Spatial coordinate transforms linking the allocentric hippocampal and egocentric parietal primate brain systems for memory,action in space,and navigation

A theory and model of spatial coordinate transforms in the dorsal visual system through the parietal cortex that enable an interface via posterior cingulate and related retrosplenial cortex to allocentric spatial representations in the primate hippocampus is described. First, a new approach to coordinate transform learning in the brain is proposed, in which the traditional gain modulation is complemented by temporal trace rule competitive network learning. It is shown in a computational model that the new approach works much more precisely than gain modulation alone, by enabling neurons to represent the different combinations of signal and gain modulator more accurately. This understanding may have application to many brain areas where coordinate transforms are learned. Second, a set of coordinate transforms is proposed for the dorsal visual system/parietal areas that enables a representation to be formed in allocentric spatial view coordinates. The input stimulus is merely a stimulus at a given position in retinal space, and the gain modulation signals needed are eye position, head direction, and place, all of which are present in the primate brain. Neurons that encode the bearing to a landmark are involved in the coordinate transforms. Part of the importance here is that the coordinates of the allocentric view produced in this model are the same as those of spatial view cells that respond to allocentric view recorded in the primate hippocampus and parahippocampal cortex. The result is that information from the dorsal visual system can be used to update the spatial input to the hippocampus in the appropriate allocentric coordinate frame, including providing for idiothetic update to allow for self‐motion. It is further shown how hippocampal spatial view cells could be useful for the transform from hippocampal allocentric coordinates to egocentric coordinates useful for actions in space and for navigation.     

Spatial deficits and hemispheric asymmetries in the rat following unilateral and bilateral lesions of posterior parietal or medial agranular cortex

Studies of spatial behavior in both the human and non-human primate have generally focused on the role of the posterior parietal and prefrontal cortices and have indicated that destruction of these regions produce allocentric and egocentric deficits, respectively. The present study examined the role of the rodent analogs of these regions, the posterior parietal (PPC) and medial agranular (AGm) cortices, in egocentric and allocentric spatial processing, and whether spatial processing in rodents is organized in a hemispatial and/or lateralized manner as has been found in the primate. Eighty male rats receiving either a unilateral or bilateral lesion of AGm or PPC were examined on an egocentric (adjacent arm) or an allocentric (cheeseboard) maze task. The results indicated that PPC and AGm have dissociable spatial functions. Bilateral AGm destruction resulted in egocentric spatial deficits, and unilateral AGm operates demonstrated an intermediate deficit. In contrast, bilateral PPC operates demonstrated a severe deficit in allocentric processing. In addition, there were lateralized differences in the performance of unilateral PPC operates. While right PPC lesions resulted in a significant deficit on the allocentric task, no such deficit was seen in left PPC operates. In addition, neither unilateral AGm nor unilateral PPC operates demonstrated a hemispatial impairment on either the egocentric or allocentric tasks.     

Common and specific neural correlates underlying the spatial congruency effect induced by the egocentric and allocentric reference frame

The spatial location of an object can be represented in two frames of reference: egocentric (relative to the observer's body or body parts) and allocentric (relative to another object independent of the observer). The object positions relative to the two frames can be either congruent (e.g., both left or both right) or incongruent (e.g., one left and one right). Most of the previous studies, however, did not discriminate between the two types of spatial conflicts. To investigate the common and specific neural mechanisms underlying the spatial congruency effect induced by the two reference frames, we adopted a 3 (type of task: allocentric, egocentric, and color) × 2 (spatial congruency: congruent vs. incongruent) within‐subject design in this fMRI study. The spatial congruency effect in the allocentric task was induced by the task‐irrelevant egocentric representations, and vice versa in the egocentric task. The nonspatial color task was introduced to control for the differences in bottom‐up stimuli between the congruent and incongruent conditions. Behaviorally, significant spatial congruency effect was revealed in both the egocentric and allocentric task. Neurally, the dorsal‐medial visuoparietal stream was commonly involved in the spatial congruency effect induced by the task‐irrelevant egocentric and allocentric representations. The right superior parietal cortex and the right precentral gyrus were specifically involved in the spatial congruency effect induced by the irrelevant egocentric and allocentric representations, respectively. Taken together, these results suggested that different subregions in the parieto‐frontal network played different functional roles in the spatial interaction between the egocentric and allocentric reference frame. Hum Brain Mapp 38:2112–2127, 2017. © 2017 Wiley Periodicals, Inc.     

Human neural systems underlying rigid and flexible forms of allocentric spatial representation

Previous studies suggest the importance of medial temporal lobe, areas of parietal cortex, and retrosplenial cortex in human spatial navigation, though the exact role of these structures in representing the relations of elements within a spatial layout (“allocentric” representation) remains unresolved. Hippocampal involvement, in particular, during memory processing is affected by whether a previously formed representation is employed in a novel fashion (“flexible” usage) or in a manner comparable with how it was encoded originally (“rigid” usage). To address whether brain systems are differentially involved during flexible vs. rigid utilization of a pre‐existing allocentric representation, subjects encoded the position of six different target buildings relative to a centrally located landmark building in a virtual city seen from an aerial view. They then actively searched for the locations of these target buildings using the landmark (rigid retrieval) or using a previously shown target building in a novel fashion (flexible retrieval) while undergoing fMRI. Activations in posterior superior parietal cortex and precuneus were greater during more rigid than flexible forms of allocentric retrieval while activation in the hippocampus decreased linearly over blocks during flexible allocentric retrieval. A functional connectivity analysis further revealed significant interactions between hippocampus and these parietal areas during flexible compared with rigid allocentric retrieval. These results extend previous models of the neural basis of spatial navigation by suggesting that while the posterior superior parietal cortex/precuneus play an important role in allocentric representation, the hippocampus, and interactions between hippocampus and these parietal areas, are important for flexible utilization of these representations. Hum Brain Mapp, 2013. © 2012 Wiley Periodicals, Inc.     

Brain Coding of Social Network Structure

Humans have large social networks, with hundreds of interacting individuals. How does the brain represent the complex connectivity structure of these networks? Here we used social media (Facebook) data to objectively map participants'' real-life social networks. We then used representational similarity analysis (RSA) of functional magnetic resonance imaging (fMRI) activity patterns to investigate the neural coding of these social networks as participants reflected on each individual. We found coding of social network distances in the default-mode network (medial prefrontal, medial parietal, and lateral parietal cortices). When using partial correlation RSA to control for other factors that can be correlated to social distance (personal affiliation, personality traits. and visual appearance, as subjectively rated by the participants), we found that social network distance information was uniquely coded in the retrosplenial complex, a region involved in spatial processing. In contrast, information on individuals'' personal affiliation to the participants and personality traits was found in the medial parietal and prefrontal cortices, respectively. These findings demonstrate a cortical division between representations of non-self-referenced (allocentric) social network structure, self-referenced (egocentric) social distance, and trait-based social knowledge.SIGNIFICANCE STATEMENT Each of us has a social network composed of hundreds of individuals, with different characteristics and different relations among them. How does our brain represent this complexity? To find out, we mapped participants'' social connections using Facebook data and then asked them to think about individuals from their network while undergoing functional MRI scanning. We found that the position of individuals within the social network, as well as their affiliation to the participant, are mapped in the retrosplenial complex, a region involved in spatial processing. Individuals'' personality traits were coded in another region, the medial prefrontal cortex. Our findings demonstrate a neural dissociation among different aspects of social knowledge and suggest a link between spatial and social cognitive mapping.     

Transcranial Direct Current Stimulation Accelerates Allocentric Target Detection

BackgroundPrevious research on hemispatial neglect has provided evidence for dissociable mechanisms for egocentric and allocentric processing. Although a few studies have examined whether tDCS to posterior parietal cortex can be beneficial for attentional processing in neurologically intact individuals, none have examined the potential effect of tDCS on allocentric and/or egocentric processing.Objective/hypothesisOur objective was to examine whether transcranial direct current stimulation (tDCS), a noninvasive brain stimulation technique that can increase (anodal) or decrease (cathodal) cortical activity, can affect visuospatial processing in an allocentric and/or egocentric frame of reference.MethodsWe tested healthy individuals on a target detection task in which the target – a circle with a gap – was either to the right or left of the viewer (egocentric), or contained a gap on the right or left side of the circle (allocentric). Individuals performed the task before, during, and after tDCS to the posterior parietal cortex in one of three stimulation conditions – right anodal/left cathodal, right cathodal/left anodal, and sham.ResultsWe found an allocentric hemispatial effect both during and after tDCS, such that right anodal/left cathodal tDCS resulted in faster reaction times for detecting stimuli with left-sided gaps compared to right-sided gaps.ConclusionsOur study suggests that right anodal/left cathodal tDCS has a facilitatory effect on allocentric visuospatial processing, and might be useful as a therapeutic technique for individuals suffering from allocentric neglect.     

Asymmetric Influence of Egocentric Representation onto Allocentric Perception

Objects in the visual world can be represented in both egocentric and allocentric coordinates. Previous studies have found that allocentric representation can affect the accuracy of spatial judgment relative to an egocentric frame, but not vice versa. Here we asked whether egocentric representation influenced the processing speed of allocentric perception. We measured the manual reaction time of human subjects in a position discrimination task in which the behavioral response purely relied on the target's allocentric location, independent of its egocentric position. We used two conditions of stimulus location: the compatible condition-allocentric left and egocentric left or allocentric right and egocentric right; the incompatible condition-allocentric left and egocentric right or allocentric right and egocentric left. We found that egocentric representation markedly influenced allocentric perception in three ways. First, in a given egocentric location, allocentric perception was significantly faster in the compatible condition than in the incompatible condition. Second, as the target became more eccentric in the visual field, the speed of allocentric perception gradually slowed down in the incompatible condition but remained unchanged in the compatible condition. Third, egocentric-allocentric incompatibility slowed allocentric perception more in the left egocentric side than the right egocentric side. These results cannot be explained by interhemispheric visuomotor transformation and stimulus-response compatibility theory. Our findings indicate that each hemisphere preferentially processes and integrates the contralateral egocentric and allocentric spatial information, and the right hemisphere receives more ipsilateral egocentric inputs than left hemisphere does.     

Visual spatial and visual pattern working memory: neuropsychological evidence for a differential role of left and right dorsal visual brain

According to neurophysiological, neuroimaging, and behavioural evidence, visual working memory (WM) can be separated into a "what" and a "where" component, reflecting the duality of visual processing. Whereas a wealth of empirical data suggests a right-sided fronto-parietal network critical for the maintenance of spatial information, the cortical structures underlying maintenance of object information have remained controversial. Although visual object processing depends on ventral, inferior temporal areas, recent neuroimaging results suggest that maintenance of visual object information involves a left-sided or bilateral fronto-parietal network. The aim of the present study is to further clarify the role of the left and right parietal lobes for pattern and spatial visual WM. Seven patients with left-sided, seven with right-sided parietal brain injury, and two age-matched healthy control groups performed a delayed-matching-to-sample task using either pattern (shape) or spatial (location) information or both. In addition, eight patients with left-sided injury sparing parietal areas were tested to further examine the specific role of the left parietal cortex in pattern WM. Left parietal injury resulted in pattern WM impairment, only, while right parietal injury was associated with pattern and spatial WM deficits. Non-parietal injury was not associated with comparable deficits. These results suggest that visual spatial WM depends critically on right parietal areas; in contrast, pattern WM depends on both, left and right parietal areas.     

Reference frames for spatial cognition: different brain areas are involved in viewer-, object-, and landmark-centered judgments about object location

Functional magnetic resonance imaging was used to compare the neural correlates of three different types of spatial coding, which are implicated in crucial cognitive functions of our everyday life, such as visuomotor coordination and orientation in topographical space. By manipulating the requested spatial reference during a task of relative distance estimation, we directly compared viewer-centered, object-centered, and landmark-centered spatial coding of the same realistic 3-D information. Common activation was found in bilateral parietal, occipital, and right frontal premotor regions. The retrosplenial and ventromedial occipital-temporal cortex (and parts of the parietal and occipital cortex) were significantly more activated during the landmark-centered condition. The ventrolateral occipital-temporal cortex was particularly involved in object-centered coding. Results strongly demonstrate that viewer-centered (egocentric) coding is restricted to the dorsal stream and connected frontal regions, whereas a coding centered on external references requires both dorsal and ventral regions, depending on the reference being a movable object or a landmark.     

Neural substrates for allocentric‐to‐egocentric conversion of remembered reach targets in humans

Targets for goal‐directed action can be encoded in allocentric coordinates (relative to another visual landmark), but it is not known how these are converted into egocentric commands for action. Here, we investigated this using a slow event‐related fMRI paradigm, based on our previous behavioural finding that the allocentric‐to‐egocentric (Allo–Ego) conversion for reach is performed at the first possible opportunity. Participants were asked to remember (and eventually reach towards) the location of a briefly presented target relative to another visual landmark. After a first memory delay, participants were forewarned by a verbal instruction if the landmark would reappear at the same location (potentially allowing them to plan a reach following the auditory cue before the second delay), or at a different location where they had to wait for the final landmark to be presented before response, and then reach towards the remembered target location. As predicted, participants showed landmark‐centred directional selectivity in occipital–temporal cortex during the first memory delay, and only developed egocentric directional selectivity in occipital–parietal cortex during the second delay for the ‘Same cue’ task, and during response for the ‘Different cue’ task. We then compared cortical activation between these two tasks at the times when the Allo–Ego conversion occurred, and found common activation in right precuneus, right presupplementary area and bilateral dorsal premotor cortex. These results confirm that the brain converts allocentric codes to egocentric plans at the first possible opportunity, and identify the four most likely candidate sites specific to the Allo–Ego transformation for reaches.     

Dissociation between egocentric and allocentric visuospatial and tactile neglect in acute stroke

Previous investigations provide evidence for distinction between egocentric (viewer-centered) and allocentric (stimulus- or object-centered) hemispatial neglect. However, it has not been determined whether this dissociation is modality-independent or modality-specific. We identify the incidence of egocentric and allocentric neglect in visual and tactile modalities, as well as the frequency of their co-occurrences in each modality. One-hundred patients with acute, right supratentorial ischemic stroke were administered tests for egocentric and allocentric hemispatial neglect in visual (n=98) and tactile (n=58) modalities. The visual test consisted of a page of 30 circles; 10 with no gap, 10 with a gap on the right side, and 10 with a gap on the left. Patients were asked to circle all complete circles and cross out all circles with gaps. A tactile version consisted of the same stimulus types presented as raised circles. Patients were asked to explore the board of circles with their dominant hand and report whether each circle had a gap. To determine the presence of egocentric or allocentric neglect, each test was analyzed for a significantly higher number of errors on the contralesional versus ipsilesional side of the page/board, or of the stimulus, using the Chi square analysis. On the visual test, 17 patients exhibited egocentric neglect; four exhibited allocentric neglect; and only two exhibited both. In the tactile modality, 19 exhibited egocentric neglect; one exhibited allocentric neglect; and none demonstrated both. Only four patients showed egocentric neglect on both visual and tactile tests. We found one patient with bilateral lesions who showed left egocentric visual neglect and right allocentric tactile neglect. These data provide strong evidence that egocentric and allocentric neglect are distinct syndromes that often dissociate and likely reflect damage to different brain areas. They also show that selective egocentric or allocentric neglect can occur in visual or tactile modalities.     

Spatial responsiveness of monkey hippocampal neurons to various visual and auditory stimuli.

To investigate involvement of the hippocampal formation in spatial information processing, activity of neurons in the hippocampal formation of the conscious monkey was recorded during presentation of various visual and auditory stimuli from several directions around the monkey. Of 1,047 neurons recorded, 106 (10.1%) responded to some stimuli from one or more directions. Of these 106 neurons with directionally differentiating responsiveness, 49 responded to visual stimulation, 35 to auditory stimulation, and 22 to both. Among 81 neurons, each tested with more than 10 different stimuli, one type responded independent of the nature of the stimulus (nonselective, n = 39), and responses of the other type depended on the nature of the stimulus (selective, n = 42). To investigate effects of change in spatial relations between test stimuli and background stimuli fixed on the monkey or fixed in the environment, 59 of 106 neurons were tested while the experimental apparatus holding the stimulus was moved relative to the monkey. Of these 59 neurons, 36 changed their responsiveness; 7 maintained the magnitude of their responses but changed the response direction with the movement of the apparatus, 5 changed direction regardless of the movement, and 24 did not change direction, but decreased or extinguished responses from the preferred direction. Thirty-two of 106 neurons were also tested by rotating the monkey. The directionally differentiating responsiveness of 11 neurons followed the monkey (egocentric neurons), that of 9 remained in place in the environment (allocentric neurons), and responses of 12 were reversibly extinguished when the monkey was rotated. The results suggest that these hippocampal neurons may be involved in identification of relations among various kinds of stimuli in different spatial frameworks (egocentric or allocentric) and this identification may be developed from multiple sensory modalities.     

Common and specific contributions of the intraparietal sulci to numerosity and length processing

Numerical and spatial magnitude processing have long been intimately associated, leading to the suggestion that they share a common system of magnitude representation. Although separate investigations on the cerebral areas involved in numerosity and spatial estimation point toward the parietal cortex, the precise anatomical overlap, if any, has not yet been directly established. Here, functional magnetic resonance imaging was used to localize the cerebral network involved in processing both numerosity and length. Blood oxygenation level‐dependent signal changes were measured while healthy volunteers were making numerosity comparisons on linear arrays of dots, and length comparisons on discrete linear arrays of dots and continuous rectangles. The results show the bilateral involvement of parietal regions around the intraparietal sulci in explicit and implicit processing of numerosity, and a right lateralized occipitoparietal network activation in length processing; numerosity and length processing both activate the right IPS and the precentral gyrus. By excluding the mandatory intrinsic spatial processing of arrays, we demonstrate that the left IPS is involved in numerosity processing only, whereas the right IPS underlies a common processing mechanism or representation of spatial and numerical magnitude. Hum Brain Mapp 2009. © 2009 Wiley‐Liss, Inc.     

Spatial coding of visual and somatic sensory information in body-centred coordinates

Because sensory systems use different spatial coordinate frames, cross-modal sensory integration and sensory-motor coordinate transformations must occur to build integrated spatial representations. Multimodal neurons using non-retinal body-centred reference frames are found in the posterior parietal and frontal cortices of monkeys. We used functional magnetic resonance imaging to reveal regions of the human brain using body-centred coordinates to code the spatial position of both visual and somatic sensory stimuli. Participants determined whether a visible vertical bar (visual modality) or a location touched by the right index finger (somatic sensory modality) lay to the left or to the right of their body mid-sagittal plane. This task was compared to a spatial control task having the same stimuli and motor responses and comparable difficulty, but not requiring body-centred coding of stimulus position. In both sensory modalities, the body-centred coding task activated a bilateral fronto-parietal network, though more extensively in the right hemisphere, to include posterior parietal regions around the intraparietal sulcus and frontal regions around the precentral and superior frontal sulci, the inferior frontal gyrus and the superior frontal gyrus on the medial wall. The occipito-temporal junction and other extrastriate regions exhibited bilateral activation enhancement related to body-centred coding when driven by visual stimuli. We conclude that posterior parietal and frontal regions of humans, as in monkeys, appear to provide multimodal integrated spatial representations in body-centred coordinates, and these data furnish the first indication of such processing networks in the human brain.     

Assessment and Functional Impact of Allocentric Neglect: A Reminder from a Case Study

SR suffered a right hemispheric stroke more than 3 years ago, and now lives with left-sided hemiparesis and chronic spatial neglect due to damaged white matter pathways connecting the frontal, temporal and parietal regions. We report here that SR suffers from both viewer-centered (i.e., egocentric) and object-centered (i.e., allocentric) spatial neglect. Notably, unlike most neuropsychological and functional assessments that focus on egocentric deficits, a specialized neuropsychological figurative discrimination test (the Apples test) revealed SR’s allocentric neglect. Further, using assessments sensitive to detect functional deficits related to allocentric neglect, we observed SR’s difficulty in reading and using clocks, reflecting his object-centered errors in these everyday activities. SR’s case suggests that allocentric-specific assessments, both neuropsychological and functional, are valuable in standard neglect examinations, particularly to predict daily function after stroke. We recommend that neglect-related functional disability be distinguished further with respect to allocentric spatial deficits, and functional assessments for allocentric neglect should be validated in future large sample studies. Identifying allocentric neglect early, and learning about its influence on daily function, may enhance care quality and facilitate effective rehabilitation planning for stroke recovery.     

Selective rotation of egocentric spatial representation following right putaminal hemorrhage

Although the role of frontoparietal cortex in spatial egocentric processing is well established, recent animal-lesion and human functional imaging studies have suggested that the neostriatum may also be a critical modulator in the processing of body-centred spatial orientation. We describe here a patient with right putamen-centred hemorrhage who exhibited a consistent counterclockwise rotation of approximately 90 degrees when drawing and writing from memory. A more detailed assessment with a series of representational clock tests demonstrated that the rotation was present only in tasks requiring the use of egocentric cues. In the absence of external cues the patient would adopt and maintain a stable but incorrectly-oriented egocentric representation of the imagined or recollected object. By contrast, performance could be rectified by presentation of correctly-oriented stimuli. These findings suggest that the putamen is part of a circuit underlying egocentric, as opposed to allocentric, representation of space in humans.     

Topographical disorientation in a patient with late-onset blindness with multiple acute ischemic brain lesions

The neurological basis for topographical disorientation has recently shifted from a model of navigation utilizing egocentric techniques alone, to multiple parallel systems of topographical cognition including egocentric and allocentric strategies. We explored if this hypothesis may be applicable to a patient with late-onset blindness. A 72-year-old male with bilateral blindness experienced a sudden inability to navigate after suffering a stroke. Multiple lesions scattered bilaterally throughout the parietal-occipital lobes were found. Deficits in the neural correlates underlying egocentric or allocentric strategies may result in topographical disorientation, even if one appears to be the predominant orientation strategy utilized.     

Sequential egocentric strategy is acquired as early as allocentric strategy: Parallel acquisition of these two navigation strategies

At least two main cognitive strategies can be used to solve a complex navigation task: the allocentric or map‐based strategy and the sequential egocentric or route‐based strategy. The sequential egocentric strategy differs from a succession of independent simple egocentric responses as it requires a sequential ordering of events, possibly sharing functional similarity with episodic memory in this regard. To question the possible simultaneous encoding of sequential egocentric and allocentric strategies, we developed a paradigm in which these two strategies are spontaneously used or imposed. Our results evidenced that sequential egocentric strategy can be spontaneously acquired at the onset of the training as well as allocentric strategy. Allocentric and sequential egocentric strategies could be used together within a trial, and bidirectional shifts (between trials) were spontaneously performed during the training period by 30% of the participants. Regardless of the strategy used spontaneously during the training, all participants could execute immediate shifts to the opposite non previously used strategy when this strategy was imposed. Altogether, our findings suggest that subjects acquire different types of spatial knowledge in parallel, namely knowledge permitting allocentric navigation as well as knowledge permitting sequential egocentric navigation. © 2009 Wiley‐Liss, Inc.     

Egocentric memory impaired and allocentric memory intact as assessed by virtual reality in subjects with unilateral parietal cortex lesions

Present evidence suggests that medial temporal cortices subserve allocentric representation and memory, whereas egocentric representation and memory mainly depends on inferior and superior parietal cortices. Virtual reality environments have a major advantage for the assessment of spatial navigation and memory formation, as computer-simulated first-person environments can simulate navigation in a large-scale space. However, virtual reality studies on allocentric memory in subjects with cortical lesions are rare, and studies on egocentric memory are lacking. Twenty-four subjects with unilateral parietal cortex lesions due to infarction or intracerebral haemorrhage (14 left-sided, 10 right-sided) were compared with 36 healthy matched control subjects on two virtual reality tasks affording to learn a virtual park (allocentric memory) and a virtual maze (egocentric memory). Subjects further received a comprehensive clinical and neuropsychological investigation, and MRI lesion assessment using T₁, T₂ and FLAIR sequences as well as 3D MRI volumetry at the time of the assessment. Results indicate that left- and right-sided lesioned subjects did not differ on task performance. Compared with control subjects, subjects with parietal cortex lesions were strongly impaired learning the virtual maze. On the other hand, performance of subjects with parietal cortex lesions on the virtual park was entirely normal. Volumes of the right-sided precuneus of lesioned subjects were significantly related to performance on the virtual maze, indicating better performance of subjects with larger volumes. It is concluded that parietal cortices support egocentric navigation and imagination during spatial learning in large-scale environments.     

A comparison of egocentric and allocentric spatial memory in a patient with selective hippocampal damage

The spatial memory of a single patient (YR) was investigated. This patient, who had relatively selective bilateral hippocampal damage, showed the pattern of impaired recall but preserved item recognition on standardised memory tests that has been suggested by Aggleton and Shaw [Aggleton JP, Shaw C. Amnesia and recognition memory: a reanalysis of psychometric data. Neuropsychologia 1996;34:51-62] to be a consequence of Papez circuit lesions. YR was tested on three recall tests and one recognition test for visuospatial information. The initial recall test assessed visuospatial memory over very short unfilled delays and YR was not significantly impaired. This test was then modified to test recall of allocentric and egocentric spatial information separately after filled delays of between 5 and 60 s. YR was found to be more impaired at recalling allocentric than egocentric information after a 60 s interval with a tendency for the impairment to increase up to this delay. Recognition of allocentric spatial information was also assessed after delays of 5 and 60 s. YR was impaired after the 60 s delay. The results suggest that the human hippocampus has a greater involvement in allocentric than egocentric spatial memory, and that this most likely concerns the consolidation of allocentric information into long-term memory rather than the initial encoding of allocentric spatial information. The findings also suggest that YR's item recognition/free recall deficit pattern reflects a problem retrieving or storing certain kinds of associative information.