Time to imagine space: a chronometric exploration of representational neglect

Colors in the mind's eye

The famous "Piazza del Duomo" paper, published in Cortex in 1978, inspired a considerable amount of research on visual mental imagery in brain-damaged patients. As a consequence, single-case reports featuring dissociations between perceptual and imagery abilities challenged the prevailing model of visual mental imagery. Here we focus on mental imagery for colors. A case study published in Cortex showed perfectly preserved color imagery in a patient with acquired achromatopsia after bilateral lesions at the borders between the occipital and temporal cortex. Subsequent neuroimaging findings in healthy participants extended and specified this result; color imagery elicited activation in both a domain-general region located in the left fusiform gyrus and the anterior color-biased patch within the ventral temporal cortex, but not in more posterior color-biased patches. Detailed studies of individual neurological patients, as those often published in Cortex, are still critical to inspire and constrain neurocognitive research and its theoretical models.

Left and right temporal-parietal junctions (TPJs) as "match/mismatch" hedonic machines: A unifying account of TPJ function

Experimental and theoretical studies have tried to gain insights into the involvement of the Temporal Parietal Junction (TPJ) in a broad range of cognitive functions like memory, attention, language, self-agency and theory of mind. Recent investigations have demonstrated the partition of the TPJ in discrete subsectors. Nonetheless, whether these subsectors play different roles or implement an overarching function remains debated. Here, based on a review of available evidence, we propose that the left TPJ codes both matches and mismatches between expected and actual sensory, motor, or cognitive events while the right TPJ codes mismatches. These operations help keeping track of statistical contingencies in personal, environmental, and conceptual space. We show that this hypothesis can account for the participation of the TPJ in disparate cognitive functions, including "humour", and explain: a) the higher incidence of spatial neglect in right brain damage; b) the different emotional reactions that follow left and right brain damage; c) the hemispheric lateralisation of optimistic bias mechanisms; d) the lateralisation of mechanisms that regulate routine and novelty behaviours. We propose that match and mismatch operations are aimed at approximating "free energy", in terms of the free energy principle of decision-making. By approximating "free energy", the match/mismatch TPJ system supports both information seeking to update one's own beliefs and the pleasure of being right in one's own' current choices. This renewed view of the TPJ has relevant clinical implications because the misfunctioning of TPJ-related "match" and "mismatch" circuits in unilateral brain damage can produce low-dimensional deficits of active-inference and predictive coding that can be associated with different neuropsychological disorders.

The connectional anatomy of visual mental imagery: evidence from a patient with left occipito-temporal damage

Most of us can use our "mind's eye" to mentally visualize things that are not in our direct line of sight, an ability known as visual mental imagery. Extensive left temporal damage can impair patients' visual mental imagery experience, but the critical locus of lesion is unknown. Our recent meta-analysis of 27 fMRI studies of visual mental imagery highlighted a well-delimited region in the left lateral midfusiform gyrus, which was consistently activated during visual mental imagery, and which we called the Fusiform Imagery Node (FIN). Here, we describe the connectional anatomy of FIN in neurotypical participants and in RDS, a right-handed patient with an extensive occipito-temporal stroke in the left hemisphere. The stroke provoked right homonymous hemianopia, alexia without agraphia, and color anomia. Despite these deficits, RDS had normal subjective experience of visual mental imagery and reasonably preserved behavioral performance on tests of visual mental imagery of object shape, object color, letters, faces, and spatial relationships. We found that the FIN was spared by the lesion. We then assessed the connectional anatomy of the FIN in the MNI space and in the patient's native space, by visualizing the fibers of the inferior longitudinal fasciculus (ILF) and of the arcuate fasciculus (AF) passing through the FIN. In both spaces, the ILF connected the FIN with the anterior temporal lobe, and the AF linked it with frontal regions. Our evidence is consistent with the hypothesis that the FIN is a node of a brain network dedicated to voluntary visual mental imagery. The FIN could act as a bridge between visual information and semantic knowledge processed in the anterior temporal lobe and in the language circuits.

Spatial Neglect Subtypes, Definitions and Assessment Tools: A Scoping Review

Objective: The objective of this scoping review was to capture the reported definitions for the subtypes of neglect post stroke and map the range of assessment tools employed for each neglect subtype.

Methods: EMBASE, Emcare, Medline, and psychINFO were searched from database inception. Searching included all allied terms and mesh headings for stroke, spatial neglect, measurement, screening tools, psychometric properties. Two reviewers independently screened studies for inclusion. Primary studies with documented protocols of a spatial neglect tool for adults post stroke, with some aspect of validity or reliability were included. Two reviewers independently reviewed the documented protocols of each tool to determine the underlying subtypes and disagreements were resolved through discussion.

Results: There were 371 articles included with 292 tools used for the screening or diagnosis of neglect. The majority of studies (67%) included a tool that did not specify the neglect subtype being assessed, therefore an analysis of the underlying subtypes for each tool is presented.

Conclusions: There is no consistency with the terms used to refer to the syndrome of spatial neglect with over 200 different terms used within the included studies to refer to the syndrome as a whole or one of its subtypes. It is essential to unify the terminology and definition for each neglect subtype. There are hundreds of neglect tools available, however many are not able to differentiate presenting subtypes. It is important for clinicians and researchers to critically evaluate the neglect tools being used for the screening and diagnosis of neglect.

From competition to cooperation: Visual neglect across the hemispheres

Visuospatial neglect is a frequent and disabling consequence of injuries to the right hemisphere. Patients with neglect show signs of impaired attention for left-sided events, which depends on dysfunction of fronto-parietal networks. After unilateral injury, such as stroke, these networks and their contralateral homologs can reorganize following multiple potential trajectories, which can be either adaptive or maladaptive. This article presents possible factors influencing the profile of evolution of neglect towards recovery or chronicity, and highlights potential mechanisms that may constrain these processes in time and space. The integrity of white matter pathways within and between the hemisphere appears to pose crucial connectivity constraints for compensatory brain plasticity from remote brain regions. Specifically, the availability of a sufficient degree of inter-hemispheric connectivity might be critical to shift the role of the undamaged left hemisphere in spatial neglect, from exerting maladaptive effects, to promoting compensatory activity.

Neuropsychological deficits in Posterior Cortical Atrophy and typical Alzheimer's disease: A meta-analytic review

AIMS

To identify cognitive tests that best differentiate between Posterior Cortical Atrophy (PCA) and typical Alzheimer's Disease (tAD), as well as PCA and healthy control (HC) participants.

METHOD

Medline, PsycInfo and Web of Science were systematically searched using terms related to PCA, tAD, and cognitive testing. Seventeen studies were identified, including 441 PCA, 391 tAD, and 284 HC participants. Standardised effect sizes of mean scores were calculated to measure performance differences on cognitive tests for PCA versus tAD and PCA versus HC groups. Meta-analyses used a random effects model.

RESULTS

The most discriminating cognitive tests for PCA and tAD presentations were measures of visuospatial function and verbal memory. Large, significant effect sizes were produced for all measures of visuospatial function, most notably for Rey-Osterrieth Copy (Hedges' g = -2.79), VOSP Fragmented letters (Hedges' g = -1.73), VOSP Dot Counting (Hedges' g = -1.74), and VOSP Cube Analysis (Hedges' g = -1.98). For measures of verbal memory, the RAVLT delay and Digit Span Backwards produced significant medium effects (Hedges' g = .62 and -.56, respectively).

CONCLUSION

Establishing a common framework for testing individuals with PCA has important implications for diagnosis and treatment, and forms a practical objective for future research. Findings from this meta-analysis suggest that measures of visuospatial function and verbal memory would form an important part of this framework.

The presence of semantic content in a visual recognition memory task reduces the severity of neglect

Patients with right hemisphere damage often show a lateral bias when asked to report the left side of mental images held in visual working memory (i.e. representational neglect). The neural basis of representational neglect is not well understood. One hypothesis suggests that it reflects a deficit in attentional-exploratory mechanisms, i.e. an inability to direct attention to the left side of the image. Another proposition states that intact visual working memory (VWM) is necessary for correctly creating a mental image. Here we examined two components of VWM in patients with unilateral spatial neglect (USN): memory for identity, and memory for spatial position. We manipulated the strength of memory representations by presenting two distinct categories of objects, in separate blocks. These were familiar namable objects (fruits, etc.), and unfamiliar abstract objects. The former category elicits stronger working-memory traces, thanks to preexisting visual and semantic representations in long-term memory. We hypothesized that if USN patients show a lateralized deficit in VWM, it should be more pronounced for abstract objects, due to their weaker working-memory traces. Importantly, to isolate a spatially lateralized deficit in memory from a failure to fully perceive the object-arrays, we ensured that all included patients perceived every item during the encoding phase. We used a working-memory task: participants viewed object arrays and had to memorize items' identities and spatial positions. Then, single objects were presented requiring 'old/new' recognition, and retrieval of 'old' items' original positions. Our results show a lateral bias in patients' recognition-memory performance. Remarkably, it was threefold milder for namable objects compared to abstract objects. We conclude that VWM lateralized deficit is substantial in USN patients and could play a role in representational neglect.

Hemispheric asymmetries in visual mental imagery

Visual mental imagery is the faculty whereby we can "visualize" objects that are not in our line of sight. Longstanding evidence dating back over thirty years has shown that unilateral brain lesions, especially in the left temporal lobe, can impair aspects of this ability. Yet, there is currently no attempt to identify analogies between these neuropsychological findings of hemispheric asymmetry and those from other neuroscientific approaches. Here, we present a critical review of the available literature on the hemispheric laterality of visual mental imagery, by looking at cross-method patterns of evidence in the domains of lesion neuropsychology, neuroimaging, and direct cortical stimulation. Results can be summarized under three main axes. First, frontoparietal networks in both hemispheres appear to be associated with visual mental imagery. Second, lateralization patterns emerge in the temporal lobes, with the left inferior temporal lobe being the most common finding in the literature for endogenously generated images, especially, but not exclusively, when orthographic material is used to ignite imagery. Third, an opposite pattern of hemispheric laterality emerges when visual mental images are induced by exogenous stimulation; direct cortical electrical stimulation tends to produce visual imagery experiences predominantly when applied to the right temporal lobe. These patterns of hemispheric asymmetry are difficult to reconcile with the dominant model of visual mental imagery, which emphasizes the implication of early sensory cortices. They suggest instead that visual mental imagery relies on large-scale brain networks, with a crucial participation of high-level visual regions in the temporal lobes.

Visual and motor neglect: Clinical and neurocognitive aspects

Attention allows us to prioritize the processing of external information according to our goals, but also to cope with sudden, unforeseen events. Attention processes rely on the coordinated activity of large-scale brain networks. At the cortical level, these systems are mainly organized in fronto-parietal networks, with functional and anatomical asymmetries in favor of the right hemisphere. Dysfunction of these right-lateralized networks often produce severe deficit of spatial attention, such as visual neglect. Other brain-damaged patients avoid moving the limbs contralateral to their brain lesion, even in the absence of sensorimotor deficits (motor neglect). This paper first summarizes past and current evidence on brain networks of attention; then, it presents clinical and experimental findings on visual and motor neglect, and on the possible mechanisms of clinical recovery.

Visual mental imagery engages the left fusiform gyrus, but not the early visual cortex: A meta-analysis of neuroimaging evidence

The dominant neural model of visual mental imagery (VMI) stipulates that memories from the medial temporal lobe acquire sensory features in early visual areas. However, neurological patients with damage restricted to the occipital cortex typically show perfectly vivid VMI, while more anterior damages extending into the temporal lobe, especially in the left hemisphere, often cause VMI impairments. Here we present two major results reconciling neuroimaging findings in neurotypical subjects with the performance of brain-damaged patients: (1) A large-scale meta-analysis of 46 fMRI studies, of which 27 investigated specifically visual mental imagery, revealed that VMI engages fronto-parietal networks and a well-delimited region in the left fusiform gyrus. (2) A Bayesian analysis showed no evidence for imagery-related activity in early visual cortices. We propose a revised neural model of VMI that draws inspiration from recent cytoarchitectonic and lesion studies, whereby fronto-parietal networks initiate, modulate, and maintain activity in a core temporal network centered on the fusiform imagery node, a high-level visual region in the left fusiform gyrus.

Visuospatial Neglect - a Theory-Informed Overview of Current and Emerging Strategies and a Systematic Review on the Therapeutic Use of Non-invasive Brain Stimulation

Visuospatial neglect constitutes a supramodal cognitive deficit characterized by reduction or loss of spatial awareness for the contralesional space. It occurs in over 40% of right- and 20% of left-brain-lesioned stroke patients with lesions located mostly in parietal, frontal and subcortical brain areas. Visuospatial neglect is a multifaceted syndrome - symptoms can be divided into sensory, motor and representational neglect - and therefore requires an individually adapted diagnostic and therapeutic approach. Several models try to explain the origins of visuospatial neglect, of which the "interhemispheric rivalry model" is strongly supported by animal and human research. This model proposes that allocation of spatial attention is balanced by transcallosal inhibition and both hemispheres compete to direct attention to the contralateral hemi-space. Accordingly, a brain lesion causes an interhemispheric imbalance, which may be re-installed by activation of lesioned, or deactivation of unlesioned (over-activated) brain areas through noninvasive brain stimulation. Research in larger patient samples is needed to confirm whether noninvasive brain stimulation can improve long-term outcomes and whether these also affect activities of daily living and discharge destination.

Neglecting the left side of a city square but not the left side of its clock: prevalence and characteristics of representational neglect

Representational neglect, which is characterized by the failure to report left-sided details of a mental image from memory, can occur after a right hemisphere lesion. In this study, we set out to verify the hypothesis that two distinct forms of representational neglect exist, one involving object representation and the other environmental representation. As representational neglect is considered rare, we also evaluated the prevalence and frequency of its association with perceptual neglect. We submitted a group of 96 unselected, consecutive, chronic, right brain-damaged patients to an extensive neuropsychological evaluation that included two representational neglect tests: the Familiar Square Description Test and the O'Clock Test. Representational neglect, as well as perceptual neglect, was present in about one-third of the sample. Most patients neglected the left side of imagined familiar squares but not the left side of imagined clocks. The present data show that representational neglect is not a rare disorder and also support the hypothesis that two different types of mental representations (i.e. topological and non-topological images) may be selectively damaged in representational neglect.

Brain networks of visuospatial attention and their disruption in visual neglect

Visual neglect is a multi-component syndrome including prominent attentional disorders. Research on the functional mechanisms of neglect is now moving from the description of dissociations in patients' performance to the identification of the possible component deficits and of their interaction with compensatory strategies. In recent years, the dissection of attentional deficits in neglect has progressed in parallel with increasing comprehension of the anatomy and function of large-scale brain networks implicated in attentional processes. This review focuses on the anatomy and putative functions of attentional circuits in the brain, mainly subserved by fronto-parietal networks, with a peculiar although not yet completely elucidated role for the right hemisphere. Recent results are discussed concerning the influence of a non-spatial attentional function, phasic alertness, on conscious perception in normal participants and on conflict resolution in neglect patients. The rapid rate of expansion of our knowledge of these systems raises hopes for the development of effective strategies to improve the functioning of the attentional networks in brain-damaged patients.

Vertical representational biases in healthy university students

In line bisection tasks neurologically intact individuals tend to bisect lines slightly left of their midpoint for horizontal lines, and above centre for vertical lines, a phenomenon known as perceptual pseudoneglect (Bowers & Heilman, 1980; Van Vugt, Fransen, Creten, & Paquiner, 2000). Recent investigations have demonstrated the leftward bias to extend to mental imagery, a finding known as representational pseudoneglect (McGeorge, Beschin, Colnaghi, Rusconi, & Della Sala, 2007). This paper examined whether the upward bias found in perceptual tasks extended to mental imagery in healthy individuals. University students studied a diagram depicting a central character and target objects that were located in six positions relative to the person in the diagram (left/right, up/down, and front/back). Following learning, participants recalled the locations of the objects from several imagined orientations. Performance on the recall task revealed faster response latencies for upward targets, providing evidence for vertical representational biases in healthy individuals.

Visual mental imagery: what the head's eye tells the mind's eye

The demonstration of an implication of attentional/eye gaze systems in visual mental imagery might help to understand why some patients with visual neglect, who suffer from severe attentional deficits, also show neglect for mental images. When normal participants generate mental images of previously explored visual scenes, their oculomotor behavior resembles that used during visual exploration. However, this could be a case of encoding specificity, whereby the probability of retrieving an event increases if some information encoded with the event (in this case its spatial location) is present at retrieval. In the present study, normal participants were invited to conjure up a mental image of the map of France and to say whether auditorily presented towns or regions were situated left or right of Paris. A perceptual version of the task was administered after the imaginal condition. Thus, in the imaginal condition participants had to retrieve information from long-term memory. Vocal response times and, unbeknownst to participants, also eye movements were recorded. Participants tended to produce similar eye movements on the imaginal and on the perceptual conditions of the task. We concluded that some mechanisms involved in spontaneous oculomotor behavior may be shared in exploration of visuospatial mental images. Deficits of these common processes participating in the oculomotor exploration might contribute to imaginal neglect.

Spatial cues affect mental number line bisections

Numerical magnitude is coded left-to-right along a mental number line (MNL). The MNL can be distorted by an attentional bias directed to the left side, known as pseudoneglect - making the left of the MNL appear longer. We investigated whether this distortion can be corrected using spatial cues. Participants (n = 17) made forced-choice discriminations of relative numerical length while spatial cues were presented to the left, right, and both sides. Overall, participants overestimated the leftward length of the MNL, consistent with the effect of pseudoneglect. The bias was present for left- and neutral-cues, but was eliminated by right-cues. The results demonstrate that low-level manipulation of attention in physical space affects attention for high-level mental representations. The effect of cueing may reflect common activation of overlapping neural circuits that are thought to underlie attention in physical and representational space.

Representation and disconnection in imaginal neglect

Patients with neglect failure to detect, orient, or respond to stimuli from a spatially confined region, usually on their left side. Often, the presence of perceptual input increases left omissions, while sensory deprivation decreases them, possibly by removing attention-catching right-sided stimuli (Bartolomeo, 2007). However, such an influence of visual deprivation on representational neglect was not observed in patients while they were imagining a map of France (Rode et al., 2007). Therefore, these patients with imaginal neglect either failed to generate the left side of mental images (Bisiach & Luzzatti, 1978), or suffered from a co-occurrence of deficits in automatic (bottom-up) and voluntary (top-down) orienting of attention. However, in Rode et al.'s experiment visual input was not directly relevant to the task; moreover, distraction from visual input might primarily manifest itself when representation guides somatomotor actions, beyond those involved in the generation and mental exploration of an internal map (Thomas, 1999). To explore these possibilities, we asked a patient with right hemisphere damage, R.D., to explore visual and imagined versions of a map of France in three conditions: (1) 'imagine the map in your mind' (imaginal); (2) 'describe a real map' (visual); and (3) 'list the names of French towns' (propositional). For the imaginal and visual conditions, verbal and manual pointing responses were collected; the task was also given before and after mental rotation of the map by 180 degrees . R.D. mentioned more towns on the right side of the map in the imaginal and visual conditions, but showed no representational deficit in the propositional condition. The rightward inner exploration bias in the imaginal and visual conditions was similar in magnitude and was not influenced by mental rotation or response type (verbal responses or manual pointing to locations on a map), thus suggesting that the representational deficit was robust and independent of perceptual input in R.D. Structural and diffusion MRI demonstrated damage to several white matter tracts in the right hemisphere and to the splenium of corpus callosum. A second right-brain damaged patient (P.P.), who showed signs of visual but not imaginal neglect, had damage to the same intra-hemispheric tracts, but the callosal connections were spared. Imaginal neglect in R.D. may result from fronto-parietal dysfunction impairing orientation towards left-sided items and posterior callosal disconnection preventing the symmetrical processing of spatial information from long-term memory.

Vocal response times to real and imagined stimuli in spatial neglect: A group study and single-case report

The relationships between spatial neglect for perceptual objects and representational for imagined items are difficult to explore because of several methodological problems, including the dearth of comparable tests for real and imagined scenes. We asked 19 patients with right brain damage and 12 healthy controls to say whether an auditorily presented French geographical location was left or right of Paris, and recorded their vocal response times. Afterwards, participants performed a similar test with visually presented items. Although several patients had asymmetries of performance on the perceptual version of the test, only one patient was more accurate for right-sided than for left-sided imagined stimuli, thus showing evidence for imaginal neglect. However, this patient performed normally on place description and on mental number line bisection, perhaps as a consequence of different strategies he employed for these tasks. Overall, our results confirm previous evidence showing that imaginal neglect is less frequent than, and often occurs in association with, perceptual neglect. Imaginal neglect may result from the contribution of deficits partly distinct from those implicated in perceptual neglect, such as impaired endogenous orienting of attention or deficits of spatial working memory.

Uses of virtual reality for diagnosis, rehabilitation and study of unilateral spatial neglect: review and analysis

Unilateral spatial neglect is a disabling condition frequently occurring after stroke. People with neglect suffer from various spatial deficits in several modalities, which in many cases impair everyday functioning. A successful treatment is yet to be found. Several techniques have been proposed in the last decades, but only a few showed long-lasting effects and none could completely rehabilitate the condition. Diagnostic methods of neglect could be improved as well. The disorder is normally diagnosed with pen-and-paper methods, which generally do not assess patients in everyday tasks and do not address some forms of the disorder. Recently, promising new methods based on virtual reality have emerged. Virtual reality technologies hold great opportunities for the development of effective assessment and treatment techniques for neglect because they provide rich, multimodal, and highly controllable environments. In order to stimulate advancements in this domain, we present a review and an analysis of the current work. We describe past and ongoing research of virtual reality applications for unilateral neglect and discuss the existing problems and new directions for development.

Pseudoneglect for the bisection of mental number lines

Patients with unilateral neglect of the left side bisect physical lines to the right whereas individuals with an intact brain bisect lines slightly to the left (pseudoneglect). Similarly, for mental number lines, which are arranged in a left-to-right ascending sequence, neglect patients bisect to the right. This study determined whether individuals with an intact brain show pseudoneglect for mental number lines. In Experiment 1, participants were presented with visual number triplets (e.g., 16, 36, 55) and determined whether the numerical distance was greater on the left or right side of the inner number. Despite changing the spatial configuration of the stimuli, or their temporal order, the numerical length on the left was consistently overestimated. The fact that the bias was unaffected by physical stimulus changes demonstrates that the bias is based on a mental representation. The leftward bias was also observed for sets of negative numbers (Experiment 2)--demonstrating not only that the number line extends into negative space but also that the bias is not the result of an arithmetic distortion caused by logarithmic scaling. The leftward bias could be caused by a rounding-down effect. Using numbers that were prone to large or small rounding-down errors, Experiment 3 showed no effect of rounding down. The task demands were changed in Experiment 4 so that participants determined whether the inner number was the true arithmetic centre or not. Participants mistook inner numbers shifted to the left to be the true numerical centre--reflecting leftward overestimation. The task was applied to 3 patients with right parietal damage with severe, moderate, or no spatial neglect (Experiment 5). A rightward bias was observed, which depended on the severity of neglect symptoms. Together, the data demonstrate a reliable and robust leftward bias for mental number line bisection, which reverses in clinical neglect. The bias mirrors pseudoneglect for physical lines and most likely reflects an expansion of the space occupied by lower numbers on the left side of the line and a contraction of space for higher numbers located on the right.

Prism adaptation overcomes pseudoneglect for the greyscales task

Visuomotor adaptation to left-shifting prisms can affect performance for a variety of tasks in neurologically intact (normal) participants. This study examined whether visuomotor adaptation affects performance on the greyscales task in normal participants. Forty-two normal participants completed a greyscales task before and after adaptation to either: left-shifting prisms, right-shifting prisms or control spectacles that did not shift the visual scene. Participants demonstrated a leftward bias (i.e., selected the stimulus that was darker on the left as being darker overall) that was reversed by a short period of visuomotor adaptation to left-shifting prisms. In contrast, this bias was unaffected by adaptation to right-shifting prisms and control spectacles. The findings demonstrate that a simple visuomotor task can alter the distribution of spatial attention for the greyscales task in normal participants.

Spatial neglect: clinical and neuroscience review: a wealth of information on the poverty of spatial attention

Hemispatial neglect (HSN) is a frequent, conspicuous neurobehavioral accompaniment of brain injury. Patients with HSN share several superficial similarities, leading earlier clinical neuroscientists to view neglect as a unitary condition associated with brain structures that mediate relatively discrete spatial cognitive mechanisms. Over the last two decades, research largely deconstructed the neglect syndrome, revealing a remarkable heterogeneity of behaviors and providing insight into multiple component processes, both spatial and nonspatial, that contribute to hemispatial neglect. This review surveys visual HSN, presenting first the means for detection and diagnosis in its manifold variations. We summarize cognitive operations relevant to spatial attention and evidence for their role in neglect behaviors and then briefly consider neural systems that may subserve the component processes. Finally, we propose several methods for rehabilitating HSN, including the challenges facing remediation of such a heterogeneous cognitive disorder.

Numerical processing overcomes left neglect for the greyscales task

This study examined whether numerical cues affect spatial attention in left neglect. Patients with right parietal damage (four with neglect, three without) completed a dual task. Stimuli consisted of two greyscales overlayed with high numbers (8, 9), low numbers (1, 2) or neutral stimuli (musical sharp, &). Participants identified if the overlay was high, low or neutral and then made a relative luminance judgement for the greyscales. Neglect patients demonstrated a rightward greyscales bias in the neutral overlay condition, which was overcome by processing low numbers. Control patients showed a leftward bias in the neutral condition, which was overcome by processing high numbers. The results demonstrate that the spatial architecture of numbers induces shifts of attention, which can overcome left neglect.

Pseudoneglect for mental alphabet lines is affected by prismatic adaptation

While patients with right parietal damage and spatial neglect bisect lines to the right, the general population bisects lines to the left; a phenomenon known as pseudoneglect. The leftward bias also occurs for mental representations, such as number and alphabet lines. Prismatic adaptation can have a dramatic effect on attentional bias and corrects neglect and pseudoneglect for physical and mental number lines. This study examined whether prismatic adaptation can correct leftward bisection biases for alphabet lines, which may have a different spatial arrangement compared to number lines. In pre-adaptation testing, students (n = 42) were shown letter trigrams (e.g. C H P) and judged whether the alphabetical distance before or after the inner-letter was larger. Participants were then split into three groups and were adapted to left-shifting, control or right-shifting prims. After adaptation, the mental alphabet bisection task was re-administered. The length of left side of the alphabet lines was overestimated by all three groups in the pre-adaptation phase. Right-shifting prisms and control spectacles had no effect on the leftward bias whereas exposure to left-shifting prisms corrected the bias. The results replicate an effect observed for mental number lines and demonstrate that low-level sensory-motor shifts can correct attentional biases associated with high-level representations, such as letters.

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.

Rightward collisions and their association with pseudoneglect

Whereas right parietal damage can result in left hemineglect, the general population shows a subtle neglect of the right hemispace-known as pseudoneglect. A recent study has demonstrated that people collide to the right more often and attributed this bias to pseudoneglect. [Nicholls, M. E. R., Loftus, A., Meyer, K., & Mattingley, J.B. (2007). Things that go bump in the right: The effect of unimanual activity on rightward collisions. Neuropsychologia, 45, 1122-1126]. Nicholls examined the effect of unimanual activation by requiring participants to fire projectiles at a target whilst walking and found that the rightward bias was exaggerated or reversed when the left and right hands were active, respectively. However, the act of aiming at a target may have inadvertently biased walking trajectory to the right. The current study addressed this issue by requiring participants (n=149) to walk through a narrow doorway three times whilst entering text into a phone using the (a) left, (b) right or (c) both hands. Despite the fact that entering text into a phone should produce no rightward bias, participants bumped to the right more often. Unlike previous research, no effect of unimanual activation was observed. This lack of effect was attributed to the smaller hand movements for entering numbers compared to firing a toy gun. Finally, this study showed an association for the first time between biases in observable bumping and line bisection performance-suggesting that unilateral bumping is related to pseudoneglect.

Seeing and imagining the "same" objects in unilateral neglect

Dissociations between perceptual and imaginal neglect are typically investigated by using very different tasks (e.g. visual target cancellation vs. place descriptions). Here we report patients' performance on imaginal and perceptual tasks which shared identical stimuli and procedure, except for the modality of stimulus presentation. In different tasks, participants either saw towns/regions on a map of France or heard their names, and pressed one of two keys according to the stimulus location (left or right of Paris). Neglect patients as a group were less accurate for left-sided stimuli in both modalities, but on single-case analysis only one patient with perceptual neglect had asymmetrical imaginal accuracy. These results confirmed that imaginal neglect is relatively rare, and is usually associated with perceptual neglect. Perceptual neglect resulted more frequent and severe than imaginal neglect, also when assessed using strictly comparable tests, consistent with the exogenous orienting bias typically observed in these patients.

Visual neglect

PURPOSE OF REVIEW

Left visual neglect is a frequent and dramatic consequence of right hemisphere lesions. Diagnosis is important because behavioural and pharmacological treatments are available. Furthermore, neglect raises important issues concerning the brain mechanisms of consciousness, perception and attention.

RECENT FINDINGS

Recent behavioural findings and new techniques, such as transcranial magnetic stimulation, direct cortical and subcortical stimulation during brain surgery, and diffusion tensor imaging tractography, have provided evidence relevant to the debate concerning the functional mechanisms and the anatomical bases of neglect.

SUMMARY

Several component deficits appear to interact in producing different forms of neglect. Rather than lesions at single cortical levels, dysfunction of large-scale brain networks, often induced by white matter disconnection, may constitute the crucial antecedent of neglect signs.

Line bisection in left neglect: the importance of starting right

When marking the subjective midpoint of a horizontal line, patients with left unilateral neglect typically deviate rightward. Different accounts of this pattern of performance refer either to a biased competition between the two hemi-segments of the line, with the right part being subjectively perceived as longer than the left part, or to a distortion of a cognitive representation of space, with spatial coordinates progressively relaxing from the right to the left. These accounts make different predictions about the role of the right part of the line, which is crucial in the biased competition account, but less important in the distortion account. To test these predictions, we asked participants to set the endpoints and the centre of perceived and imaginary lines. Contrary to previous studies, we controlled for the direction of performance of the endpoint task, with left-to-right trials and right-to-left trials being performed in separate blocks. Five patients with right-hemisphere lesions and left neglect demonstrated the typical asymmetries when a right-sided stimulus (segment or endpoint) was present, but showed either normal performance or a reversed (leftward) bias while setting the endpoints and the centre of an imaginary line starting from the left side, when no right-sided visual stimulus was present until completion of each trial. We concluded that the right-sided portion of the line has a crucial importance in determining patients' rightward deviations in line bisection, consistent with the biased competition hypothesis and with neurocognitive models of attentional orienting.

A lateralized bias in mental imagery: evidence for representational pseudoneglect

One hundred right-handed healthy individuals were asked to imagine a familiar scene (the Piazza del Duomo, Milan) from two opposite viewpoints and report what they could see. For elements that should be visible from the participants' viewpoint, more elements were reported from the left side of the image than from the right, irrespective of view. These results establish that there is a lateralized bias in reporting the details in mental images--representational pseudoneglect. This bias is in the opposite direction and significantly smaller than the bias seen in individuals with representational neglect following right hemisphere damage. Representational pseudoneglect appears analogous to perceptual pseudoneglect and the two may share an underlying mechanism. The results are interpreted as indicating that pseudo-representational neglect arises as the result of a bias in the allocation of attention to the imagined scene.

Pseudoneglect and neglect for mental alphabet lines

While patients with right parietal damage neglect the left side of stimuli, the intact-brain population shows a slight neglect of the right side--known as pseudoneglect. Although pseudoneglect occurs for physical stimuli, it is not certain whether the bias extends to mental representations. To investigate this issue, we examined spatial distortions in the representation of length for mental alphabet lines, which are thought to have a left-to-right arrangement. In Expt. 1, participants (n=10) were presented with letter strings (e.g. C_H_P) and estimated whether the letter length was greater on the left or right side of the inner-letter. The strings were presented simultaneously along a line or sequentially in the centre of the screen in either an ascending (i.e. A-Z) or descending (i.e. Z-A) sequence. Participants reliably overestimated the length on the left regardless of presentation mode. In Expt. 2, participants (n=20) judged whether the inner-letter was the true centre. Responses were biased such that inner-letters shifted to the left of true centre were perceived to be the centre. Combined, both studies demonstrate that the length on left side of the mental alphabet line is overestimated relative to the right. In Expt. 3, a reversal of the bias towards the right was found for a group of neglect patients. The data demonstrate that letters have a left-to-right mental representation and that the left side of this representation is overrepresented in a manner similar to the overestimation associated with pseudoneglect for physical stimuli.

Négligence spatiale unilatérale : une conséquence dramatique mais souvent négligée des lésions de l’hémisphère droit

INTRODUCTION

Unilateral Spatial Neglect (USN) is a common consequence of right brain damage. In the most severe cases, behavioral signs of USN can last several years and compromise patients' autonomy and social rehabilitation. These clinical facts stress the need for reliable procedures of diagnosis and rehabilitation.

STATE OF THE ART

The last 3 decades have witnessed an explosion of studies on USN, which raises issues related to complex cognitive activities such as mental representation, spatial attention and consciousness. USN is probably a heterogeneous syndrome, but some of its underlying mechanisms might be understood as an association of disorders of spatial attention. A bias of automatic orienting towards right-sided objects seems typical of left USN. Afterwards, patients find it difficult to disengage their attention in order to explore the rest of the visual scene. Neglected objects are sometimes processed in an "implicit" way.

PERSPECTIVES

The development of behavioural paradigms and of neuroimaging techniques and their application to the study of USN has advanced our understanding of the functional mechanisms of attention and spatial awareness, as well as of their neural bases. A number of new procedures for rehabilitation have recently been proposed.

CONCLUSION

The present review describes the clinical presentation of USN, its anatomical basis and some of possible accounts of different aspects of neglect behavior. Results of computer simulations and of rehabilitation techniques are also presented with implications for the functioning of normal neurocognitive systems.

A disengagement deficit in representational space

Some patients with hemispatial neglect show a deficit of "disengagement", i.e., their left-sided inattention is largely a consequence of an inability to move the spotlight of attention away from right-sided stimuli. We report a neglect patient with a failure of disengagement in imagined space, a feature not previously described. The patient was repeatedly moved along a hallway and had to memorize 20 objects placed alongside the walls (alternating starting points). Each learning run was followed by a recall run, in which objects had to be named in their correct sequence from one imaginary starting point. Initially, when performance was still poor, only right-sided items were named, a response pattern mimicking a neglect of representational space. However, as recall improved over successive runs, left-sided objects were as well memorized as right-sided, but the latter were named before the former. By contrast, if photographs of single objects were presented in the center of a screen for laterality decisions, neither accuracy nor latency of the patient's decisions differentiated between left-sided and right-sided items. We interpret the sticking to the right side during initial periods of free recall, in the absence of side-differences during cued recognition, as a failure to disengage from the right side of a mental image. In view of the extensive cortical and subcortical lesions in our patient the current debate about the functional neuroanatomy of this deficit cannot be resolved. However, the present report adds to our understanding of the heterogeneous nature of deficiencies in the representation of space.

Time and neglect: abnormal temporal dynamics in unilateral spatial neglect

Temporality enters our immediate experience as passage and becoming: the role time plays in the construction of a world of enduring entities tends to go unnoticed. This paper examines the relation between time and ontology in the context of unilateral neglect, a neuropsychological syndrome in which patients fail to perceive or respond to stimuli in the contralateral hemifield, behaving as if that half of space does not exist. Traditional models characterize neglect exclusively in spatial terms. Based on recent investigations suggesting abnormal temporal dynamics, here we highlight the impact of time factors on the presentation of the disorder. Neglect patients do not simply miss the presence of stimuli on the left: they also ignore the past as well as the future of neglected stimuli. We claim that, if this occurs, it is because time, and not only space, is impaired.