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The ability to distinguish left from right has been shown to vary substantially within healthy individuals, yet its characteristics and mechanisms are poorly understood. In three experiments, we focused on a detailed description of the ability to distinguish left from right and the role of individual differences, and further explored the potential underlying mechanisms. In Experiment 1, a questionnaire concerning self-reported left-right identification (LRI) and strategy use was administered. Objective assessment was used in Experiment 2 by means of vocal responses to line drawings of a figure, with the participants' hands in a spatially neutral position. In Experiment 3, the arm positions and visibility of the hands were manipulated to assess whether bodily posture influences left-right decisions. Results indicate that 14.6% of the general population reported insufficient LRI and that 42.9% of individuals use a hand-related strategy. Furthermore, we found that spatial alignment of the participants' arms with the stimuli increased performance, in particular with a hand-related strategy and females. Performance was affected only by the layout of the stimuli, not by the position of the participant during the experiment. Taken together, confusion about left and right occurs within healthy population to a limited extent, and a hand-related strategy affects LRI. Moreover, the process involved appears to make use of a stored body representation and not bottom-up sensory input. Therefore, we suggest a top-down body representation is the key mechanism in determining left and right, even when this is not explicitly part of the task.

The neurocognitive architecture of navigation ability has been investigated by extensively studying the navigation problems of individual neurological patients. These neuropsychological case reports have applied highly variable approaches to establish navigation impairment in their patients. This review provides a systematic and up-to-date inventory of all relevant case studies and presents an analysis of the types of navigation impairments that have been described. The systematic literature search revealed 58 relevant papers reporting on 67 neurological patients. Close analysis of their patterns of navigation performance suggests three main categories of navigation impairments. These categories are related to three types of representations that are considered highly relevant for accurate navigation: knowledge of landmarks, locations, and paths. The resulting model is intended to serve both clinical and theoretical advances in the study of navigation ability and its neural correlates., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Recent studies have shown that navigation impairment is a common complaint after brain injury. Effective training programmes aiming to improve navigation ability in neurological patients are, however, scarce. The few reported programmes are merely focused on recalling specific routes rather than encouraging brain-damaged patients to use an alternative navigation strategy, applicable to any route. Our aim was therefore to investigate the feasibility of a (virtual reality) navigation training as a tool to instruct chronic stroke patients to adopt an alternative navigation strategy. Navigation ability was systematically assessed before the training. The training approach was then determined based on the individual pattern of navigation deficits of each patient. The use of virtual reality in the navigation strategy training in six middle-aged stroke patients was found to be highly feasible. Furthermore, five patients learned to (partially) apply an alternative navigation strategy in the virtual environment, suggesting that navigation strategies are mouldable rather than static. In the evaluation of their training experiences, the patients judged the training as valuable and proposed some suggestions for further improvement. The notion that the navigation strategy people use can be influenced after a short training procedure is a novel finding and initiates a direction for future studies.

The aim of this study was to explore how people use egocentric (i.e., with respect to their body) and allocentric (i.e., with respect to another element in the environment) references in combination with coordinate (metric) or categorical (abstract) spatial information to identify a target element. Participants were asked to memorize triads of 3D objects or 2D figures, and immediately or after a delay of 5 s, they had to verbally indicate what was the object/figure: (1) closest/farthest to them (egocentric coordinate task); (2) on their right/left (egocentric categorical task); (3) closest/farthest to another object/figure (allocentric coordinate task); (4) on the right/left of another object/figure (allocentric categorical task). Results showed that the use of 2D figures favored categorical judgments over the coordinate ones with either an egocentric or an allocentric reference frame, whereas the use of 3D objects specifically favored egocentric coordinate judgments rather than the allocentric ones. Furthermore, egocentric judgments were more accurate than allocentric judgments when the response was Immediate rather than delayed and 3D objects rather than 2D figures were used. This pattern of results is discussed in the light of the functional roles attributed to the frames of reference and spatial relations by relevant theories of visuospatial processing.

Objectives: An increasing number of studies have presented evidence that various patient groups with acquired brain injury suffer from navigation problems in daily life. This skill is, however, scarcely addressed in current clinical neuropsychological practice and suitable diagnostic instruments are lacking. Real-world navigation tests are limited by geographical location and associated with practical constraints. It was, therefore, investigated whether virtual navigation might serve as a useful alternative., Methods: To investigate the convergent validity of virtual navigation testing, performance on the Virtual Tubingen test was compared to that on an analogous real-world navigation test in 68 chronic stroke patients. The same eight subtasks, addressing route and survey knowledge aspects, were assessed in both tests. In addition, navigation performance of stroke patients was compared to that of 44 healthy controls., Results: A correlation analysis showed moderate overlap (r = .535) between composite scores of overall real-world and virtual navigation performance in stroke patients. Route knowledge composite scores correlated somewhat stronger (r = .523) than survey knowledge composite scores (r = .442). When comparing group performances, patients obtained lower scores than controls on seven subtasks. Whereas the real-world test was found to be easier than its virtual counterpart, no significant interaction-effects were found between group and environment., Conclusions: Given moderate overlap of the total scores between the two navigation tests, we conclude that virtual testing of navigation ability is a valid alternative to navigation tests that rely on real-world route exposure.

In this large scale, individual differences study (N = 521), the effects of cardinal axes of an environment and the path taken between locations on distance comparisons were assessed. The main goal was to identify if and to what extent previous findings in simple 2D tasks can be generalized to a more dynamic, three-dimensional virtual reality environment. Moreover, effects of age and gender were assessed. After memorizing the locations of six objects in a circular environment, participants were asked to judge the distance between objects they encountered. Results indicate that categorization (based on the cardinal axes) was present, as distances within one quadrant were judged as being closer together, even when no visual indication of the cardinal axes was given. Moreover, strong effects of the path taken between object locations were found; objects that were near on the path taken were perceived as being closer together than objects that were further apart on this path, regardless of the metric distance between the objects. Males outperformed females in distance comparison, but did not differ in the extent of the categorization and path effects. Age also affected performance; the categorization and path effects were highly similar across the age range tested, but the general ability to estimate distances does show a clear pattern increase during development and decrease with aging.

Route memory is frequently assessed in virtual environments. These environments can be presented in a fully controlled manner and are easy to use. Yet they lack the physical involvement that participants have when navigating real environments. For some aspects of route memory this may result in reduced performance in virtual environments. We assessed route memory performance in four different environments: real, virtual, virtual with directional information (compass), and hybrid. In the hybrid environment, participants walked the route outside on an open field, while all route information (i.e., path, landmarks) was shown simultaneously on a handheld tablet computer. Results indicate that performance in the real life environment was better than in the virtual conditions for tasks relying on survey knowledge, like pointing to start and end point, and map drawing. Performance in the hybrid condition however, hardly differed from real life performance. Performance in the virtual environment did not benefit from directional information. Given these findings, the hybrid condition may offer the best of both worlds: the performance level is comparable to that of real life for route memory, yet it offers full control of visual input during route learning.

The tablet computer initiates an important step toward computerized administration of neuropsychological tests. Because of its lack of standardization, the Corsi Block-Tapping Task could benefit from advantages inherent to computerization. This task, which requires reproduction of a sequence of movements by tapping blocks as demonstrated by an examiner, is widely used as a representative of visuospatial attention and working memory. The aim was to validate a computerized version of the Corsi Task (e-Corsi) by comparing recall accuracy to that on the standard task. Forty university students (Mage = 22.9 years, SD = 2.7 years; 20 female) performed the standard Corsi Task and the e-Corsi on an iPad 3. Results showed higher accuracy in forward reproduction on the standard Corsi compared with the e-Corsi, whereas backward performance was comparable. These divergent performance patterns on the 2 versions (small-to-medium effect sizes) are explained as a result of motor priming and interference effects. This finding implies that computerization has serious consequences for the cognitive concepts that the Corsi Task is assumed to assess. Hence, whereas the e-Corsi was shown to be useful with respect to administration and registration, these findings also stress the need for reconsideration of the underlying theoretical concepts of this task.

Any spatial situation can be approached either categorically - the window is to my left - or coordinately - the glass is 20cm away from the bottle. Since the first description of the distinction between categorical and coordinate spatial relation processing, it has often been shown that they are processed by at least partially different underlying mechanisms, mainly located in the left and right hemisphere, respectively. A number of recent studies have suggested that spatial attention plays a particularly important part in the perception of space: categorical processing benefits from a local focus of attention, and coordinate processing profits from a global focus of attention. This suggests that the lateralization pattern is modified by the concurrent size of the attentional focus, and is consequently more dynamic than previously thought. Therefore, a thorough revision of earlier theories on spatial relation processing is in order. In this review, we present a new model on lateralization of spatial relation processing that explicitly describes the role of spatial attention., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Objective: The aim of this study was to measure the prevalence of navigation problems in patients with mild stroke, using a navigation questionnaire (the Wayfinding Questionnaire; WQ). In addition, the correlations between WQ scores and quality of life measures and neuropsychological test scores were studied., Methods: A sample of 62 patients with mild stroke completed a questionnaire measuring self-reported navigation ability and spatial anxiety. A subset of this sample (n = 31) also completed a questionnaire on quality of life. Additional relevant neuropsychological data were retrieved from medical files and correlated with WQ and quality of life scores., Results: The results indicate that self-reported navigation impairment occurs in a substantial proportion of patients (29.0%), compared with a large control group (n = 384) of which 19.9% showed impairment. Moreover, these ratings are closely linked to quality of life and negatively correlated with spatial anxiety. The neuro-psychological data show that there is very little correlation between scores on commonly administered tests and navigation ability, which is in line with the results of a previous study., Conclusion: As our data indicate that navigation impairment is common among patients with mild stroke, we recommend a specific focus on navigation ability as part of neuropsychological assessment. This focus is currently lacking. Furthermore, the use of dedicated, experimental navigation tests in cases of explicit problems with navigation should be considered, in order objectively to measure such impairments.

Patient NC showed impairment on several tasks making use of coordinate spatial information, while categorical processing was at control level. Her assessment of local and global features of visual stimuli indicated that she had a local bias of attention, whereas controls showed a global bias. Her problems with coordinate tasks can be explained by this reduced global attentional focus. These findings confirm previous reports suggesting that the processing of categorical spatial relations benefits from a small scope of attention, whereas a relatively large scope of attention enhances coordinate spatial relation processing.

Spatial relations between objects can be represented in a categorical and in a coordinate manner. Categorical representations reflect abstract relations, like 'left of' or 'under', whereas coordinate representations concern exact metric distances between objects. These two types of spatial relations are thought to be linked to a left hemisphere and a right hemisphere advantage, respectively. This lateralization pattern was examined in a visual search task, making use of natural scenes, in patients with unilateral brain damage and healthy controls. In addition, all participants performed a low-level spatial relation processing task. The results suggest that the lateralization pattern commonly found for spatial relation processing in low-level perceptual tasks is also applicable to the processing of complex visual scenes.

Spatial relations are typically divided into categorical and coordinate spatial relations. Categorical relations are abstract and show a left hemisphere (LH) advantage, whereas coordinate relations are metric and related to a right hemisphere (RH) advantage. In the current study a working memory task was used to asses categorical and coordinate performance with two different stimulus sets. In this task, participants had to compare two sequentially presented stimuli, consisting of a dot and a cross. The cross size used in the stimuli was either large or small; a direct manipulation of the amount of information provided to determine a category, or to assess a distance. Patients with damage in the LH or the RH and highly comparable controls were tested. In control participants, categorical processing is faster with the use of a large cross, i.e., more visual information about category boundaries. In contrast, coordinate performance was more accurate with a small cross, i.e., presenting less unnecessary visual information. LH patients showed a specific defect in processing categorical stimuli with a small cross and coordinate stimuli with a large cross. The RH patients were impaired in all conditions except for the categorical small cross condition. We conclude that a larger amount of information present in stimuli increases categorical processing performance and decreases coordinate processing performance, while opposite effects are found for less stimulus information., (Copyright © 2011 Elsevier Srl. All rights reserved.)

Spatial relations are commonly divided in two global classes. Categorical relations concern abstract relations which define areas of spatial equivalence, whereas coordinate relations are metric and concern exact distances. Categorical and coordinate relation processing are thought to rely on at least partially separate neurocognitive mechanisms, as reflected by differential lateralization patterns, in particular in the parietal cortex. In this study we address this textbook principle from a new angle. We studied retinotopic activation in early visual cortex, as a reflection of attentional distribution, in a spatial working memory task with either a categorical or a coordinate instruction. Participants were asked to memorize a dot position, with regard to a central cross, and to indicate whether a subsequent dot position matched the first dot position, either categorically (opposite quadrant of the cross) or coordinately (same distance to the centre of the cross). BOLD responses across the retinotopic maps of V1, V2, and V3 indicate that the spatial distribution of cortical activity was different for categorical and coordinate instructions throughout the retention interval; a more local focus was found during categorical processing, whereas focus was more global for coordinate processing. This effect was strongest for V3, approached significance in V2 and was absent in V1. Furthermore, during stimulus presentation the two instructions led to different levels of activation in V3 during stimulus encoding; a stronger increase in activity was found for categorical processing. Together this is the first demonstration that instructions for specific types of spatial relations may yield distinct attentional patterns which are already reflected in activity early in the visual cortex.

This research is about the role of categorical and coordinate spatial relations and allocentric and egocentric frames of reference in processing spatial information. To this end, we asked whether spatial information is firstly encoded with respect to a frame of reference or with respect to categorical/coordinate spatial relations. Participants had to judge whether two vertical bars appeared on the same side (categorical) or at the same distance (coordinate) with respect to the centre of a horizontal bar (allocentric) or with respect to their body midline (egocentric). The key manipulation was the timing of the instructions: one instruction (reference frame or spatial relation) was given before stimulus presentation, the other one after. If spatial processing requires egocentric/allocentric encoding before coordinate/categorical encoding, then spatial judgements should be facilitated when the frame of reference is specified in advance. In contrast, if categorical and coordinate dimensions are primary, then a facilitation should appear when the spatial relation is specified in advance. Results showed that participants were more accurate and faster when the reference frame rather than the type of spatial relation was provided before stimulus presentation. Furthermore, a selective facilitation was found for coordinate and categorical judgements after egocentric and allocentric cues, respectively. These results suggest a hierarchical structure of spatial information processing where reference frames play a primary role and selectively interact with subsequent processing of spatial relations.

The corpus callosum is the largest white matter structure in the human brain, connecting cortical regions of both hemispheres. Complete and partial callosotomies or callosal lesion studies have granted more insight into the function of the corpus callosum, namely the facilitation of communication between the cerebral hemispheres. How the corpus callosum mediates this information transfer is still a topic of debate. Some pose that the corpus callosum maintains independent processing between the two hemispheres, whereas others say that the corpus callosum shares information between hemispheres. These theories of inhibition and excitation are further explored by reviewing recent behavioural studies and morphological findings to gain more information about callosal function. Additional information regarding callosal function in relation to altered morphology and dysfunction in disorders is reviewed to add to the discussion of callosal involvement in interhemispheric transfer. Both the excitatory and inhibitory theories seem likely candidates to describe callosal function, however evidence also exists for both functions within the same corpus callosum. For future research it would be beneficial to investigate the functional role of the callosal sub regions to get a better understanding of function and use more appropriate experimental methods to determine functional connectivity when looking at interhemispheric transfer., (Copyright © 2011 Elsevier B.V. All rights reserved.)

The majority of research on functional cerebral lateralization in primates revolves around vocal abilities, addressing the evolutionary origin of the human language faculty and its predominance in the left hemisphere of the brain. Right hemisphere specialization in spatial cognition is commonly reported in humans. This functional asymmetry is especially evident in the context of the unilateral neglect, a deficit in attention to and awareness of one side of space, that more frequently occurs after right-side rather than left-side brain damage. Since most of the research efforts are concentrated on vocalization in primates, much less is known about the presence or absence of spatial functions lateralization. Obtaining this knowledge can provide insight into the evolutionary aspect of the functionally lateralized brain of Homo sapiens and deliver refinement and validation of the nonhuman primate unilateral neglect model. This paper reviews the literature on functional brain asymmetries in processing spatial information, limiting the search to nonhuman primates, and concludes there is no clear evidence that monkeys process spatial information with different efficiency in the two hemispheres. We suggest that lateralization of spatial cognition in humans represents a relatively new feature on the evolutionary time scale, possibly developed as a by-product of the left hemisphere intrusion of language competence. Further, we argue that the monkey model of hemispatial neglect requires reconsideration., (Copyright © 2010 Elsevier B.V. All rights reserved.)

Numerous studies have focused on the distinction between categorical and coordinate spatial relations. Categorical relations are propositional and abstract, and often related to a left hemisphere advantage. Coordinate relations specify the metric information of the relative locations of objects, and can be linked to right hemisphere processing. Yet, not all studies have reported such a clear double dissociation; in particular the categorical left hemisphere advantage is not always reported. In the current study we investigated whether verbal and spatial strategies, verbal and spatial cognitive abilities, and gender could account for the discrepancies observed in hemispheric lateralization of spatial relations. Seventy-five participants performed two visual half field, match-to-sample tasks (Van der Ham, van Wezel, Oleksiak, & Postma, 2007; Van der Ham, Raemaekers, van Wezel, Oleksiak, and Postma, 2009) to study the lateralization of categorical and coordinate relation processing. For each participant we determined the strategy they used in each of the two tasks. Consistent with previous findings, we found an overall categorical left hemisphere advantage and coordinate right hemisphere advantage. The lateralization pattern was affected selectively by the degree to which participants used a spatial strategy and by none of the other variables (i.e., verbal strategy, cognitive abilities, and gender). Critically, the categorical left hemisphere advantage was observed only for participants that relied strongly on a spatial strategy. This result is another piece of evidence that categorical spatial relation processing relies on spatial and not verbal processes., (Copyright © 2011 Elsevier Inc. All rights reserved.)

We report two experiments on the relationship between allocentric/egocentric frames of reference and categorical/coordinate spatial relations. Jager and Postma (2003) suggest two theoretical possibilities about their relationship: categorical judgements are better when combined with an allocentric reference frame and coordinate judgements with an egocentric reference frame (interaction hypothesis); allocentric/egocentric and categorical/coordinate form independent dimensions (independence hypothesis). Participants saw stimuli comprising two vertical bars (targets), one above and the other below a horizontal bar. They had to judge whether the targets appeared on the same side (categorical) or at the same distance (coordinate) with respect either to their body-midline (egocentric) or to the centre of the horizontal bar (allocentric). The results from Experiment 1 showed a facilitation in the allocentric and categorical conditions. In line with the independence hypothesis, no interaction effect emerged. To see whether the results were affected by the visual salience of the stimuli, in Experiment 2 the luminance of the horizontal bar was reduced. As a consequence, a significant interaction effect emerged indicating that categorical judgements were more accurate than coordinate ones, and especially so in the allocentric condition. Furthermore, egocentric judgements were as accurate as allocentric ones with a specific improvement when combined with coordinate spatial relations. The data from Experiment 2 showed that the visual salience of stimuli affected the relationship between allocentric/egocentric and categorical/coordinate dimensions. This suggests that the emergence of a selective interaction between the two dimensions may be modulated by the characteristics of the task.

While neurons in posterior parietal cortex have been found to signal the presence of a salient stimulus among multiple items in a display, spatial summation within their receptive field in the absence of an attentional bias has never been investigated. This information, however, is indispensable when one investigates the mechanisms of spatial attention and competition between multiple visual objects. To examine the spatial summation rule in parietal area 7a neurons, we trained rhesus monkeys to fixate on a central cross while two identical stimuli were briefly displayed in a neuron's receptive field. The response to a pair of dots was compared with the responses to the same dots when they were presented individually. The scaling and power parameters of a generalized summation algorithm varied greatly, both across neurons and across combinations of stimulus locations. However, the averaged response of the recorded population of 7a neurons was consistent with a winner-take-all rule for spatial summation. A control experiment where a monkey covertly attended to both stimuli simultaneously suggests that attention introduces additional competition by facilitating the less optimal stimulus. Thus an averaging stage is introduced between ∼ 200 and 300 ms of the response to a pair of stimuli. In short, the summation algorithm over the population of area 7a neurons carries the signature of a winner-take-all operation, with spatial attention possibly influencing the temporal dynamics of stimulus competition, that is the moment that the "winner" takes "victory" over the "loser" stimulus.

Spatial relation information can be encoded in two different ways: categorically, which is abstract, and coordinately, which is metric. Although categorical and coordinate spatial relation processing is commonly conceived as relying on spatial representations and spatial cognitive processes, some suggest that representations and cognitive processes involved in categorical spatial relation processing can be verbal as well as spatial. We assessed the extent to which categorical and coordinate spatial relation processing engages verbal and spatial representations and processes using a dual-task paradigm. Participants performed the classical dot-bar paradigm and simultaneously performed either a spatial tapping task, or an articulatory suppression task. When participants were requested to tap blocks in a given pattern (spatial tapping), their performance decreased in both the categorical and coordinate tasks compared to a control condition without interference. In contrast, articulatory suppression did not affect performance in either spatial relation task. A follow-up experiment indicated that this outcome could not be attributed to different levels of difficulty of the two interference tasks. These results provide strong evidence that both coordinate and categorical spatial relation processing relies mainly on spatial mechanisms. These findings have implications for theories on why categorical and coordinate spatial relations processing are lateralized in the brain.

Spatial relations can be represented categorically, by means of abstract labels, or coordinately, in metric, absolute measures. These representations have been associated to the left and the right hemispheres respectively (Kosslyn, 1987). Recent studies have focused on the temporal dynamics of spatial relation processing, with working memory task designs. In this light, we examined the suggested lateralization effect in an ERP study incorporating a visual half field match-to-sample design, in which two sequentially presented stimuli were compared. By manipulating the length of the retention intervals between the two stimuli (500 ms, 2000 ms, and 5000 ms), spatial working memory effects were studied at three separate stages of working memory; encoding, memorization, and retrieval. The hypothesized interaction of instruction and visual field was found in the behavioural data, restricted to the 2000 ms retention interval. The EEG data indicate a strong overall right hemisphere effect, which is likely related to spatial working memory in general. Categorical and coordinate processing appears to already differentiate during the encoding stage in the P300 complex (300-500 ms after presentation of the first stimulus), where instruction interacts significantly with hemisphere in the parietal area. We found a clear right hemisphere advantage for coordinate processing and no lateralization for categorical processing. We argue that the outcome indicates qualitative rather than quantitative differences between categorical and coordinate processing., (Copyright 2010 Elsevier B.V. All rights reserved.)

We present two cases (A.C. and W.J.) with navigation problems resulting from parieto-occipital right hemisphere damage. For both the cases, performance on the neuropsychological tests did not indicate specific impairments in spatial processing, despite severe subjective complaints of spatial disorientation. Various aspects of navigation were tested in a new virtual reality task, the Virtual Tübingen task. A double dissociation between spatial and temporal deficits was found; A.C. was impaired in route ordering, a temporal test, whereas W.J. was impaired in scene recognition and route continuation, which are spatial in nature. These findings offer important insights in the functional and neural architecture of navigation.

Many reports show that spatial relations between and within objects show differences in hemispheric lateralization. Coordinate, metric relations concerning distances are processed with a right-hemisphere advantage, whereas a left-hemisphere advantage is thought to be related to categorical, abstract relations (Kosslyn, 1987). Kemmerer and Tranel (2000) argued that the left-hemisphere advantage for categorical processing might apply only for verbal spatial categories, however, whereas a right-hemisphere advantage is related to visuospatial categories. To test this idea, we examined categorical processing for stimuli in both verbal and visuospatial formats, with a visual half-field, match-to-sample design. In Experiment 1, we manipulated the format of the second stimulus to compare response patterns for both verbal and visuospatial stimuli. In Experiment 2, we varied the expectancy of the format of the second stimulus, allowing for an assessment of strategy use. The results showed that a left-hemisphere advantage was related to verbal stimulus format only, but not in all conditions. A right-hemisphere advantage was found only with a visuospatial expectancy, visuospatial format, and brief interval. The theory we present to explain these results proposes that the lateralization related to basic categorical processing can be strongly influenced by verbal characteristics and, to some extent, by additional coordinate processing. The lateralization measured in such cases does not represent lateralization related purely to categorical processing, but to these additional effects as well. This stresses the importance of careful task and stimulus design when examining categorical processing in order to reduce the influence of those additional processes.

Background: It is well established that foveating a behaviorally relevant part of the visual field improves localization performance as compared to the situation where the gaze is directed elsewhere. Reduced localization performance in the peripheral encoding conditions has been attributed to an eccentricity-dependent increase in positional uncertainty. It is not known, however, whether and how the foveal and peripheral encoding conditions can influence spatial interval estimation. In this study we compare observers' estimates of a distance between two co-planar dots in the condition where they foveate the two sample dots and where they fixate a central dot while viewing the sample dots peripherally., Methodology/principal Findings: Observers were required to reproduce, after a short delay, a distance between two sample dots based on a stationary reference dot and a movable mouse pointer. When both sample dots are foveated, we find that the distance estimation error is small but consistently increases with the dots-separation size. In comparison, distance judgment in peripheral encoding condition is significantly overestimated for smaller separations and becomes similar to the performance in foveal trials for distances from 10 to 16 degrees., Conclusions/significance: Although we find improved accuracy of distance estimation in the foveal condition, the fact that the difference is related to the reduction of the estimation bias present in the peripheral condition, challenges the simple account of reducing the eccentricity-dependent positional uncertainty. Contrary to this, we present evidence for an explanation in terms of neuronal populations activated by the two sample dots and their inhibitory interactions under different visual encoding conditions. We support our claims with simulations that take into account receptive fields size differences between the two encoding conditions.

Spatial relations within and between objects can be represented either coordinately or categorically. Coordinate representations concern metric and precise relations, and are strongly associated with right parietal cortex activity, while categorical representations relate to more qualitative, abstract relations, and have shown to have a, somewhat weaker, relationship with left parietal cortex activation [Trojano et al., 2002. Coordinate and categorical judgements in spatial imagery. An fMRI study. Neuropsychologia, 40, 1666-1674]. In the current study, a functional magnetic resonance imaging (fMRI) experiment enabled a closer examination of this proposed hemispheric lateralization within a working memory paradigm. A visual half field task in a match-to-sample format was conducted to examine these lateralization effects with a short (500 ms) and a long (2000 ms) interval between two stimuli, with either a categorical or a coordinate instruction. In the behavioural data, the hypothesized hemispheric specialization was found for the brief interval. The imaging data support the hemispheric lateralization as well. The proposed lateralization effect is present during spatial relation processing, but only within the superior parietal cortex and with certain temporal constraints. Additionally, categorical trials show a clear involvement of the left and right premotor and posterior parietal areas during the brief interval, while coordinate trials are related to higher activity in the left and right insula, during the long interval. We propose a refined view on lateralization of spatial relation processing, keeping in mind the temporal restrictions shown by this study.

It has been proposed that spatial relations can be encoded in two different ways: categorically, where the relative position of objects can be described in prepositional terms (to the left/right, above/below, etc.) and coordinately, where a precise distance between the objects is assessed. Processing of categorical and coordinate spatial relations is believed to rely on the parvo- and magnocellular pathways or small and large receptive fields, respectively. We employed the response signal speed-accuracy trade-off procedure to obtain a description of temporal dynamics of information transfer for categorical and coordinate spatial decisions. In the two tasks the same procedure and stimuli were used, while the instructions called for different types of discrimination. We found no differences in information accrual speed between the tasks as would be expected from the parvo/magno cells or small/large receptive fields distinction. Theoretical consequences of these findings are discussed.

Spatial relations between objects can be represented either categorically or coordinately. The metric, coordinate representation is associated with predominant right hemisphere activity, while the abstract, qualitative categorical representation is thought to be processed more in the left hemisphere [Kosslyn, S. M. (1987). Seeing and imagining in the cerebral hemispheres: A computational analysis. Psychological Review, 94, 148-175]. This hypothesized lateralization effect has been found in a number of studies, along with indications that specific task demands can be crucial for these outcomes. In the current experiment a new visual half field task was used which explores these hemispheric differences and their time course by means of a match-to-sample design. Within retention intervals that were brief (500 ms), intermediate (2000 ms), or long (5000 ms), the processing of categorical and coordinate representations was studied. In the 500 ms interval, the hemispheric effect suggested by Kosslyn (1987) was found, but in the longer intervals it was absent. This pattern of the lateralization effect is proposed to be caused by the differential effect the retention interval has on coordinate and categorical representations. Coordinate spatial relations appear susceptible to changes in retention interval and decay very quickly over time, congruent with previous findings about accurate location memory. The processing of categorical spatial relations showed less decay and only between 2000 ms and 5000 ms. Qualitative self reports suggest that the decay found for categorical relations might be caused by a switch from a visual to a more verbal memorization strategy.

Evidence from neuropsychological patients suggests that multiple body representations exist. The most common dissociation is between body schema to guide limb movements, and body image used to make perceptual judgements. In the current study we employed a kinaesthetic illusion in two experiments to dissociate body representations in healthy individuals. Tendon vibration creates an illusory lengthening of the muscle and an illusive displacement of the limb. In Experiment 1 two conditions were used. In the 'direct' condition the biceps of the dominant right arm of blindfolded participants was vibrated, creating illusory elbow extension. In the 'indirect' condition the right knee was held with the vibrated right arm, creating illusive lowering of the leg and knee. In both conditions, subjects performed with the non-vibrated arm a reaching as well as a matching response, theorized to be based on the body schema and body image, respectively. Results showed that the illusion was significantly larger for the matching as compared to the reaching response, with the most pronounced difference observed in the direct condition. In Experiment 2 reaching and matching without vibration and a passive matching response were implemented in the direct condition. The same differential effect of the illusion was found. Results further showed that passive and active matching were statistically similar but significantly different from the reaching response. In conclusion, these findings suggest that the effect of the kinaesthetic illusion on reaching and matching differed, consistent with the idea of separate underlying body representations for both responses.