish pure forms of blindsight from forms of conscious, yet degraded, vision. It has, in fact, been recently shown that 1) dichotomous and graded measures to assess awareness can lead to different behavioural results in patients with hemianopia and that 2) different grades of perceptual clarity show different electrophysiological correlates in healthy participants. Here, in hemianopic patients, we assessed awareness by means of the four-point Perceptual Awareness Scale (PAS) and investigated its neural correlates with Event Related Potentials (ERPs). Results showed that patients, in most of the cases, can rate the clarity of their perceptual experience in a graded manner. Moreover, graded perceptual experiences correlated with the amplitude of deflections in ERPs. These results call for the need to assess perceptual awareness with graded measures and for the importance to use electrophysiological data to correlate behaviour with neural processing.

Neurorehabilitation of saccadic ocular movement in a patient with a homonymous hemianopia postgeniculate caused by an arteriovenous malformation: A Case Report

RATIONALE

Visual therapy, which includes a restorative and compensatory approach, seems to be a viable treatment option for homonymous defects of the visual field in patients with postgeniculate injury of the visual pathway, due to occipital arteriovenous malformation (AVM). Until now, the Mexican population suffering from homonymous hemianopia did not have health services that provided any type of visual therapy for their condition.

PATIENT CONCERNS

A 31-year-old patient, who underwent a surgical procedure for resection of the AVM, was referred with posterior low vision on the left side.

DIAGNOSES

The patient was diagnosed with left homonymous hemianopia.

INTERVENTIONS

Visual neurorehabilitation therapy (NRT), which integrated restorative and compensatory approaches, was administered for 3 hours each week. NRT included fixation, follow-up, search, peripheral vision, and reading.

OUTCOMES

The NRT did not change visual field defects and, retinotopocally, the same campimetric defects remained. However, after training the tracking ocular movements improved to standard values on the ENG, further, the visual search became more organized. The reading reached a level without mistakes, with rhythm and goog intonation. The Beck test demostrated an improvement in depression symptoms. Regarding the daily life activities, the patient reported significant improvements.

LESSONS

Visual NRT can significantly improve eye movements, as well as the quality of life and independence of the patient. This integral approach could be an effective therapeutic option for homonymous defects of the visual field.

An atypical presentation of functional visual loss: A case report

RATIONALE

Functional visual loss (FVL) can manifest as various symptoms. Decreased distant visual acuity is the most common symptom and visual field defect is the second most common symptom. Hemianopsia is rarely reported. In an atypical situation of FVL, it is important to prove that no organic pathology exists, through detailed history taking and appropriate examinations.

PATIENT CONCERNS

This review presents the case of a 48-year-old male patient presented with decreased bilateral visual acuity and visual field defect after a traffic accident 3 weeks ago. Visual field test showed atypical features of FVL in which visual field change from binasal hemianopsia to left homonymous hemianopsia.

DIAGNOSIS

The best corrected visual acuities (BCVA) were 20/63 in both eyes and binasal hemianopsia was observed on a Humphrey visual field test. Brain computed tomography (CT) scan and magnetic resonance imaging (MRI) showed no abnormalities in the brain and optic chiasm. Two weeks after presentation, however, the patient's visual field defect changed from binasal hemianopsia to left homonymous hemianopsia. We diagnosed it as FVL due to conversion disorder.

INTERVENTION AND OUTCOMES

We decided to cooperate with a psychiatrist for cognitive behavioral therapy and the patient is under observation.

LESSONS

Binasal hemianopsia and homonymous hemianopsia are rare; however, it may occur simultaneously in 1 patient with FVL. The possibility of FVL should be considered when there is atypical visual field defect and no organic abnormalities are observed. Repeated Humphrey field test and VEP may be helpful in diagnosis of FVL.

Neural Substrates of Blindsight After Hemispherectomy

Blindsight is a visual phenomenon whereby hemianopic patients are able to process visual information in their blind visual field without awareness. Previous research demonstrating the existence of blindsight in hemianopic patients has been criticized for the nature of the paradigms used, for the presence of methodological artifacts, and for the possibility that spared islands of visual cortex may have sustained the phenomenon because the patients generally had small circumscribed lesions. To respond to these criticisms, the authors have been investigating for several years now residual visual abilities in the blind field of hemispherectomized patients in whom a whole cerebral hemisphere has been removed or disconnected from the rest of the brain. These patients have offered a unique opportunity to establish the existence of blindsight and to investigate its underlying neuronal mechanisms because in these cases, spared islands of visual cortex cannot be evoked to explain the presence of visual abilities in the blind field. In addition, the authors have been using precise behavioral paradigms, strict control for potential methodological artifacts such as light scatter, fixation, criterion effects, and macular sparing, and they have utilized new neuroimaging techniques such as diffusion tensor imaging tractography to enhance their understanding of the phenomenon. The following article is a review of their research on the involvement of the superior colliculi in blindsight in hemispherectomized patients. NEUROSCIENTIST 13(5):506—518, 2007.

A User and Their Family's Perspective of The Use of a Low-Tech Vs A High-Tech AAC System

This qualitative case study describes a 9-year-old child, diagnosed with homonymous hemianopia, left side weakness and seizures that has been followed by Access to Communication and Technology Unit in Malta for 5 years. The child previously used a communication book and now uses an iPad as a speech generating device. A semi-structured interview was utilised with the parent to explore preference for each AAC system and the reasons for it. The impact of each AAC system on the family and on the child's communication skills, and perceived barriers in the implementation of the AACs were also explored. The child's own experience using the AAC systems was also investigated using a structured interview format. Talking Mats was used to support the child's understanding of the questions and to explore her perspectives on the two AAC systems using Yes-No responses. The parent interview was analysed thematically and represented visually using a thematic network. This was compared with child responses. Four organising themes emerged including barriers, benefits, facilitators, and expectations. Specific barriers included self-funding in order to provide the child with the best fit high-tech AAC. Perceived benefits for both AAC systems were that it increased her communicative intent. The child's mother perceived access to increased vocabulary and capacity for sentence building, operational autonomy as well as voice output as a benefit of the SGD. The child's results indicated a preference for the high-tech AAC because she found it easier to navigate than the low-tech AAC.

Pathways involved in human conscious vision contribute to obstacle-avoidance behaviour

Human patients with visual field defects following damage to their primary visual cortex (V1) will often misperceive the midpoint of a horizontal line. They tend to shift the midpoint away from the real position towards their blind field. In patients with unilateral neglect, where midpoint shifts can also be observed, these perceptual errors do not lead to errors in an obstacle-avoidance task, which also requires the ability to find the midpoint between two obstacles. This dissociation in neglect patients was taken as evidence that obstacle-avoidance performance is guided by visual information from the dorsal visual stream. Recently it was shown that a patient with hemianopia could avoid an obstacle presented in his blind field. This suggests that obstacle-avoidance behaviour can be guided by subconscious vision alone involving a direct route from extrageniculate structures in the brain to dorsal stream areas. To investigate whether obstacle avoidance relies only on this subconscious route or also uses information from pathways involved in conscious vision, we examined the effect of the hemianopic shift on obstacle-avoidance behaviour. This shift is found in tasks where a conscious visual judgement is required and presumably arises in pathways underlying conscious vision (V1 and ventral stream areas). We compared the performance of six patients with left hemianopia with the performance of six patients with right hemianopia. We found a clear bias in both groups, which also affected obstacle-avoidance performance. It is thus concluded that obstacle avoidance does not bypass the system for conscious vision completely.

Neuropsychology of acute stroke

Neuropsychology includes both the psychiatric manifestations of neurological illness (primary brain-based disorders) and neurobiology of "idiopathic" psychiatric disorders. Neurological primary brain disorders provoke broad spectrum of brain pathophysiology that cause deficit sin human behaviour, and the magnitude of neurobehavioral-related problems is a world wide health concern. Speech disorders of aphasic type, unilateral neglect, anosognosia (deficit disorders), delirium and mood disorders (productive disorders) in urgent neurology, first of all in acute phase of stroke are more frequent disorders then it verified in routine exam, not only in the developed and large neurological departments. Aphasia is common consequence of left hemispheric lesion and most common neuropsychological consequence of stroke, with prevalence of one third of all stroke patients in acute phase although exist reports on greater frequency. Unilateral neglect is a disorder that mostly effects the patient after the lesion of the right hemisphere, mostly caused by a cerebrovascular insult (infarct or haemorrhage affecting a large area - up to two thirds of the right hemisphere), and in general the left-side neglect is the most widespread neuropsychological deficit after the lesion of the right cerebral hemisphere. Reports on the incidence of visual neglect vary and they range from 13 to 85%. Anosognosia is on the second place as neuropsychological syndrome of stroke in right hemisphere, characterized by the denial of the motor, visual or cognitive deficit. This syndrome, defined as denial of hemiparesis or hemianopsia, is a common disorder verified in 17-28% of all patents with acute brain stoke. There are different reports on frequency of delirium in acute stroke, from 24 to 48%, and it is more frequent in hemorrhagic then ischemic stoke. Post stroke depression (PSD) is one of the more frequent consequences on the stroke, and the prevalence of PSD has ranged from 5 to 63% of patients in several cross-sectional studies, peaking three to six months after a stroke.

Linking physiology with behaviour: Functional specialisation of the visual field is reflected in gaze patterns during visual search

Based on neurophysiological findings and a grid to score binocular visual field function, two hypotheses concerning the spatial distribution of fixations during visual search were tested and confirmed in healthy participants and patients with homonymous visual field defects. Both groups showed significant biases of fixations and viewing time towards the centre of the screen and the upper screen half. Patients displayed a third bias towards the side of their field defect, which represents oculomotor compensation. Moreover, significant correlations between the extent of these three biases and search performance were found. Our findings suggest a new, more dynamic view of how functional specialisation of the visual field influences behaviour.

Charles Bonnet syndrome in hemianopia, following antero-mesial temporal lobectomy for drug-resistant epilepsy

Charles Bonnet syndrome (CBS) is a disorder characterized by the occurrence of complex visual hallucinations in patients with acquired impairment of vision and without psychiatric disorders. In spite of the high incidence of visual field defects following antero-mesial temporal lobectomy for refractory temporal lobe epilepsy, reports of CBS in patients who underwent this surgical procedure are surprisingly rare. We describe a patient operated on for drug-resistant epilepsy. As a result of left antero-mesial temporal resection, she presented right homonymous hemianopia. A few days after surgery, she started complaining of visual hallucinations, such as static or moving "Lilliputian" human figures, or countryside scenes, restricted to the hemianopic field. The patient was fully aware of their fictitious nature. These disturbances disappeared progressively over a few weeks. The incidence of CBS associated with visual field defects following epilepsy surgery might be underestimated. Patients with post-surgical CBS should be reassured that it is not an epileptic phenomenon, and that it has a benign, self-limiting, course which does not usually require treatment.

Rightward and leftward bisection biases in spatial neglect: two sides of the same coin?

Neglect patients, when asked to bisect a horizontal line, typically show large rightward errors with long lines and a decreased error with medium length lines. With very short lines the bisection bias reverses from the right to left side of the line physical centre (the so-called crossover effect). It is commonly pointed out that such a leftward bias is difficult to explain by traditional theories of neglect. Several accounts propose two distinct mechanisms, one that works for short lines and one that works for long. In the present study we demonstrated that the crossover effect can be explained by means of a unitary mechanism that derives from the space anisometry hypothesis. This hypothesis postulates that in neglect patients representational space is progressively 'relaxed' contralesionally and progressively 'compressed' ipsilesionally. In a series of five experiments, we investigated the crossover effect in 26 right-brain damaged patients: 17 with neglect without hemianopia, 4 with neglect and hemianopia and 6 without neglect or hemianopia. Patients were to bisect or extend lines of objectively and subjectively different lengths. The modulation of subjective length was created by an Oppel-Kundt illusion that is thought to resemble the distortion of representational space that occurs with neglect. All groups, except for the patients with neglect and hemianopia, were prone to the illusion. The rightward bias was reduced when the illusion induced a perceptual distortion opposite to that thought to underlie neglect. Importantly, the strength of the illusion decreased with reducing the physical line length and reversed with very short lines. These results argue for a simple and unitary explanation of the crossover effect in spatial neglect within the framework of the space anisometry hypothesis.

Delayed neuropsychiatric syndrome in a child following carbon monoxide poisoning

Here, we report the case of a five-year-old boy with carbonic monoxide (CO) poisoning. The patient initially recovered after the initiation of hyperbaric oxygen (HBO) therapy, but lethargy as well as visual and gait disturbances appeared two days later. Left hemiparesis and mood lability also subsequently appeared. Slow frontal activity was noted on electroencephalography, while fluid-attenuation inversion recovery and diffusion-weighted magnetic resonance imaging (MRI) revealed high signal-intensity lesions in the hippocampus and deeper layers of the occipital and frontal cerebral cortex. The neurological symptoms subsided gradually during the 10-day course of HBO therapy, but the left-hand paresis and quadrantic hemianopsia persisted, in association with impaired attention, slow mental processing, and incontinence. Lesions in the globus pallidum were noted on follow-up MRI at 14 days, and cortical lesions became evident as linear, low signal-intensity areas on T1-weighted imaging 4 months after presentation. Delayed neuropsychiatric syndrome in CO poisoning is rare in childhood, although children should be carefully monitored after CO exposure. The finding of cortical laminar necrosis in this patient is quite atypical in CO poisoning, and suggests a broader and previously nonpredicted pathomechanism in this condition.

Lateralized Visual Hallucinations: An Analysis of Affective Valence

Visual formaesthesias (hallucinations) are a common symptom of neurological insult. Research on emotional valances and the laterality of emotional valence extends to clinical populations experiencing visual hallucinations. The purpose of this project was to evaluate the relationship of reported hemispace of visual formaesthesia and associated emotional valence. Based on current literature exploring asymmetries in the processing of emotional valence, it was predicted that right visual formaesthesias would be predominantly associated with positive emotion and that left visual formaesthesias would largely be associated with negative emotion. Review of archival data on 150 neuropsychological patients from a rehabilitation unit in a tertiary care regional medical center resulted in 25 men and women, ages 25 to 93, endorsing visual formaesthesia upon neuropsychological interview. Analysis of variance results indicate the associated affective valence of the visual formaesthesia is a function of location, F(1, 24) = 8.33, p < .008. Additionally, the majority (84%) of sensory deficits detected were left-sided, regardless of the location of the formaesthesia. Patients specifically evidencing visual and tactile deficits had essentially exclusive left-sided deficits regardless of the location of the formaesthesia.

Visual search improvement in hemianopic patients after audio-visual stimulation

One of the most effective techniques in the rehabilitation of visual field defects is based on implementation of oculomotor strategies to compensate for visual field loss. In the present study we develop a new rehabilitation approach based on the audio-visual stimulation of the visual field. Since it has been demonstrated that audio-visual interaction in multisensory neurons can improve temporally visual perception in patients with hemianopia, the aim of the present study was to verify whether a systematic audio-visual stimulation might induce a long-lasting amelioration of visual field disorders. Eight patients with chronic visual field defects were trained to detect the presence of visual targets. During the training, the visual stimulus could be presented alone, i.e. unimodal condition, or together with an acoustic stimulus, i.e. crossmodal conditions. In the crossmodal conditions, the spatial disparity between the visual and the acoustic stimuli were systematically varied (0, 16 and 32 degrees of disparity). Furthermore, the temporal interval between the acoustic stimulus and the visual target in the crossmodal conditions was gradually reduced from 500 to 0 ms. Patients underwent the treatment for 4 h daily, over a period of nearly 2 weeks. The results showed a progressive improvement of visual detections during the training and an improvement of visual oculomotor exploration that allowed patients to efficiently compensate for the loss of vision. More interesting, there was a transfer of treatment gains to functional measures assessing visual field exploration and to daily-life activities, which was found stable at the 1 month follow-up control session. These findings are very promising with respect to the possibility of taking advantage of human multisensory capabilities to recover from unimodal sensory impairments.

Causes of cross-over in unilateral neglect: between-group comparisons, within-patient dissociations and eye movements

Patients with left unilateral neglect bisect long horizontal lines to the right of the true centre. However, when given short lines, many of the same patients mark the midpoint to the left of the true centre, towards the otherwise neglected space. This paradoxical phenomenon has been termed 'cross-over' and is difficult to explain based on current accounts of the neglect syndrome. To explore the causes of cross-over, in a first study we evaluated bisection of 20, 100 and 200 mm horizontal lines in groups of unilateral brain-damaged patients with neglect and hemianopia, with neglect and no hemianopia, with hemianopia and no neglect and without neglect or hemianopia. Cross-over of 20 mm lines was found only in neglect patients with hemianopia. To ascertain further the influence of visual field defects on cross-over, in a second study we compared the performance of two right-brain-damaged patients with contralesional neglect and inferior quadrantanopia with that of a patient with inferior quadrantanopia and no neglect. Patients bisected lines oriented so as to cross or uncross the blind quadrant of the visual field. When short 20 mm lines crossed the blind quadrant, neglect patients showed cross-over; when the same lines crossed the seeing quadrants cross-over was absent. These findings were confirmed by the examination of a neglect patient with sparing of the central 5 degrees of the contralesional left visual hemifield in the right eye and no sparing in the left eye. In monocular viewing, cross-over was present when 20 mm lines were bisected with the left eye and absent when bisected with the right eye. Recording of eye movements showed that at the moment of bisection left eye fixations shifted towards the contralesional line endpoint whereas right eye fixations remained anchored to the centre of the line. With long lines, both eyes deviated ipsilesionally. These results show that in neglect patients ipsilesional deviation in the bisection of long lines turns into apparently paradoxical contralesional bisection of short ones only when these cross a retinotopically blind sector of the neglected space. Cross-over seems to depend on the small spatial effects produced by reflexive contralesional gaze shifts allowing eccentric fixations with the seeing hemifield. During the bisection of long lines, these effects are cancelled out by the strong attentional deviation induced by the marked extension of the ipsilesional line segment. This explanation establishes coherence between cross-over and current accounts of the neglect syndrome.

Impaired Spatial Working Memory: One Component of the Visual Neglect Syndrome?

Both impaired spatial working memory (SWM) and unilateral neglect may follow damage to the right parietal lobe. We propose that impaired SWM can exacerbate visual neglect, due to failures in remembering locations that have already been searched. When combined with an attentional bias to the ipsilesional right side, such a SWM impairment should induce recursive search of ipsilesional locations. Here we studied a left neglect patient with a right temporoparietal haemorrhage. On a nonlateralised, purely vertical SWM task, he was impaired in retaining spatial locations. In a visual search task, his eye position was monitored while his spatial memory was probed. He recursively searched through right stimuli, re-fixating previously inspected items, and critically treated them as if they were new discoveries, consistent with the SWM deficit. When his recovery was tracked over several months, his SWM deficit and left neglect showed concurrent improvements. We argue that impaired SWM may be one important component of the visual neglect syndrome.

Stimulus cueing in blindsight

When visual stimuli are presented in the cortically blind visual field of patients or monkeys with verified destruction of striate cortex, many subjects can voluntarily respond to them. In studies of this blindsight, the on- and/or offset of the visual stimulus is usually known to the subject, either because it is signaled in some way or because the subject can present the stimulus himself. To study the effect of stimulus uncertainty on the responses of four hemianopic monkeys and one human hemianope, we compared trials on which the subjects themselves could instantly trigger the stimulus with trials on which the same stimulus appeared 1-7 s after the start-light that normally served as the trigger was first touched. The latter manipulation diminished both the percentage of trials on which the subjects responded and the percentage correct when they did respond. As the start-light disappeared when touched in the first but not second condition, we interpret our results as indicating an influential role for attention in blindsight. Although keeping attention focused on the start-light and delaying the target impaired performance especially in the monkeys, localization was still significant in three and hardly affected in GY.

[Unawareness for homonymous visual field defect]

BACKGROUND

There was no report which dealt with the relationship between emotional state, degree of defective visual search, severity of hemianopic dyslexia, the episode when the patient became aware of the defect, and unawareness of visual loss in homonymous hemifield.

OBJECTIVE

To investigate the relationship between degree of awareness and those factors that might be responsible for the unawareness, including the aspects listed above.

METHODS

Four patients with visual field defects caused by a brain lesion after a stroke was investigated. Self rating of emotional state, search performance for an object among many placed on a table, and for text reading, as well as visual field, visual positive phenomena, and hemispatial neglect were evaluated. Degree of unawareness for field loss was evaluated by modified version of the method of Bisiach et al. (1985). In addition, the episodes when the patient became aware of the defect were asked.

RESULT

In accordance with the previous studies, we found no relationship between the degree of awareness of field defect and anatomic lesions, co-existence of hemispatial neglect, or the degree of awareness of hemiplegia. However, the patient with neglect was unaware of their troubles in vision at all, whereas the patients without neglect were aware of the troubles but misinterpreted them as problems of the eyes including acuity. In accordance with previous studies, co-existence of visual hallucinations or illusions seemed to be associated with awareness of visual field defect. No relationship was found between the degree of awareness of field defect and emotional state, degree of field loss, degree of defective visual search, or severity of hemianopic dyslexia. Their responses to the inquiry about the degree of awareness of field defect were not consistent. Thus, the awareness of the field defect seemed to be difficult to be kept firmly in their mind. On the other hand, the patients could remember the episode when they became aware of the defect for the first time, being able to specify time, place, and situation.

CONCLUSION

Levine (1990) suggested that the sensory loss in this sort of patients was never phenomenally immediate but instead must be discovered by observation and inference. Non-specificity of the lesion, qualitative difference in awareness between the patient with and without hemispatial neglect, association of positive visual phenomena and awareness, fluctuation of awareness, and dependence of awareness on personal experiences found in our patients, can be explained with this 'discovery' hypothesis.

Principles of cross-modal competition: evidence from deficits of attention

How does the attentional system coordinate the processing of stimuli presented simultaneously to different sensory modalities? We investigated this question with individuals with neurological damage who suffered from deficits of attention. In these individuals, we examined how the processing of tactile stimuli is affected by the simultaneous presentation of visual or auditory stimuli. The investigation demonstrated that two stimuli from different modalities are in competition when attention is directed to the perceptual attributes of both, but not when attention is directed to the perceptual attributes of one and the semantic attributes of the other. These findings reveal a differentiated attentional system in which competition is modulated by the level of stimulus representation to which attention is directed.

Pure topographical disorientation--the anatomical basis of landmark agnosia

We used MRI studies of four patients to investigate the lesions responsible for landmark agnosia. A detailed investigation of the relationship between the symptoms and the lesions suggests that the right posterior part of the parahippocampal gyrus is critical for the acquisition of novel information about buildings and landscapes, and that the same region plus the anterior half of the lingual gyrus and the adjacent fusiform gyrus play an important role in the identification of familiar buildings and landscapes. Furthermore, the lesion responsible for prosopagnosia, which frequently occurs with landmark agnosia, seems to involve the posterior half of the lingual and fusiform gyri. This suggests that the lesions responsible for landmark agnosia and prosopagnosia are close to each other, but distinct.

Fast responses to neglected targets in visual search reflect pre-attentive processes: an exploration of response times in visual neglect

AE is a patient who suffered a right hemisphere stroke resulting in visual neglect symptoms. In the first experiment, AE neglected a single visual target that was present in half of the trials and appeared in variable and unpredictable positions on the computer screen. The contrast of the target to the screen's background was also varied. AE demonstrated severe neglect for left-sided targets, and yet his RTs to targets reported incorrectly as absent were faster than correct rejections and even right-sided hits. AEs fast "neglect" responses seem to indicate that the target was detected but that he remained unaware of its presence. Counter intuitively, his fast misses got faster as the discriminability of the target decreased. The possibility that fast responses to neglected targets reflected a guessing strategy, used proportionally to the degree of uncertainty of a target presence, was examined. AEs fast misses were indeed faster at lower level of contrast of the stimulus, but his error rate did not tend to approach the chance level as the guessing model would predict. In a second experiment, AE searched for the letter Z, present on half of the trials, among variable sets of distractor letters. In one condition the distractors were all O's and therefore differed from the target by an elementary feature. In the other condition, the distractors were various letters that differed from the target by combinations of features. The key finding was that fast responses to neglected targets occurred only in the simple feature search task and not in the complex features (conjunction) task. We interpret these findings as indicating that AEs pre-attentive processing can detect pop-out targets on the left-hand side, but that the attentional search is faulty and is aborted early. Hence, the patient's attentional system has an "early start" when "pop-out" forms are present, but can also fail to "grab" the detected target; consequently, by not attending to a stimulus, the patient remains unaware of its presence and will quickly respond "no" to present targets.

Task-dependent differences in the exploratory behaviour of patients with spatial neglect

The present study analysed task-dependent effects on the exploratory behaviour of neglect patients during their spontaneous search of the surroundings. We were asking whether different tasks would be associated with different structuring of the visual display and, therefore, would result in different forms of neglect in one and the same brain-damaged subjects. Neglect patients' eye and head movements were recorded when they searched for a target within a homogeneous stimulus array surrounding the subjects. Subsequently, they explored the same array which was now segmented into different areas. When the patients' attention was allocated to the whole surrounding space, all patients completely neglected the left hemispace and spontaneously attended to the right hemispace. No significant left-right asymmetry was detected in a selected segment located in the periphery of the attended, right hemispace. However, all patients completely ignored the left part of this segment when they had to concentrate visual search on this segment alone. The results suggest an important influence of task-dependent effects on the exploratory behaviour of neglect patients. They show that one and the same physical stimulus at one and the same location in a scene might be attended or, in another situation, neglected, just depending on the behavioural goal of the subject. The findings support the idea that the brain organises and reorganises continuously the representation of the same physical input according to the changing task requirements.

Aware or unaware: assessment of cortical blindness in four men and a monkey

In four patients and one monkey with unilateral visual field defects caused by retro-geniculate lesions we measured forced-choice localization of square-wave gratings as a function of contrast, and compared results from the patients' absolutely and relatively blind fields. In addition, the patients indicated verbally whether they were aware of the stimuli. We then switched to a signal detection task in which the subjects had to signal a stimulus as in the localization task, by touching it, no matter whether it appeared in the good or bad hemifield, and in addition to signal a blank trial by touching an outlined square now constantly present on the monitor, and designated the no-stimulus response area. In this way, we could compare a non-verbal procedure that we had previously used in hemianopic monkeys with a verbal one commonly used to assess visual awareness. The results showed a close correspondence between the two measures of awareness in the human subjects who signalled 'stimulus' only for targets that also evoked verbal aware responses, validating the non-verbal approach. The hemianopic monkey behaved more like a patient with an absolute rather than a relative defect, and perfectly localized high-contrast stimuli which she nevertheless treated as blanks in the vast majority of presentations in the signal detection task.

'Seeing' in the blind hemifield following hemispherectomy

Destruction of the striate cortex has traditionally been thought to lead to permanent blindness in the contralateral visual field and to the dogma that this region is indispensable for vision in primates. For over 25 years now, evidence has been accumulating that hemianopic human subjects and monkeys possess wide-ranging residual visual capacities or 'blindsight' in the blind part of their visual field. For some researchers, isolated islands of the striate cortex have been associated with patches of degraded vision and made responsible for blindisight. Artefacts such as light scatter, criterion effects, macular sparing, eccentric fixation and minute eye movements have also been linked with the residual vision. For others, the fact that certain aspects of the visual information can be processed without the geniculostriate pathway suggests mediation by the visual subsystems such as extrastriate visual cortical areas which receive visual information via subcortical pathways, that escaped the cortical damage. Subjects who have had a whole cerebral hemisphere removed or disconnected (for the treatment of uncontrollable epilepsy) and who show residual vision in their blind field offer a remarkable opportunity to help clear the controversy regarding the neural substrate of blindsight. Because it is certain that no functional striate or extrastriate cortex remains on the ablated side, these subjects have contributed significantly to identifying the critical pathways involved in blindsight.

Visual imagery in hemianopic patients

In this article we report some findings about visual imagery in patients with stable homonymous hemianopia compared to healthy control subjects. These findings were obtained by analyzing the gaze control through recording of eye movements in different phases of viewing and imagery. We used six different visual stimuli for the consecutive viewing and imagery phases. With infrared oculography, we recorded eye movements during this presentation phase and in three subsequent imagery phases in absence of the stimulus. Analyzing the basic parameters of the gaze sequences (known as "scanpaths"), we discovered distinct characteristics of the "viewing scanpaths" and the "imagery scanpaths" in both groups, which suggests a reduced extent of the image within the cognitive representation. We applied different similarity measures (string/vector string editing, Markov analysis). We found a "progressive consistency of imagery," shown through raising similarity values for the comparison of the late imagery scanpaths. This result suggests a strong top-down component in picture exploration: In both groups, healthy subjects and hemianopic patients, a mental model of the viewed picture must evolve very soon and substantially determine the eye movements. As our hemianopic patients showed analogous results to the normal subjects, we conclude that these patients are well adjusted to their deficit and, despite their perceptual defect, have a preserved cognitive representation, which follows the same top-down vision strategies in the process of visual imagery.

Word-centred neglect dyslexia: evidence from a new case

Neglect dyslexia resulting from damage to word-centred representations is extremely rare. We report on a new case. A left-handed subject, SVE, presented with aphasia and neglect dyslexia/dysgraphia following a right hemisphere stroke. In tachistoscopic reading tasks, some of his errors resulted from retina-centred neglect, as he responded more accurately to words flashed in the left visual field than to words flashed in the right visual field. However, the critical aspects of his reading performance indicated word-centred neglect. SVE incorrectly produced the initial elements of four-letter words, regardless of stimulus location (to the left and to the right of fixation, or at fixation), and orientation (horizontal and vertical presentation). A similar distribution of errors was demonstrated in writing (very inaccurate performance on initial letters). This pattern of performance suggests damage to an abstract letter string representation defined by spatial coordinates, rather than to an ordering mechanism. It is most naturally accommodated by models of word recognition which assume a word-centred level of representation, and cannot be explained by models which do not include such a representational level. Consideration of our subject in the light of other similar reports prompts hypotheses on the neural mechanisms involved in computing word-centred representations.

Computer-based training of stimulus detection improves color and simple pattern recognition in the defective field of hemianopic subjects

In a previously conducted randomized placebo-controlled trial, we were able to demonstrate significant visual field enlargement induced by restitution therapy in patients with cerebral lesions [Kasten, E., Wuest, S., Behrens-Bamann, W., & Sabel, B. A. (1998c). Computer-based training for the treatment of partial blindness. Nature Medicine, 4, 1083-1087.]. Visual field training was performed on a computer monitor for 1 hr per day over a period of 6 months. Since the procedure included only stimulation with white light, in the present study we investigated if this simple detection training had a transfer effect on color or form recognition in the trained area (i.e., in the absence of modality specific training). Answering this question would be crucial for planning optimal restitution therapy: In case there is no transfer of training effects to other visual modalities, a specific treatment of each visual function must be performed in order to achieve maximum benefit. Therefore, we analyzed the data from 32 patients with visual field defects who had participated in the original trial and whose form and color recognition had been investigated. The experimental group (n = 19, restitution training) experienced not only an increase of 12.8% correctly detected stimuli (PeriMa program, p <.05), but also an improvement of 5.6% in pattern recognition (PeriForm) and of 6.1% in color perception (PeriColor), respectively. In contrast, the placebo group (n = 13, fixation training) showed no significant changes from baseline to final outcome in any of the visual modalities (PeriMa: 0.3%; PeriForm: -0.3%; PeriColor: 0.4%). Conventional perimetry yielded an increase of 7.8% detected stimuli in the experimental group, but only of 1.2% in the placebo group (p <.05). For form recognition and color perception, the differences between the results of the experimental and the placebo groups narrowly missed significance. However, correlations of diagnostic results showed that mainly those patients who had achieved visual field enlargement also improved in color and form perception: r =.67 (p <.05) between PeriMa and PeriForm and r =.32 between PeriMa and PeriColor. We conclude that visual restitution training using a simple white light stimulus has at least some influence on improving other visual functions such as color and pattern recognition. This result supports the "bottleneck theory" of visual restitution, i.e., training effects can be explained as a process of perceptual learning and increased processing of information by residual structures surviving lesions of the primary visual pathways.

Variability not ability: another basis for performance decrements in neglect

OBJECTIVE

To determine reaction time (RT) and its variability, as a function of horizontal spatial position, in subjects with neglect.

BACKGROUND

In neglect, performance is frequently reported a as mean and a decreased ability to perform the task inferred by comparison to control groups. Few studies have examined how consistency and optimal performance relate to spatial neglect.

METHODS

Ten subjects with brain damage, five with and five without spatial neglect, were assessed on a RT task. Subjects responded by pushing a computer key to the onset of a white square appearing on a black screen. The locations of stimuli were randomly varied along the horizontal meridian.

RESULTS

For three of five neglect subjects, optimal RT showed no or little relation to horizontal location. Four of five neglect subjects demonstrated an increased variability in RT that correlated with spatial position and which was not present in our brain damaged subjects without neglect. The relationship was not an artifact of left sided stimuli, in general, being processed differently. For the two neglect subjects with the most trials, a significant correlation between RT variability and spatial position existed for left-sided trials alone. Increased variability was not a consequence of simply looking left proportionately less often, nor could a model of multiple compensatory systems operating in parallel explain the enhanced variability. Neither hemianopsia alone nor brain damage per se could account for the spatial modulation of RT variability.

CONCLUSIONS

That neglect subjects perform the RT task normally on some trials, even in their 'neglected field', challenges the notion that neglect must reflect an irreparably damaged cognitive system. Performance decrements in neglect can reflect an inability to consistently detect and respond. Evaluating optimal performance and variability of performance can indicate if a capacity has been lost absolutely or merely degraded such that normal performance cannot be sustained.

Non-conscious recognition of affect in the absence of striate cortex

Functional neuroimaging experiments have shown that recognition of emotional expressions does not depend on awareness of visual stimuli and that unseen fear stimuli can activate the amygdala via a colliculopulvinar pathway. Perception of emotional expressions in the absence of awareness in normal subjects has some similarities with the unconscious recognition of visual stimuli which is well documented in patients with striate cortex lesions (blindsight). Presumably in these patients residual vision engages alternative extra-striate routes such as the superior colliculus and pulvinar. Against this background, we conjectured that a blindsight subject (GY) might recognize facial expressions presented in his blind field. The present study now provides direct evidence for this claim.

Visual perception of motion, luminance and colour in a human hemianope

Human patients rendered cortically blind by lesions to V1 can nevertheless discriminate between visual stimuli presented to their blind fields. Experimental evidence suggests that two response modes are involved. Patients are either unaware or aware of the visual stimuli, which they are able to discriminate. However, under both conditions patients insist that they do not see. We investigate the fundamental difference between percepts derived for the normal and affected hemifield in a human hemianope with visual stimuli of which he was aware. The psychophysical experiments we employed required the patient, GY, to make comparisons between stimuli presented in his affected and normal hemifields. The subject discriminated between, and was allowed to match, the stimuli. Our study reveals that the stimulus parameters of colour and motion can be discriminated and matched between the normal and blind hemifields, whereas brightness cannot. We provide evidence for associations between the percepts of colour and motion, but a dissociation between the percepts of brightness, derived from the normal and hemianopic fields. Our results are consistent with the proposal that the perception of different stimulus attributes is expressed in activity of functionally segregated visual areas of the brain. We also believe our results explain the patient's insistence that he does not see stimuli, but can discriminate between them with awareness.

Visual search and visual target detection in patients with infarctions of the left or right posterior or the right middle brain artery

We compared visual target detection and search performance of patient groups with infarctions of (1) the right middle brain artery (R-MBA) and with neglect; (2) the left posterior artery (L-PBA), (3) the right posterior artery (R-PBA), the latter two groups had contralesional hemianopias, or (4) with right hemisphere lesions without hemianopia or neglect. We found that: (1) The first three groups differed from the fourth (control) group in omissions. (2) The first three groups differed only in horizontal search but not in target detection. (3) No vertical search deficit was present for either group. (4) R-MBA patients found increasingly more targets in visual search from left to right, R-PBA patients had problems with the outermost contralesional column, L-PBA patients showed a generally slowed and more variable search pattern. Infarctions of left and R-PBA therefore resulted in different visual search patterns. The behavior of the patients with R-MBA is consistent with Kinsbourne's (1992) interactive inhibition theory of neglect.

To see better to the left when looking more to the right: effects of gaze direction and frames of spatial coordinates in unilateral neglect

Unilateral spatial neglect entails a failure to detect or respond to stimuli in the space opposite to a brain lesion. However, the contralesional hemispace can be determined by different frames of spatial coordinates, such as eyes-, head-, body-, or environment-centered coordinates. We observed 2 patients with a right hemisphere stroke whose left spatial neglect was modulated by distinct coordinates systems depending on the task. Four tasks were given in different conditions of central gaze and either the eyes or the head rotated 30 degrees to the right or 30 degrees to the left. While the 2 patients had a retinotopic defect in 1 visual field quadrant that remained the same irrespective of gaze direction (upper or lower quadrant in 1 case each), the other quadranopic field defect improved with eyes rotation to the right but not with head rotation, suggesting a head-centered spatiotopic deficit. Performance on line bisection was influenced both by eyes and head rotation, as well as by the position of the lines with respect to the trunk midline, suggesting the involvement of both head-centered and body-centered coordinates. Visual imagery and auditory extinction were not modified by changing the eyes or head position. These findings suggest that distinct spatial coordinates are brought into play depending on the tasks demands.

Dissociation between recency and span: neuropsychological and experimental evidence

This article reports dissociations between verbal span and the recency portion of the serial position curve in immediate free recall, in 2 neuropsychological case studies and in 3 experiments with normal participants. Patient A. N. presented with an impaired serial verbal span while showing an intact recency effect. The opposite pattern was observed in patient G. C., who despite a poor recency showed normal span in verbal serial recall tasks. Experiments 1 and 2 showed a recency effect with visually and auditory presented lists and written recall was resistant to the effects of articulatory suppression and of irrelevant speech, but was disrupted by the suffix effect. Experiment 3 showed that in contrast with recency, memory span was affected by articulatory suppression and irrelevant speech during presentation but not by a suffix. These findings are not consistent with the idea that span and recency measure aspects of the same memory system. Moreover, in clinical practice, they should not be used as equivalent alternatives.

Volitional control of homonymous hemianopsia: a single case study

A 52-year-old woman suffered a posterior left hemisphere infarction involving the geniculo-striate system with right homonymous hemianopsia. Eighteen months after the stroke, we used perimetry and PET imaging of cerebral blood flow to demonstrate the patient's volitional control over the size of the scotoma. The volitional control could be eliminated with competing cognitive tasks. Significant increases in cerebral blood flow in contralateral medial prefrontal cortex, and in ipsilateral dorsolateral frontal convexity, temporoparietal and insular cortex were associated with volitional control of the visual field defect.

Line bisection in hemianopia

The effect of hemianopia on line bisection is not known. To study this, manual line bisection in 30 patients with unilateral cerebral hemispheric lesions was examined. The mean bisection point in a group of eight patients with left hemineglect was biased rightward (ipsilaterally), as expected. Among the remaining 22 patients, eight had right hemianopic visual defects, eight had left hemianopic visual defects, and six had normal visual fields. Both groups of patients with contralateral visual field defects had mean bisection points biased contralaterally, compared with 68 normal subjects. This bias was less than the ipsilateral (opposite) bias of patients with hemineglect. Contralateral bisection bias was more evident in those whose field defect involved the macular region. No bias was seen in patients with neither field defects nor hemineglect. The contralateral bias in hemianopia may represent either non-veridical spatial representation within a visual hemifield or a consequence of the strategic adaptation of attention into contralateral hemispace after hemianopia.

Hemispatial neglect on visual search tasks in Alzheimer's disease

Abnormal visual attention may underlie certain visuospatial difficulties in patients with Alzheimer's disease (AD). These patients have hypometabolism and neuropathology in parietal cortex. Given the role of parietal function for visuospatial attention, patients with AD may have relative hemispatial neglect masked by other cognitive disturbances. Fifteen patients with-to-moderate AD and 15 healthy elderly controls matched for age, sex, and education were compared on four measures of neglect the visual search of a complex picture, a letter cancellation task, the Schenkenberg line bisection test, and a computerized line bisection task. Compared with controls, the group with AD was significantly impaired overall in attending to left hemispace on both picture search (F[1,56] = 11.27, p < 0.05) and cancellation tasks (F[1,112] = 12.68, p < 0.01); however, a subgroup of patients with AD had disproportionate difficulty in attending to right hemispace. The performance of the groups did not differ on either of the line bisection tasks regardless of the hand used. In AD, hemispatial neglect on visual search tasks may relate to difficulty in disengaging attention or in visual exploration, as well as to the severity of the disease. Future investigations may implicate neglect in visually related deficits in AD, for example, the prominent difficulty with left turns on driving a car.

[Hemi-neglect versus hemianopia. Differential diagnosis]

Neglect and hemianopia represent frequent disorders in brain damaged patients. Differential diagnosis of both disorders may be difficult since both may present in diagnostic tests and daily life as a failure to perceive or react to stimuli in the contralesional hemispace or hemifield. The present paper summarizes several useful techniques in order to achieve a correct distinction. After a short review of the clinical phenomenology of hemianopia and -neglect the following topics are described: (1) subjective complaints and awareness of deficits (2) aetiology and lesion localisation, (3) uni-versus multimodal deficits; (4) extinction, (5) drawing from memory; (6) visual-spatial disorders; (7) line bisection, (8) effectiveness of attentional "cueing" strategies; (9) specific perimetric techniques; (10) visual evoked potentials and eye movement registration. The differences between hemianopia and -neglect are contrasted for all these topics. Taken all these possible features together the association and/or dissociation of hemineglect and postchiasmatic scotomata can efficiently be diagnosed. The results are summarized in a table at the end of the paper.

Anatomical and neurological correlates of acute and chronic visuospatial neglect following right hemisphere stroke

Anatomical and neurological correlates of visuospatial neglect were studied in 53 patients with a CT-documented right hemisphere stroke. Evidence of neglect at the acute stage poststroke was strongly related to large lesions involving the middle temporal gyrus and/or the temporo-parietal paraventricular white matter. Thus, out of 18 patients with evidence of visuospatial neglect at the acute stage, 12 showed a lesion in the middle temporal gyrus and/or the deep temporo-parietal white matter. Among the 35 patients that failed to show visuospatial neglect, only one patient had a lesion within these areas. Comparing those patients who recovered from neglect with those that did not, a high correlation was found between persisting neglect and a lesion involving the paraventricular white matter in the temporal lobe. On the basis of above findings, it was suggested that a simultaneous damage to the cortico-thalamic system for regulation of arousal and to the neural systems mediating visual orienting, is likely to be followed by persisting neglect symptoms.

The novelty effect in recovered hemineglect

Left neglect patients, patients who had recovered from left neglect and control subjects performed a task of simple motor reaction times (RTs) to lateralised visual stimuli. Neglect and recovered patients were slower than controls on left-sided targets. To explore the time course of the allocation of attention across space, an analysis of responses as a function of the serial order of the trials was performed. While neglect patients' performance did not substantially change over time, recovered patients showed a stereotyped 'novelty effect', consisting of larger left/right RT differences at the beginning of the task than at the end of it. To explain this practice-related change, a trade-off is hypothesised between the process of learning the motor task and the mechanisms involved in recovery from neglect, such as the reorienting of attention toward the contralesional side following the initial ipsilesional orienting. A possible role is proposed for the prefrontal cortex as the crucial neural structure that mediates both processes.

Preserved imagery for colours in a patient with cerebral achromatopsia

We report the case of a patient who, after sequential bilateral strokes in the occipital regions sparing the primary visual cortex, developed a severe deficit of colour perception. At variance with other reports of acquired achromatopsic patients, she showed a perfectly vivid visual imagery for colours. These findings, together with similar data in domains other than colour processing, challenge the theories which posit that the same cognitive processes are involved in both the perception and the retrieval from memory of a given stimulus.

Are there sex differences in hemispatial visual neglect after unilateral stroke?

This study examined the hypothesis of greater functional asymmetry in the male compared with the female brain for contralesional spatial neglect. One hundred thirty-eight consecutive patients with computed tomography verified unilateral first strokes were examined within two months of onset. An aggregate measure of neglect was based on four clinical tasks: copying/drawing, line bisection, line cancellation, and figure cancellation. The incidence and severity of neglect were significantly greater after right-than left-hemisphere lesions and equal in men and women. Sex differences were not found between anterior and posterior groups after left- or right-hemisphere strokes. When neglect was based on different scores between ipsilateral versus contralesional response times on a Visual Search Task, the incidence was higher in females than males with right-hemisphere lesions.

Sustained attention and awareness of disability in chronic neglect

The possible causative role of defective sustained attention and awareness of disability on the persistence of neglect was explored. The study included stroke patients who had had moderate or severe neglect 1-5 years before the start of the present examination. Questionnaire responses showed that the patients were aware of their disability. Impaired sustained attention was associated with poor performance in two out of three tests most sensitive in detecting neglect. This, together with indications of compensation on neglect tests, is interpreted as providing support for the hypothesis that chronic neglect is related to an impaired sustained attention.