What the eyes perceive, the brain ignores: a case of pure unilateral 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.

Allochiria for spatial landmarks as the presenting feature of posterior cortical atrophy

Allochiria refers to the mislocation of stimuli to the corresponding position on the opposite side of the body or hemispace. It is most often, although not exclusively, reported in the tactile modality and typically in association with unilateral neglect. We describe a patient presenting with a 2-year history of topographical disorientation without other cognitive complaints. We conducted a systematic exploration of his topographical problems to identify their cognitive substrate. Standard neuropsychological examination revealed no abnormalities. Notably, he performed well on perceptual, spatial, and constructional tasks. No signs of neglect were elicited. A tailored battery of tests was administered, involving road maps and landmarks, and designed to replicate the situations in which he experienced symptoms. The experimental tests showed no evidence of topographical agnosia or amnesia for landmarks and their spatial relationships and no hemispatial neglect. Nevertheless, the patient exhibited a systematic tendency to translocate topographical landmarks sited on the left to the right side. The phenomenon, consistent with representational allochiria, occurred exclusively for topographical landmarks, and was present along both personally familiar and new learned routes. Over the next two years more widespread visuoperceptual and spatial deficits emerged, with Balint and Gerstmann syndromes. Functional imaging revealed hypoperfusion of the occipito-parietal regions and amyloid PET the presence of amyloid plaques. A diagnosis was made of posterior cortical atrophy, the visual variant of Alzheimer's Disease. To our knowledge this is the first case of topographical disorientation presenting with selective representational allochiria and the first report of allochiria as an early sign of posterior cortical atrophy. The case sheds light on the cognitive basis of allochiria and on a puzzling clinical presentation of neurodegenerative brain disease.

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.

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.

Working Memory in Unilateral Spatial Neglect: Evidence for Impaired Binding of Object Identity and Object Location

Working memory (WM) is known to be impaired in patients with stroke experiencing unilateral spatial neglect (USN). Here, we examined in a systematic manner three WM components: memory of object identity, memory of object location, and binding between object identity and location. Moreover, we used two different retention intervals to isolate maintenance from other mnemonic and perceptual processes. Fourteen USN first-event stroke patients with right-hemisphere damage were tested in two different WM experiments using long and short retention intervals and an analog response scale. Patients exhibited more identification errors for items displayed on the contralesional side. Localization errors were also more prominent in the contralesional side, especially after a long retention interval. These localization errors were often a result of swap errors, that is, erroneous localizations of correctly identified contralesional objects in correctly memorized locations of ipsilesional objects. We conclude that a key WM deficit in USN is a lateralized impairment in binding between the identity of an object and its spatial tag.

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.

Neuropsychological Changes in Complex Regional Pain Syndrome (CRPS)

Complex Regional Pain Syndrome (CRPS) is a poorly understood chronic pain condition of multifactorial origin. CRPS involves sensory, motor, and autonomic symptoms primarily affecting one extremity. Patients can also present with neuropsychological changes such as reduced attention to the CRPS-affected extremity, reminiscent of hemispatial neglect, yet in the absence of any brain lesions. However, this "neglect-like" framework is not sufficient to characterise the range of higher cognitive functions that can be altered in CRPS. This comprehensive literature review synthesises evidence of neuropsychological changes in CRPS in the context of potential central mechanisms of the disorder. The affected neuropsychological functions constitute three distinct but not independent groups: distorted body representation, deficits in lateralised spatial cognition, and impairment of non-spatially-lateralised higher cognitive functions. We suggest that many of these symptoms appear to be consistent with a broader disruption to parietal function beyond merely what could be considered "neglect-like." Moreover, the extent of neuropsychological symptoms might be related to the clinical signs of CRPS, and rehabilitation methods that target the neuropsychological changes can improve clinical outcomes in CRPS and other chronic pain conditions. Based on the limitations and gaps in the reviewed literature, we provide several suggestions to improve further research on neuropsychological changes in chronic pain.

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.

The 'Neglected' Personal Neglect

A review of patients with brain injury showing personal neglect is presented. The aim is to shed light on this aspect of neglect often unresearched or only indirectly investigated, and to discuss recent findings concerning the methods used to assess personal neglect, its neural correlates and its association with the more often explored aspect of extrapersonal neglect. The review was performed using PubMed and PsychInfo databases to search for papers published in the last 123 years (until January 2018). We reviewed 81 papers describing either single or group studies for a total of 2247 patients. The results of this review showed that various aspects of personal neglect are still controversial and outcomes potentially contradictory. Despite the data reported in the present review suggest that personal neglect is more frequently associated with lesions of the right hemisphere, the left hemisphere may also play an important role. Not surprisingly, personal neglect and extrapersonal neglect seem to co-occur. However double dissociations of these two forms of neglect have been reported, and they seem to dissociate both from a functional and an anatomical perspective. More recent interpretations of personal neglect suggest that it may result from a disrupted body representation. The development of reliable psychometric tools with shared diagnostic criteria is essential to identify different degrees of personal neglect for different body parts and to better refine personal neglect in comparison to extrapersonal neglect and disorders related to distortions of personal domain.

Fly on the right: Lateral preferences when choosing aircraft seats

A small preference has been observed for people to choose seats on the left of aircraft when booking via an online system. Although this is consistent with pseudoneglect-the known leftward bias in perception and representation-rightward preferences have been commonly observed in seating selection tasks in other environments. Additionally, the previous research in aircraft seating was unable to dissociate a bias to one side of the screen from a bias to one side of the cabin of the aircraft. Here, we present a study in which participants were asked to select seats for a range of fictional flights. They demonstrated a preference for seats on the right of the cabin, irrespective of whether the right of the cabin appeared to either the right or the left of the screen, a preference for seats towards the front of the aircraft and a preference to favour window and aisle seats. This suggests, in contrast to previous research, that participants demonstrated a rightward lateral bias to representations of an aircraft. These results may have implications for our understanding of asymmetries in cognition as well as having potentially important practical implications for airlines.

Unconscious Imagination and the Mental Imagery Debate

Traditionally, philosophers have appealed to the phenomenological similarity between visual experience and visual imagery to support the hypothesis that there is significant overlap between the perceptual and imaginative domains. The current evidence, however, is inconclusive: while evidence from transcranial brain stimulation seems to support this conclusion, neurophysiological evidence from brain lesion studies (e.g., from patients with brain lesions resulting in a loss of mental imagery but not a corresponding loss of perception and vice versa) indicates that there are functional and anatomical dissociations between mental imagery and perception. Assuming that the mental imagery and perception do not overlap, at least, to the extent traditionally assumed, then the question arises as to what exactly mental imagery is and whether it parallels perception by proceeding via several functionally distinct mechanisms. In this review, we argue that even though there may not be a shared mechanism underlying vision for perception and conscious imagery, there is an overlap between the mechanisms underlying vision for action and unconscious visual imagery. On the basis of these findings, we propose a modification of Kosslyn's model of imagery that accommodates unconscious imagination and explore possible explanations of the quasi-pictorial phenomenology of conscious visual imagery in light of the fact that its underlying neural substrates and mechanisms typically are distinct from those of visual experience.

Related but different: Examining pseudoneglect in audition, touch and vision

Although researchers have consistently demonstrated a leftward attentional bias in visual and representational (e.g. tactile/mental number line) line bisection tasks, the results from audition have been mixed. Differences in methodology between auditory and visual bisection tasks, especially with regards to the location of stimuli of peripersonal versus extrapersonal space, have also meant that researchers have not been able to compare performance in visual, tactile and auditory line bisection directly. In this research, 39 neurologically typical individuals participated in standard visual and tactile line bisection tasks, together with a newly developed auditory line bisection task. Results demonstrated significant leftward bisection biases across all three modalities. Hence, we demonstrate auditory pseudoneglect in peripersonal space for the first time. Tactile and auditory line bisections showed a relatively small but statistically reliable correlation, but neither task correlated with visual line bisection. This suggests that the processes underlying auditory line bisection are not synonymous to those involved in visual perceptual bisection, and further we argue that this bias may be related to representational pseudoneglect.

Cognitive Mechanisms of Visual Memories and Visual Images

Two experiments are reported that investigate recent claims that a visual memory and a visual image access different mechanisms within working memory and are differentially susceptible to interference. Experiment 1 demonstrates that when participants are required to hold the visual memory in consciousness, the representation is liable to interference in the same way as a visual image. Experiment 2 strengthens this interpretation by showing that the interference is specifically visual rather than general. Additionally, the experiments argue that visual noise interference is front loaded and does not require access to long-term memory structures prior to causing disruption.

Visual Memory in Unilateral Spatial Neglect: Immediate Recall versus Delayed Recognition

Patients with unilateral spatial neglect (USN) often show impaired performance in spatial working memory tasks, apart from the difficulty retrieving “left-sided” spatial data from long-term memory, shown in the “piazza effect” by Bisiach and colleagues. This study's aim was to compare the effect of the spatial position of a visual object on immediate and delayed memory performance in USN patients. Specifically, immediate verbal recall performance, tested using a simultaneous presentation of four visual objects in four quadrants, was compared with memory in a later-provided recognition task, in which objects were individually shown at the screen center. Unlike healthy controls, USN patients showed a left-side disadvantage and a vertical bias in the immediate free recall task (69% vs. 42% recall for right- and left-sided objects, respectively). In the recognition task, the patients correctly recognized half of “old” items, and their correct rejection rate was 95.5%. Importantly, when the analysis focused on previously recalled items (in the immediate task), no statistically significant difference was found in the delayed recognition of objects according to their original quadrant of presentation. Furthermore, USN patients were able to recollect the correct original location of the recognized objects in 60% of the cases, well beyond chance level. This suggests that the memory trace formed in these cases was not only semantic but also contained a visuospatial tag. Finally, successful recognition of objects missed in recall trials points to formation of memory traces for neglected contralesional objects, which may become accessible to retrieval processes in explicit memory.

Representational pseudoneglect: a review

Pseudoneglect, the tendency to be biased towards the left-hand side of space, is a robust and consistent behavioural observation best demonstrated on the task of visuospatial line bisection, where participants are asked to centrally bisect visually presented horizontal lines at the perceived centre. A number of studies have revealed that a representational form of pseudoneglect exists, occurring when participants are asked to either mentally represent a stimulus or explore a stimulus using touch in the complete absence of direct visuospatial processing. Despite the growing number of studies that have demonstrated representational pseudoneglect there exists no current and comprehensive review of these findings and no discussion of a theoretical framework into which these findings may fall. An important gap in the current representational pseudoneglect literature is a discussion of the developmental trajectory of the bias. The focus of the current review is to outline studies that have observed representational pseudoneglect in healthy participants, consider a theoretical framework for these observations, and address the impact of lifespan factors such as cognitive ageing on the phenomenon.

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.

Environment and object mental images in patients with representational neglect: two case reports

The aim of this study was to shed light on the nature of the imagery deficits in two patients with representational neglect and to determine whether representational neglect is affected by the content of the mental images the patients have to generate, inspect and manipulate. In particular, we submitted two patients with different types of representational neglect to a battery of visual mental imagery tests to assess the different kinds of imagery processes. We found that Patient 1, whose performance was asymmetrical on the O'Clock Test, performed poorly on tasks involving the mental generation, inspection and manipulation of objects but showed no deficit on tasks involving the mental generation, inspection and manipulation of environments. On the other hand, Patient 2, whose performance was asymmetrical on the Familiar Squares Description Test, performed poorly on tasks involving the mental generation, inspection and manipulation of environments, but not on tasks involving the mental generation, inspection and manipulation of objects. Our results demonstrate that environments and objects in the imagery domain can be represented separately and can be selectively affected by damage following brain lesions.

Does hemineglect affect visual mental imagery? Imagery deficits in representational and perceptual neglect

To give new insight about the relationship between imagery processes and different types of hemispatial neglect, we assessed different mental imagery abilities in a sample of right- and left-brain-damaged patients. Furthermore, because of reports of a mental representation disorder for environments in patients affected by representational neglect we also tested their navigational imagery ability. We found that patients with no signs of perceptual or representational neglect performed flawlessly on our imagery tasks regardless of whether they had left- or right-sided lesions. By contrast, patients affected by neglect failed most of the tests; in particular, representational neglect patients failed one test of mental transformation and tests requiring the manipulation of cognitive maps. These results suggest there is a specific relationship between hemispatial neglect and deficits in visual mental imagery and demonstrate that the right hemisphere plays a specific role in visual mental imagery.

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.

Items on the left are better remembered

Neurologically intact individuals show a spatial processing bias in perception tasks, specifically showing a bias towards the left in bisecting lines. We present evidence for a novel finding that a leftwards bias occurs in short-term memory for recently presented arbitrary bindings of visual features. Three experiments are reported, two of which involve a total of over 60,000 participants with a small number of trials for each. Experiment 3 involved a larger number of trials for each of 144 participants. Participants reproduced from immediate memory arrays of shape–colour–location bindings. In all three experiments, significantly more errors were observed in reproduction of items presented on the right of the array than on the left. Results could not be accounted for by perceptual errors, or by order of presentation or order of reproduction. Findings suggest that items presented on the left are better remembered, indicating a spatial asymmetry in forming or retrieving feature bindings in visual short-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.

Selective interference with image retention and generation: evidence for the workspace model

We address three types of model of the relationship between working memory (WM) and long-term memory (LTM): (a) the gateway model, in which WM acts as a gateway between perceptual input and LTM; (b) the unitary model, in which WM is seen as the currently activated areas of LTM; and (c) the workspace model, in which perceptual input activates LTM, and WM acts as a separate workspace for processing and temporary retention of these activated traces. Predictions of these models were tested, focusing on visuospatial working memory and using dual-task methodology to combine two main tasks (visual short-term retention and image generation) with two interference tasks (irrelevant pictures and spatial tapping). The pictures selectively disrupted performance on the generation task, whereas the tapping selectively interfered with the retention task. Results are consistent with the predictions of the workspace model.

The two sides of the mental clock: the Imaginal HemiSpatial Effect in the healthy brain

When subjects are asked to compare the mental images of two analog clocks telling different times (the mental clock test), they are faster to process angles formed by hands located in the right than in the left half of the dial. In the present paper, we demonstrate that this Imaginal HemiSpatial Effect (IHSE) can be also observed in two modified versions of the mental clock test: in Experiment 1 subjects had to imagine the position occupied by two numbers within a clock face and to judge if the subtended angle was smaller or greater than 90 degrees , while in Experiment 2 subjects were asked to match two imagined positions on a clock face with two visually presented locations. The finding of the IHSE in these tasks suggested that it is a genuine phenomenon related to spatial imagery tasks, and is consistent with recent neuroimaging evidence that the left parietal lobe is mainly involved in the generation of spatial mental images.

Movement and visual coding: the structure of visuo-spatial working memory

The influential model of verbal working memory (WM) introduced by Baddeley and Hitch (Recent advances in learning and motivation. Academic, New York, 1974) comprised three interacting component parts; an executive controller and two subservient systems. The two subservient systems, one underpinning verbal processing and the other underpinning visual processing are themselves subdivided. In the verbal system, a passive phonological store is maintained by an active phonological loop, which is able to rehearse the material in the passive store. The visual working memory system has traditionally been thought of as having a similar architecture with a passive visual store being maintained by an active store, which codes in terms of movement over space. The paper discusses the evidence for this relationship in visuo-spatial WM and concludes that the architecture does not fit well with the experimental literature. A direction for future research is suggested.

Remembering the past and imagining the future: a neural model of spatial memory and imagery

The authors model the neural mechanisms underlying spatial cognition, integrating neuronal systems and behavioral data, and address the relationships between long-term memory, short-term memory, and imagery, and between egocentric and allocentric and visual and ideothetic representations. Long-term spatial memory is modeled as attractor dynamics within medial-temporal allocentric representations, and short-term memory is modeled as egocentric parietal representations driven by perception, retrieval, and imagery and modulated by directed attention. Both encoding and retrieval/imagery require translation between egocentric and allocentric representations, which are mediated by posterior parietal and retrosplenial areas and the use of head direction representations in Papez's circuit. Thus, the hippocampus effectively indexes information by real or imagined location, whereas Papez's circuit translates to imagery or from perception according to the direction of view. Modulation of this translation by motor efference allows spatial updating of representations, whereas prefrontal simulated motor efference allows mental exploration. The alternating temporal-parietal flows of information are organized by the theta rhythm. Simulations demonstrate the retrieval and updating of familiar spatial scenes, hemispatial neglect in memory, and the effects on hippocampal place cell firing of lesioned head direction representations and of conflicting visual and ideothetic inputs.

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.

Alignment effect: primary-secondary learning and cognitive styles

The degree to which the way of learning spatial information (primary/secondary learning) and spatial cognitive style (landmark/route/survey) affect orientation specificity (alignment effect) is studied. We think that the most important factor explaining the absence of the alignment effect is the spatial cognitive style. We hypothesise that while landmark participants show an alignment effect after both primary and secondary learning, route participants show this effect only after secondary learning, and survey participants do not show it at all. Participants performed three tasks in order to distinguish their cognitive style; they were then randomly assigned to primary or secondary learning and submitted to directional judgment tasks to verify whether the alignment effect was present. The results confirm our hypothesis.

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.

Productive and Optic Prism Exposureproductive and Defective Impairments in the Neglect Syndrome: Graphic Perseveration, Drawing Productions and Optic Prism Exposure

The effects of adaptation to prisms displacing rightwards the field of vision on omission errors, and on perseveration and other graphic productions in a line cancellation task, were assessed in nine right-brain-damaged patients with left unilateral spatial neglect. Prism adaptation improved both neglect, as indexed by omission errors, and perseveration behaviour, up to a delay of 60 min. No correlation was found between omission and perseveration errors in all assessments. The suggestion is made that perseveration and other complex graphic productions made by right brain-damaged-patients with left spatial unilateral neglect is due to a defective monitoring of complex motor behaviour, frequently associated to cerebral damage involving the right frontal lobe. Interpretations of perseveration behaviour in terms of allochiria and directional hypokinesia are considered, and their limits discussed.

Neural mechanisms underlying spatial judgements on seen and imagined visual stimuli in the left and right hemifields in men

We investigated the neural networks involved in spatial judgements on visual and imagined stimuli in the left and right hemifields. Twenty healthy male right-handers were scanned using fMRI. In an adaptation of the Mental Clock Test, subjects judged whether the angle between visually presented or to be imagined clock hands at a given time (e.g. "12:20") was >90 degrees or <90 degrees . The angle formed by the clock hands was always located either on the left or right hemifield of the clock face, constituting a 2x2 factorial design with factors stimulus type (visual "V", imagined "I") and attention to spatial hemifield (left "L", right "R"). The contrast I>V revealed a bilateral neural network which included the superior parietal, prefrontal and antero-dorsal cingulate cortex. While right superior parietal cortex was activated during both RV and LV, left superior parietal cortex was only active during RV. The contrast RI versus LI (and vice versa) did not reveal significant differences with respect to activation height. However, a masking procedure and the calculation of a laterality index showed that RI recruited larger areas in the left than in the right hemisphere, while LI led to symmetrical activations in both hemispheres. Our data thus confirm that there are hemifield-specific asymmetries in brain activity during spatial processing of both visual and imagined stimuli. Our data indicate, however, that these asymmetries are less clear and of a different nature in the latter. Overall, our findings converge with neuropsychological data showing that representational and visuospatial neglect do not always co-occur.