Different patterns of dissociation in unilateral spatial neglect

Recovery of Visuospatial Neglect Subtypes and Relationship to Functional Outcome Six Months After Stroke

Background/Objective. This study aims to investigate how complex visuospatial neglect behavioural phenotypes predict long-term outcomes, both in terms of neglect recovery and broader functional outcomes after 6 months post-stroke. Methods. This study presents a secondary cohort study of acute and 6-month follow-up data from 400 stroke survivors who completed the Oxford Cognitive Screen's Cancellation Task. At follow-up, patients also completed the Stroke Impact Scale questionnaire. These data were analysed to identify whether any specific combination of neglect symptoms is more likely to result in long-lasting neglect or higher levels of functional impairment, therefore warranting more targeted rehabilitation. Results. Overall, 98/142 (69%) neglect cases recovered by follow-up, and there was no significant difference in the persistence of egocentric/allocentric (X2 [1] = .66 and P = .418) or left/right neglect (X2 [2] = .781 and P = .677). Egocentric neglect was found to follow a proportional recovery pattern with all patients demonstrating a similar level of improvement over time. Conversely, allocentric neglect followed a non-proportional recovery pattern with chronic neglect patients exhibiting a slower rate of improvement than those who recovered. A multiple regression analysis revealed that the initial severity of acute allocentric, but not egocentric, neglect impairment acted as a significant predictor of poor long-term functional outcomes (F [9,300] = 4.742, P < .001 and adjusted R2 = .098). Conclusions. Our findings call for systematic neuropsychological assessment of both egocentric and allocentric neglect following stroke, as the occurrence and severity of these conditions may help predict recovery outcomes over and above stroke severity alone.

A novel computerized assessment of manual spatial exploration in unilateral spatial neglect

Unilateral spatial neglect is a neuropsychological syndrome commonly observed after stroke and defined by the inability to attend or respond to contralesional stimuli. Typically, symptoms are assessed using clinical tests that rely upon visual/perceptual abilities. However, neglect may affect high-level representations controlling attention in other modalities as well. Here we developed a novel manual exploration test using a touch screen computer to quantify spatial search behaviour without visual input. Twelve chronic stroke patients with left neglect and 27 patients without neglect (based on clinical tests) completed our task. Four of the 12 "neglect" patients exhibited clear signs of neglect on our task as compared to "non-neglect" patients and healthy controls, and six other patients (from both groups) also demonstrated signs of neglect compared to healthy controls only. While some patients made asymmetrical responses on only one task, generally, patients with the strongest neglect performed poorly on multiple tasks. This suggests that representations associated with different modalities may be affected separately, but that severe forms of neglect are more likely related to damage in a common underlying representation. Our manual exploration task is easy to administer and can be added to standard neglect screenings to better measure symptom severity.

The Fluff test: Improved scoring system to account for different degrees of contralesional and ipsilesional personal neglect in brain damaged patients

The Fluff test is a simple test to assess evidence of personal neglect (PN) in brain-damaged patients. While blindfolded, patients are asked to remove targets previously attached to their body and the number of targets detached provides information about possible spatial bias. This test has been widely used for clinical and research purposes. However, the current scoring system presents some limitations, which make difficult to interpret patients' performance in terms of both contralalesional and ipsilesional PN when they omit targets on the ipsilesional side. Moreover, it does not consider possible confounding variables, such as non-spatial cognitive deficits or lack of compliance that may affect patients' performance and lead to incorrect diagnosis. The present paper proposes a new scoring method overcoming the limitations mentioned above and it analyses data from a large sample of 243 brain-damaged patients. Findings showed that contralesional PN was significantly more severe, but not more frequent, following right (31%) than left (21%) brain damage. We also found evidence of left ipsilesional PN and cases of potential mis-diagnosis that would have passed unnoticed with the original scoring system. The new scoring method allows to identify different degrees of contralesional and ipsilesional PN and potential confounding variable.

Dissociating spatial attention from neglect dyslexia: A single case study

Word-centred neglect dyslexia is generally thought to be caused by a visuospatial neglect-like attentional deficit which impacts orientation-canonical representations of visual stimuli. However, the relationship between word-centred neglect dyslexia and more general attentional processes is not well described. Here, we investigated the impact of attentional load manipulations within a case of word-centred neglect dyslexia. This study presents data from a single case, Patient CD, who exhibited ipsilesional word-centred neglect dyslexia in conjunction with severe, contralesional allocentric neglect. CD demonstrated an intact ability to name all letters in visually presented words, but committed neglect dyslexia errors when subsequently asked to read the same word as a whole. The severity of patient CD's neglect dyslexia was not found to be impacted by attentional manipulations. We found no effect of exposure time or visual crowding on the frequency of neglect dyslexia errors. This absence of an apparent, right-lateralised perceptual deficit, comorbid left-lateralised object-centred neglect, and insensitivity to attentional load manipulations suggests that the deficit underlying word-centred neglect dyslexia is not related to broad visuo-spatial attention. These findings suggest that neglect dyslexia and domain-general visuospatial neglect may not be as related as previously asserted.

The importance of time limits in detecting signs of left visual peripersonal neglect: a multiple single-case, pilot study

We developed a new visual search test to assess signs of left visual peripersonal neglect. Five right-hemisphere-damaged patients, 10 healthy controls, and 10 orthopedic controls were administered the test in four conditions: easy task (no distractors)/time-limited (45''), easy task (no distractors)/time-unlimited, difficult task (distractors)/time-limited (45''), difficult task (distractors)/time-unlimited. With respect to controls, most RHDP showed signs of left visual peripersonal neglect in the time-limited condition, but not in the time-unlimited condition, particularly on the difficult task. We suggest that the presence of appropriate time limits, in difficult visual search tasks, could considerably improve the diagnosis of left visual peripersonal neglect.

Unilateral spatial neglect after posterior parietal damage

Unilateral spatial neglect is a disabling neurologic deficit, most frequent and severe after right-hemispheric lesions. In most patients neglect involves the left side of space, contralateral to a right-hemispheric lesion. About 50% of stroke patients exhibit neglect in the acute phase. Patients fail to orient, respond to, and report sensory events occurring in the contralateral sides of space and of the body, to explore these portions of space through movements by action effectors (eye, limbs), and to move the contralateral limbs. Neglect is a multicomponent higher-level disorder of spatial awareness, cognition, and attention. Spatial neglect may occur independently of elementary sensory and motor neurologic deficits, but it can mimic and make them more severe. Diagnostic tests include: motor exploratory target cancellation; setting the midpoint of a horizontal line (bisection), that requires the estimation of lateral extent; drawing by copy and from memory; reading, assessing neglect dyslexia; and exploring the side of the body contralateral to the lesion. Activities of daily living scales are also used. Patients are typically not aware of neglect, although they may exhibit varying degrees of awareness toward different components of the deficit. The neural correlates include lesions to the inferior parietal lobule of the posterior parietal cortex, which was long considered the unique neuropathologic correlate of neglect, to the premotor and to the dorsolateral prefrontal cortices, to the posterior superior temporal gyrus, at the temporoparietal junction, to subcortical gray nuclei (thalamus, basal ganglia), and to parietofrontal white-matter fiber tracts, such as the superior longitudinal fascicle. Damage to the inferior parietal lobule of the posterior parietal cortex is specifically associated with the mainly egocentric, perceptual, and exploratory extrapersonal, and with the personal, bodily components of neglect. Productive manifestations, such as perseveration, are not a correlate of posterior parietal cortex damage.

Semiology of neglect: An update

Hemispatial neglect is a common disabling condition following brain damage to the right hemisphere. Generally, it involves behavioral bias directed ipsilaterally to the damaged hemisphere and loss of spatial awareness for the contralesional side. In this syndrome, several clinical subtypes were identified. The objective of this article is to provide a nosological analysis of the recent data from the literature on the different subtypes of neglect (visual, auditory, somatosensory, motor, egocentric, allocentric and representational neglect), associated ipsilesional and contralesional productive manifestations and their anatomical lesion correlates. These different anatomical-clinical subtypes can be associated or dissociated. They reflect the heterogeneity of this unilateral neglect syndrome that cannot be approached or interpreted in a single manner. We propose that these subtypes result from different underlying deficits: exogenous attentional deficit (visual, auditory neglect); representational deficit (personal neglect, representational neglect, hyperschematia); shift of the egocentric reference frame (egocentric neglect); attentional deficit between objects and within objects (allocentric neglect), endogenous attentional deficit (representational neglect) and transsaccadic working memory or spatial remapping deficit (ipsilesional productive manifestations). Taking into account the different facets of the unilateral neglect syndrome should promote the development of more targeted cognitive rehabilitation protocols.

Dissociations between developmental dyslexias and attention deficits

We examine whether attention deficits underlie developmental dyslexia, or certain types of dyslexia, by presenting double dissociations between the two. We took into account the existence of distinct types of dyslexia and of attention deficits, and focused on dyslexias that may be thought to have an attentional basis: letter position dyslexia (LPD), in which letters migrate within words, attentional dyslexia (AD), in which letters migrate between words, neglect dyslexia, in which letters on one side of the word are omitted or substituted, and surface dyslexia, in which words are read via the sublexical route. We tested 110 children and adults with developmental dyslexia and/or attention deficits, using extensive batteries of reading and attention. For each participant, the existence of dyslexia and the dyslexia type were tested using reading tests that included stimuli sensitive to the various dyslexia types. Attention deficit and its type was established through attention tasks assessing sustained, selective, orienting, and executive attention functioning. Using this procedure, we identified 55 participants who showed a double dissociation between reading and attention: 28 had dyslexia with normal attention and 27 had attention deficits with normal reading. Importantly, each dyslexia with suspected attentional basis dissociated from attention: we found 21 individuals with LPD, 13 AD, 2 neglect dyslexia, and 12 surface dyslexia without attention deficits. Other dyslexia types (vowel dyslexia, phonological dyslexia, visual dyslexia) also dissociated from attention deficits. Examination of 55 additional individuals with both a specific dyslexia and a certain attention deficit found no attention function that was consistently linked with any dyslexia type. Specifically, LPD and AD dissociated from selective attention, neglect dyslexia dissociated from orienting, and surface dyslexia dissociated from sustained and executive attention. These results indicate that visuospatial attention deficits do not underlie these dyslexias.

Finding the answer in space: the mental whiteboard hypothesis on serial order in working memory

Various prominent models on serial order coding in working memory (WM) build on the notion that serial order is achieved by binding the various items to-be-maintained to fixed position markers. Despite being relatively successful in accounting for empirical observations and some recent neuro-imaging support, these models were largely formulated on theoretical grounds and few specifications have been provided with respect to the cognitive and/or neural nature of these position markers. Here we outline a hypothesis on a novel candidate mechanism to substantiate the notion of serial position markers. Specifically, we propose that serial order WM is grounded in the spatial attention system: (I) The position markers that provide multi-item WM with a serial context should be understood as coordinates within an internal, spatially defined system; (II) internal spatial attention is involved in searching through the resulting serial order representation; and (III) retrieval corresponds to selection by spatial attention. We sketch the available empirical support and discuss how the hypothesis may provide a parsimonious framework from which to understand a broad range of observations across behavioral, neural and neuropsychological domains. Finally, we pinpoint what we believe are major questions for future research inspired by the hypothesis.

Perceptual and response-related components of unilateral neglect may evolve independently of one another: evidence from five single-case studies

The Milner Landmark Task allows the disentanglement of perceptual and response-related components of unilateral neglect. If these two components reflect separate functional systems, then cases should be observed in which the two components evolve differently across time. To test this hypothesis we surveyed a continuous series of 21 right hemisphere stroke patients. Five patients from the sample were affected by unilateral neglect at the outset and could be submitted to repeated administrations of the Landmark task in the first weeks post stroke. Two versions of the task were used, Landmark-Manual and Landmark-Verbal, differing in the type of response required. Two patients showed independent changes in the perceptual and the response-related component of neglect, hence confirming the view of separate functional systems underlying them. Dissociations between the task versions were found, witnessing a role of the type of response. Unexpectedly, one patient showed an initial leftward deviation of the subjective midpoint of the stimulus line, which later reversed to a classical rightward deviation. We interpreted such a pattern in terms of co-existing "productive" and "negative" components of perceptual neglect.

Compound reading in Hebrew text-based neglect dyslexia: the effects of the first word on the second word and of the second on the first

In many Hebrew compounds, which are two-word phrases, the first word is marked morphophonologically, and often also orthographically, as the head of the compound. Because Hebrew is read from right to left, this allowed us to ask whether a right-hand word that is marked orthographically as a compound-head, and hence signals that another word is expected, causes readers with text-based neglect to continue shifting attention to the left and read the second word. We also asked whether the second, left-hand, word affects the reading of the first word. The effect of the second word was assessed in a condition in which the second word semantically disambiguated the first word, a biased heterophonic homograph, and a condition in which the second word formed a compound with the first and hence required reading the first in the morphophonological form of a compound-head. The two participants were Hebrew-speaking men with acquired left text-level neglect dyslexia, without word-based neglect dyslexia. They read 294 two-word compounds and control phrases, composed of five conditions that assessed the effect of the first word on the second word, and of the second on the first. The results indicated that morphosyntax modulates reading in neglect dyslexia. When the first, right-hand, word included an orthographic cue indicating that a second word follows, fewer words on the left were omitted than when no such cue existed. The second word, however, did not affect the reading of the first, and the first word was read as if the patients did not look ahead to the second.

Neglect dyslexia: a matter of "good looking"

Brain-damaged patients with right-sided unilateral spatial neglect (USN) often make left-sided errors in reading single words or pseudowords (neglect dyslexia, ND). We propose that both left neglect and low fixation accuracy account for reading errors in neglect dyslexia. Eye movements were recorded in USN patients with (ND+) and without (ND-) neglect dyslexia and in a matched control group of right brain-damaged patients without neglect (USN-). Unlike ND- and controls, ND+ patients showed left lateralized omission errors and a distorted eye movement pattern in both a reading aloud task and a non-verbal saccadic task. During reading, the total number of fixations was larger in these patients independent of visual hemispace, and most fixations were inaccurate. Similarly, in the saccadic task only ND+ patients were unable to reach the moving dot. A third experiment addressed the nature of the left lateralization in reading error distribution by simulating neglect dyslexia in ND- patients. ND- and USN- patients had to perform a speeded reading-at-threshold task that did not allow for eye movements. When stimulus exploration was prevented, ND- patients, but not controls, produced a pattern of errors similar to that of ND+ with unlimited exposure time (e.g., left-sided errors). We conclude that neglect dyslexia reading errors may arise in USN patients as a consequence of an additional and independent deficit unrelated to the orthographic material. In particular, the presence of an altered oculo-motor pattern, preventing the automatic execution of the fine saccadic eye movements involved in reading, uncovers, in USN patients, the attentional bias also in reading single centrally presented words.

Error types and error positions in neglect dyslexia: comparative analyses in neglect patients and healthy controls

Unilateral spatial neglect frequently involves a lateralised reading disorder, neglect dyslexia (ND). Reading of single words in ND is characterised by left-sided omissions and substitutions of letters. However, it is unclear whether the distribution of error types and positions within a word shows a unique pattern of ND when directly compared to healthy controls. This question has been difficult to answer so far, given the usually low number of reading errors in healthy controls. Therefore, the present study compared single word reading of 18 patients with left-sided neglect, due to right-hemisphere stroke, and 11 age-matched healthy controls, and adjusted individual task difficulty (by varying stimulus presentation times in participants) in order to reach approximately equal error rates between neglect patients and controls. Results showed that, while both omission and substitution errors were frequently produced in neglect patients and controls, only omissions appeared neglect-specific when task difficulty was adapted between groups. Analyses of individual letter positions within words revealed that the spatial distribution of reading errors in the neglect dyslexic patients followed an almost linear increase from the end to the beginning of the word (right-to-left-gradient). Both, the gradient in error positions and the predominance of omission errors presented a neglect-specific pattern. Consistent with current models of visual word processing, these findings suggest that ND reflects sublexical, visuospatial attentional mechanisms in letter string encoding.

Spatial analysis after perinatal stroke: patterns of neglect and exploration in extra-personal space

This study was conducted to determine whether school-aged children who had experienced a perinatal stroke demonstrate evidence of persistent spatial neglect, and if such neglect was specific to the visual domain or was more generalized. Two studies were carried out. In the first, 38 children with either left hemisphere (LH) or right hemisphere (RH) damage and 50 age-matched controls were given visual cancellation tasks varying in two factors: target stimuli and stimulus array. In the second study, tactile neglect was evaluated in 41 children with LH or RH damage and 72 age-matched controls using a blindfolded manual exploration task. On the visual cancellation task, LH subjects omitted more target stimuli on the right, but also on the left, compared with controls. Children with RH lesions also produced a larger number of omissions on both the left and right sides than controls, but with poorer performance on the left. On the manual exploration task, LH children required significantly longer times to locate the target on both sides of the board than did controls. RH children had significantly prolonged search times on the left side, but not on the right, compared with controls. In both tasks, LH subjects employed unsystematic search strategies more often than both control and RH children. The search strategy of RH children also tended to be erratic when compared to controls, but only in the random arrays of the visual cancellation tasks; structure of the target stimuli improved their organization. These results demonstrate that children with early LH brain damage display bilateral difficulties in visual and tactile modalities; a pattern that is in contrast to that seen in adults with LH damage. This may result from disorganized search strategies or other subtle spatial or attentional deficits. Results of performance of RH children suggests the presence of contralateral neglect in both the visual and tactile modalities; a finding that is similar to the neglect in adult stroke patients with RH lesions. The fact that deficits in spatial attention and organizational strategies are present after very early focal damage to either the LH or the RH broadens our understanding of the differences in functional lateralization between the immature and mature brain. These results also add to evidence for limitations to plasticity in the developing brain. Our findings may have therapeutic and rehabilitative implications for the management of children with early focal brain lesions.

Ipsilesional 'where' with contralesional 'what' neglect

Whereas contralesional spatial neglect is usually caused by right temporo-parietal lesions, ipsilesional spatial neglect is induced primarily by right frontal lesions. This report describes a 73-year-old woman with a right inferior parietal lesion who on 'where' tasks (line bisection and midline pointing) demonstrated ipsilesional neglect, but on 'what' tests (gap vs. no-gap detection cancellation and clothing tape removal) demonstrated contralesional neglect. This 'what' and 'where' directional dissociation provides evidence for independent 'what' and 'where' attentional networks; however, the reason this parietal lesion causes this contralesional vs. ipsilesional spatial attentional 'what' and 'where' dichotomy remains to be determined.

Visuo-haptic interactions in unilateral spatial neglect: the cross modal judd illusion

Unilateral spatial neglect (USN) has been mainly investigated in the visual modality; only few studies compared spatial neglect across different sensory modalities, and explored their multisensory interactions, with controversial results. We investigated the integration between vision and haptics, through a bisection task of a cross modal illusion, the Judd variant of the Müller-Lyer illusion. We examined right-brain-damaged patients with (n = 7) and without (n = 7) left USN, and neurologically unimpaired participants (n = 14) in the bisection of Judd stimuli under visual, haptic, and visuo-haptic presentation. Neglect patients showed the characteristic rightward bias in the bisection of the baseline stimuli in the visual modality, but not in the haptic and visuo-haptic conditions. The illusory effects were preserved in each group and in each modality, indicating that the processing of the cross modal illusion is independent of the presence of deficits of spatial attention and representation. Spatial neglect can be modality-specific, but visual and tactile sensory inputs are properly integrated.

When connectedness increases hemispatial neglect

Patients with left neglect were tested with “chimeric” figures composed of the right and left halves of two different objects. The connectivity relation was modulated between the two half figures. For some displays, the two chimeric halves were separated by a small gap, while in others, the separate halves were connected by a line segment. In line with previous reports, performance on reporting the left half improved when the chimera were separated; but when a line connected the two separated halves the advantage was lost. If the connecting line was broken, the performance was again enhanced. The results suggest an important role for connectedness in the representation of perceptual objects and in the distribution of attention in neglect.

The effect of syntax on reading in neglect dyslexia

Individuals with text-based neglect dyslexia omit words on the neglected side of the sentence or text, usually on the left side. This study tested whether the syntactic structure of the target sentence affects reading in this type of neglect dyslexia. Because Hebrew is read from right to left, it enables testing whether the beginning of the sentence and its syntactic properties determine if the final, leftmost, constituent is omitted or not. The participants were 7 Hebrew-speaking individuals with acquired left text-based neglect dyslexia, without syntactic impairments. Each participant read 310 sentences, in which we compared 5 types of minimal pairs of sentences that differed in the obligatoriness of the final (left) constituent. Complements were compared with adjuncts, obligatory pronouns were compared with optional resumptive pronouns, and the object of a past tense verb was compared with the object of a present tense verb, which can also be taken to be an adjective, which does not require an object. Questions that require a verb were compared with questions that can appear without a verb, and clauses that serve as sentential complements of a verb were compared with coordinated clauses, which are not required by the verb. In addition, we compared the reading of noun sequences to the reading of meaningful sentences, and assessed the neglect point in reading 2 texts. The results clearly indicated that the syntactic knowledge of the readers with neglect dyslexia modulated their sentence reading. They tended to keep on reading as long as the syntactic and lexical-syntactic requirements of the sentence had not been met. In 4 of the conditions twice as many omissions occurred when the final constituent was optional than when it was obligatory. Text reading was also guided by a search for a "happy end" that does not violate syntactic or semantic requirements. Thus, the syntactic structure of the target sentence modulates reading and neglect errors in text-based neglect dyslexia, suggesting that the best stimuli to diagnose mild text-based neglect dyslexia are sentences in which the leftmost constituent is optional, and not required by syntax. Another finding of this study is dissociation between neglect dyslexia at the text and at the word levels. Two of the participants had neglect dyslexia at the text level, manifested in omissions of words on the left side of text, without neglect dyslexia at the word level (namely, without omissions, substitutions, or additions of letters on the left side of words).

Two different mechanisms for omission and substitution errors in neglect dyslexia

Neglect dyslexia is a reading disorder often associated with right-sided brain lesions. In reading single words, errors are mostly substitutions or omissions of letters that occupy the left-sided positions. Typically, these errors have been thought to depend on a single mechanism. Conversely, we propose that they are due to different mechanisms. In particular, a visuo-spatial mechanism is responsible for omissions and a perceptual integration process for substitution errors. We measured the performance of six patients with both neglect and neglect dyslexia, analyzing their reading errors as a function of letter spacing. According to our conjecture, letter spacing should increase omissions by moving part of the string further in the unattended space, while it should reduce substitutions by restoring the integration processes. Furthermore, we predict that letter spacing should be more effective with pseudowords compared to words, in that in this latter case lexical effects are supposed to influence attentional and perceptual processes. Accordingly, we found that for pseudowords only the two types of errors are differently affected by this manipulation and only omissions correlate with the severity of the disorder in visuo-spatial tasks.

Representational neglect in "invisible" drawing from memory

We describe the case of a patient with right hemisphere damage and left unilateral neglect. The patient was asked to draw from memory common objects, either with or without visual feedback. In the conditions without visual feedback the patient was either blindfolded or he made "invisible" drawings using a pen with the cap on, the drawings being recorded with carbon paper underneath. Results showed more neglect without than with visual feedback, contrary to previously published cases. This patient's pattern of performance may result from the contribution of a deficit of spatial working memory. Alternatively or in addition, the patient, who was undergoing cognitive rehabilitation for neglect, may have found easier to compensate for his neglect with visual feedback, which allowed him to visually explore the left part of his drawings.

Unilateral neglect: A theory of proprioceptive space of a stimulus as determined by the cerebellar component of motor efference copy (and is autism a special case of neglect)

Unilateral neglect is a devastating condition, which manifests as a loss of a person's spatial awareness opposite the damaged side of the brain. It challenges our conception of the seat of the soul and its explanation is at the heart of the mind-body problem. A heuristic definition of the dorsal stream of a modality is here based on the categorization of parietal networks by the cerebellar component of motor efference copy. Taking this premise, the proprioceptive space of a stimulus is established as a concept in this paper. It is proposed that unilateral neglect is typically a dysfunction of proprioceptive space of a stimulus associated with lesions of the dorsal stream. Furthermore, most experimental findings of unilateral visual neglect (and by extrapolation, other sensory modalities), can be explained by two developmental mechanisms by which the proprioceptive space of a stimulus is encoded in the parietal cortex. Its right and left hemisphere representation can be dissociated from the hemifield of presentation of perceptual information, such that the left hemifield can have a left hemisphere representation through callosal connections and likewise, the right hemifield can have a right hemifield representation. The processing of a sensory stimulus in either parietal hemisphere is dynamically determined as shown by experimental modulation of performance. A theory of historical precedence will provide a developmental background to the organization of proprioceptive space and will invoke separate models according to specified terms of engagement. A model based on the expansion and contraction of the proprioceptive space of a stimulus as a gradient across both hemispheres and modulated by concurrent proprioceptive state will be differentiated from a model that is non-graduating but competitive and lacks such modulation. In other words, the dorsal representation of a sensory stimulus in the former case is shared to a varying degree by the two parietal hemispheres, whereas in the latter case the representation of left and right aspects of the same object stimulus is strictly divided between the two hemispheres. A further hypothesis of a dorsoventral gradient of the peripheral and foveal components of proprioceptive space characterizing dorsal stream networks will predict the double dissociation revealed by experimental paradigms. It will explain why some patients show neglect only in foveal while others only in peripheral vision. The paper proposes to unify neglect, extinction and optic ataxia on the one hand, and spatial and object-based neglect on the other hand, under a singularly proficient paradigmatic structure. A binding model as described is a component theory that acknowledges how the involved pathways or transitional zones in a pathway may contribute to a differential clinical picture as one progresses from posterior to anterior parietal cortex. Finally, a brief discussion is given on how autistic subjects neglect spatial cues and the inability of a spatial cognitive transformation underlies the impairments postulated for 'a theory of mind'.

Right hemispatial neglect: frequency and characterization following acute left hemisphere stroke

The frequency of various types of unilateral spatial neglect and associated areas of neural dysfunction after left hemisphere stroke are not well characterized. Unilateral spatial neglect (USN) in distinct spatial reference frames have been identified after acute right, but not left hemisphere stroke. We studied 47 consecutive right handed patients within 48h of left hemisphere stroke to determine the frequency and distribution of types of right USN using cognitive testing and MRI imaging. The distribution of USN types was different from the previously reported distribution following acute right hemisphere stroke. In this left hemisphere stroke population, allocentric neglect was more frequent than egocentric neglect.

Left ipsilesional neglect for visual imagery: a mental image generation impairment?

In this article, we describe a patient, CN, with unilateral left posterior brain damage, who shows a rare occurrence of left ipsilesional neglect limited to mental representations. CN's clinical pattern is consistent with the hypothesis that different mechanisms underlie perceptual and representational neglect. We also discuss an interpretation of representational neglect based on the different involvement of the two hemispheres in visual imagery processes. In particular, left brain lesions do not present the typical strong contralesional bias and may lead to an impairment of mental generation of either side of the image, depending on pre-morbid and methodological factors.

The role of vision in spatial representation

A complex link exists between vision and unilateral spatial neglect (USN). Firstly, USN is not a perceptual deficit, secondly, USN is not necessarily accompanied by a visual deficit and finally, USN can be observed in non-visual modalities as well as in mental spatial imagery. This apparent supramodality of USN stands in sharp contrast to the fact that neglect signs are often more severe and more durable in the visual than in other sensory modalities (Chokron et al., 2002). The influence of vision on spatial representation has rarely been studied. In the present study we assessed six right brain-damaged patients suffering from left USN on two tasks involving spatial representations: a clock-drawing task and a drawing from memory task in two experimental conditions, with and without visual control. We confirm that even in mental imagery, the absence of visual feedback may decrease and even suppress left neglect signs (Bartolomeo and Chokron, 2001b; 2002). Since vision is largely involved in the orientation of attention in space, suppressing visual control could reduce the magnetic attraction towards the right ipsilesional hemispace and in this way could allow a re-orientation of attention towards the left neglected hemispace. We discuss the theoretical and therapeutic implications of these findings.

Visual and Tactile Length Matching in Spatial Neglect

Previous studies have shown that many patients with spatial neglect underestimate the horizontal extent of leftwardly located shapes (presented on screen or on paper) relative to rightwardly located shapes. This has been used to help explain their leftward biases in line bisection. In the present study we have tested patients with right hemisphere damage, either with or without neglect, on a comparable length matching task, but using 3-dimensional objects. The task was executed first visually without tactile contact, and second through touch without vision. In both sense modalities, we found that patients with neglect, but not those without, tended to underestimate leftward located objects relative to rightward located objects, differing significantly in this regard from healthy subjects. However these lateral biases were not as frequent or as pronounced as in previous studies using 2-D visual shapes. Despite the similar asymmetries in the two sense modalities, we found only a small correlation between them, and clear double dissociations were observed among our patients. We conclude that leftward length underestimation cannot be attributed to any one single cause. First it cannot be entirely due to impairments in the visual pathways, such as hemianopia and/or processing biases, since the disorder is also seen in the tactile modality. At the same time, however, length underestimation phenomena cannot be fully explained as a disruption of a supramodal central size processor, since they can occur in either vision or touch alone. Our data would fit best with a multiple-factor model in which some patients show leftward length underestimation for modality-specific reasons, while others do so due to a more high-level disruption of size judgements.

Left on the Right or Viceversa: A Case of “Alternating” Constructional Allochiria

We describe a patient with an ischemic right frontal lesion and mild left neglect who showed a systematic tendency to transpose drawings on one side of the page, which varied depending on the starting point (left or right) of his graphic productions. When not specifically cued, the patient started to draw in the ipsilesional (right) side and tended to show allochiria on the right, but occasionally, or under specific instructions, the patient started drawing from the left side and then showed a complete reversion of his spatial transpositions. To clarify the basic mechanisms underlying such a peculiar constructional phenomenon, we performed a series of experimental investigations, including extended copying tasks, a clock-marking test (to mark the position of single hours on a clock-face), and a line bisection task with progressive left-toright or right-to-left stimulus presentation. Findings suggested that "alternating" allochiria in copying and drawing from memory tasks is an epiphenomenon of a basic inability to move attention and action away from the starting point of graphic productions. The present case study, contrasted with observations on other brain-damaged patients, demonstrates that allochiria may have different neuro-cognitive bases and offers new insights for theoretical interpretations of unilateral spatial neglect.

Developmental Neglect Dyslexia in a Hebrew-Reading Child

This paper describes for the first time a detailed study of a child with neglect dyslexia. NT is 10-year-old child, with left word-based neglect dyslexia, without clinical signs of visuo-spatial neglect. Since he is a native speaker of Hebrew, which is read from right to left, his neglect dyslexia manifests in omissions and substitutions of final letters. He is severely impaired in single words, with 96% of his errors being omissions and substitutions of final letters. When presented with more than one word, in word pairs, sentences or text, he neglects the left part of each word, but never omit whole words on the left side of the page. His reading improves considerably when the same word is presented vertically or when manipulations are done to shift his attention to the left--with coloured final letters, flashing light, or tapping his finger to the left of each word. NT's neglect dyslexia is very selective, with good reading of numbers and symbols, and even good performance on letter sequences when no reading is required. A dissociation is also detected between his impaired reading due to neglect dyslexia and his normal performance on conventional clinical tests of general visual neglect visual of line, object and letter cancellation, line bisection, object drawing and copying. His neglect dyslexia seems to be developmental as no abrupt onset is reported.

Attention, spatial representation, and visual neglect: simulating emergent attention and spatial memory in the selective attention for identification model (SAIM)

The selective attention for identification model (SAIM) is presented. This uses a spatial window to select visual information for recognition, binding parts to objects and generating translation-invariant recognition. The model provides a qualitative account of both normal and disordered attention. Simulations of normal attention demonstrate 2-object costs and effects of object familiarity on selection, global precedence, spatial cueing, and inhibition of return. When lesioned, SAIM demonstrated either view- or object-centered neglect or spatial extinction, depending on the type and extent of lesion. The model provides a framework to unify (a) object- and space-based theories of normal selection, (b) dissociations within the syndrome of unilateral neglect, and (c) attentional and representational accounts of neglect.

On selective left neglect during walking in a child

In this article we describe a child affected by right parieto-occipital lesion due to head injury. The patient showed left hemianopia, but not unilateral spatial neglect on traditional paper-and-pencil tests and on "ecological" tests. However, his parents reported frequent collisions with obstacles on the left side. A specific test was set up: The patient had to kick down skittles put on both sides of a route traced on the floor. He kicked down 89% of skittles on the right, but only 38% on the left side. These findings are discussed in light of recent theories on unilateral neglect.

Graphic Neglect—More Than the Sum of the Parts

Patients who seem to "ignore" objects or people on one side of space have been described in the medical literature for well over a century. The term "visuospatial neglect" is now used to describe the cluster of behaviors whereby patients after unilateral cerebral lesions (most frequently of right parietal cortex) fail to attend or explore (predominantly) the side of space contralateral to the lesion. Although the condition comprises a complex disruption of space-related behaviors, the prevailing view was that the different symptoms could be accommodated in terms of damage to one of three different cognitive mechanisms mediating attention (e.g., K. M. Heilman and E. Valenstein, Ann. Neurol. 5: 166-170, 1979), intention (R. T. Watson, E. Valenstein, and K. Heilman, Ann. Neurol. 3: 505-508, 1978), and/or representation (E. Bisiach, Q. J. Exp. Psychol. 46: 435-461, 1993). The general consensus favors an attentional deficit but the notion of attention has always proved conceptually slippery and difficult to operationalize (P. W. Halligan and J. C. Marshall, Cogn. Neuropsychol. 11: 167-206, 1994a). In this paper, we consider how drawing performance after right brain damage in patients with "visual neglect" reveals the involvement and interplay of several cognitive deficits, including aspects of mental representation and spatial awareness.

Simulating a lesion in a basis function model of spatial representations: comparison with hemineglect

The basis function theory of spatial representations explains how neurons in the parietal cortex can perform nonlinear transformations from sensory to motor coordinates. The authors present computer simulations showing that unilateral parietal lesions leading to a neuronal gradient in basis function maps can account for the behavior of patients with hemineglect, including (a) neglect in line cancellation and line bisection experiments; (b) neglect in multiple frames of reference simultaneously; (c) relative neglect, a form of what is sometime called object-centered neglect; and (d) neglect without optic ataxia. Contralateral neglect arises in the model because the lesion produces an imbalance in the salience of stimuli that is modulated by the orientation of the body in space. These results strongly support the basis function theory for spatial representations in humans and provide a computational model of hemineglect at the single-cell level.

How the brain encodes the order of letters in a printed word: the SERIOL model and selective literature review

This paper describes a novel theoretical framework of how the position of a letter within a string is encoded, the SERIOL model (sequential encoding regulated by inputs to oscillations within letter units). Letter order is represented by a temporal activation pattern across letter units, as is consistent with current theories of information coding based on the precise timing of neural spikes. The framework specifies how this pattern is invoked via an activation gradient that interacts with subthreshold oscillations and how it is decoded via contextual units that activate word units. Using mathematical modeling, this theoretical framework is shown to account for the experimental data from a wide variety of string-processing studies, including hemispheric asymmetries, the optimal viewing position, and positional priming effects.

Perceiving left and imagining right: dissociation in neglect

Signor Piazza, a patient with a left parieto-occipital haemorrhage and a right thalamic stroke, showed severe right personal neglect (e.g. touching own body parts) and right perceptual neglect in tasks with (e.g. cancelling tasks) or without (e.g. description of a complex picture) motor response. He had also right-sided neglect dyslexia (including single words), without language impairments. However, the patient also presented with a clear left-sided deficit in the representational domain (e.g. imagery tasks). Signor Piazza's pattern of performance suggests dissociation between imagery and perception within the neglect syndrome.