Topographic disorientation--a case report

Running speed and REM sleep control two distinct modes of rapid interhemispheric communication

Rhythmic gamma-band communication within and across cortical hemispheres is critical for optimal perception, navigation, and memory. Here, using multisite recordings in both rats and mice, we show that even faster ~140 Hz rhythms are robustly anti-phase across cortical hemispheres, visually resembling splines, the interlocking teeth on mechanical gears. Splines are strongest in superficial granular retrosplenial cortex, a region important for spatial navigation and memory. Spline-frequency interhemispheric communication becomes more coherent and more precisely anti-phase at faster running speeds. Anti-phase splines also demarcate high-activity frames during REM sleep. While splines and associated neuronal spiking are anti-phase across retrosplenial hemispheres during navigation and REM sleep, gamma-rhythmic interhemispheric communication is precisely in-phase. Gamma and splines occur at distinct points of a theta cycle and thus highlight the ability of interhemispheric cortical communication to rapidly switch between in-phase (gamma) and anti-phase (spline) modes within individual theta cycles during both navigation and REM sleep.

Gamma-rhythmic communication within and across cortical hemispheres is critical for optimal perception, navigation, and memory. Here, Ghosh et al. identify even faster ~140 Hz rhythms, named splines, that reflect anti-phase neuronal synchrony across hemispheres. The balance of anti-phase spline and in-phase gamma communication is dynamically controlled by behavior and sleep.

Hyperexcitable Neurons Enable Precise and Persistent Information Encoding in the Superficial Retrosplenial Cortex

The retrosplenial cortex (RSC) is essential for memory and navigation, but the neural codes underlying these functions remain largely unknown. Here, we show that the most prominent cell type in layers 2/3 (L2/3) of the mouse granular RSC is a hyperexcitable, small pyramidal cell. These cells have a low rheobase (LR), high input resistance, lack of spike frequency adaptation, and spike widths intermediate to those of neighboring fast-spiking (FS) inhibitory neurons and regular-spiking (RS) excitatory neurons. LR cells are excitatory but rarely synapse onto neighboring neurons. Instead, L2/3 is a feedforward, not feedback, inhibition-dominated network with dense connectivity between FS cells and from FS to LR neurons. Biophysical models of LR but not RS cells precisely and continuously encode sustained input from afferent postsubicular head-direction cells. Thus, the distinct intrinsic properties of LR neurons can support both the precision and persistence necessary to encode information over multiple timescales in the RSC.

Cortical networks dynamically emerge with the interplay of slow and fast oscillations for memory of a natural scene

Neural oscillations are crucial for revealing dynamic cortical networks and for serving as a possible mechanism of inter-cortical communication, especially in association with mnemonic function. The interplay of the slow and fast oscillations might dynamically coordinate the mnemonic cortical circuits to rehearse stored items during working memory retention. We recorded simultaneous EEG-fMRI during a working memory task involving a natural scene to verify whether the cortical networks emerge with the neural oscillations for memory of the natural scene. The slow EEG power was enhanced in association with the better accuracy of working memory retention, and accompanied cortical activities in the mnemonic circuits for the natural scene. Fast oscillation showed a phase-amplitude coupling to the slow oscillation, and its power was tightly coupled with the cortical activities for representing the visual images of natural scenes. The mnemonic cortical circuit with the slow neural oscillations would rehearse the distributed natural scene representations with the fast oscillation for working memory retention. The coincidence of the natural scene representations could be obtained by the slow oscillation phase to create a coherent whole of the natural scene in the working memory.

Topographical disorientation after ischemic mini infarct in the dorsal hippocampus: whispers in silence

Silent focal ischemic mini infarcts in the brain are thought to cause no clinically overt symptoms. Some populations of hippocampal cells are particularly sensitive to ischemic events, however, rendering hippocampal functions especially vulnerable to ischemia-induced deficits. The present study investigated whether an otherwise silent ischemic mini infarct in the hippocampus (HPC) can produce impairments in spatial performance in rats. Spatial performance was assessed in the ziggurat task (ZT) using a 10-trial spatial learning protocol for 4 days prior to undergoing hippocampal ischemic lesion or sham surgery. Hippocampal silent ischemia was induced by infusion of endothelin-1 (ET-1), a potent vasoconstrictor, into either the dorsal or the ventral hippocampus (dHPC and vHPC). When tested postoperatively in the ZT using a standard testing protocol for 8 days, rats with hippocampal lesions exhibited no spatial deficit. Although spatial learning and memory in the ZT were not affected by the ET-1-induced silent ischemia, rats with dHPC stroke showed more returns when navigating the ZT as opposed to the vHPC rats. Comparison of region-specific HPC lesions in the present study indicated that dorsal hippocampal function is critically required for topographic orientation in a complex environment. Topographic disorientation as reflected by enhanced return behaviors may represent one of the earliest predictors of cognitive decline after silent ischemic insult that may be potentially traced with sensitive clinical examination in humans.

Parahippocampal and retrosplenial contributions to human spatial navigation

Spatial navigation is a core cognitive ability in humans and animals. Neuroimaging studies have identified two functionally defined brain regions that activate during navigational tasks and also during passive viewing of navigationally relevant stimuli such as environmental scenes: the parahippocampal place area (PPA) and the retrosplenial complex (RSC). Recent findings indicate that the PPA and RSC have distinct and complementary roles in spatial navigation, with the PPA more concerned with representation of the local visual scene and RSC more concerned with situating the scene within the broader spatial environment. These findings are a first step towards understanding the separate components of the cortical network that mediates spatial navigation in humans.

Macaque monkey retrosplenial cortex: III. Cortical efferents

We have investigated the cortical efferent projections of the macaque monkey retrosplenial and posterior cingulate cortices by using (3)H-amino acids as anterograde tracers. All the injections produced extensive local connections to other portions of this region. There were also a number of extrinsic efferent cortical connections, many of which have not hitherto been reported. Major projections from the retrosplenial cortex were directed to the frontal lobe, with heaviest terminations in areas 46, 9, 10, and 11. There were also very substantial projections to the entorhinal cortex, presubiculum, and parasubiculum of the hippocampal formation, as well as to areas TH and TF of the parahippocampal cortex. Some injections led to labeling of area V4, the dorsal bank of the superior temporal sulcus, and area 7a of the parietal cortex. Projections from the posterior cingulate cortex innervated all these same regions, although the density of termination was different from the retrosplenial projections. The posterior cingulate cortex gave rise to additional projections to parietal area DP and to the cortex along the convexity of the superior temporal gyrus. The ventral portion of the posterior cingulate cortex (area 23v) gave rise to much denser efferent projections to the hippocampal formation than the dorsal portions (areas 23e and i). These connections are discussed in relation to the clinical syndromes of retrosplenial amnesia and topographic disorientation in humans commonly caused by lesions in the caudoventral portions of the retrosplenial and posterior cingulate cortices.

Pure topographical disorientation following a right forceps major of the splenium lesion: a case study

A 72-year-old man with pure topographical disorientation following a focal hemorrhage in the right forceps major of splenium was assessed at 2 weeks and 3 months after the onset. Initially, he could identify familiar buildings and landmarks, but noted topographical disorientation, dysfunction in sense of quarters, and in visuo-spatial function. The improvement of topographical disorientation was attained in 3 months, while the inability of the sense of quarters and manipulating visuo-spatial information remained unchanged. These results suggested the heading disorientation was accompanied with impaired sense of quarters, although disabled sense of quarters continued beyond the recovery of heading disorientation.

Topographical, autobiographical and semantic memory in a patient with bilateral mesial temporal and retrosplenial infarction

According to Consolidation Theory (Squire, 1992, Psychological Review, 99, 195; Squire & Alvarez, 1995, Current Opinion in Neurobiology, 5, 169), the mesial temporal lobes have a time-limited role in the maintenance, storage and retrieval of retrograde declarative memories, such that they are not necessary for recalling remote memories. In contrast, proponents of the Multiple Trace Theory (Fuji, Moscovitch, & Nadel, 2000, Handbook of neuropsychology, 2nd ed., p 223, Amsterdam, New York: Elsevier; Nadel & Moscovitch, 1999, Current Opinion in Neurobiology, 7, 217) posit that the mesial temporal lobe (MTL) is necessary for remembering detailed autobiographical and topographical material from all time periods. A third theory of hippocampal function, the Cognitive Map Theory (O'Keefe & Nadel, 1978, The hippocampus as a cognitive map. Oxford: Clarendon), states that the hippocampus is involved in the processing of allocentric spatial representations. The precise role of the MTL in remote memory has been difficult to elucidate, as the majority of studies present cases with widespread brain damage that often occurred many years prior to testing. We investigated retrograde autobiographical, semantic and topographical memories in a subject (SG) who had recently sustained infarctions confined to the MTL and retrosplenial region bilaterally. Inconsistent with the predictions of Cognitive Map Theory, memory for spatial maps that were learned in the past was preserved. Additional testing indicated that SG suffered from a landmark agnosia, which affected remotely and recently acquired information equally. SG was also poor at imagining which direction he would have to turn his body to move from one landmark to another. In accordance with Consolidation Theory, SG performed similarly to control subjects for remote time periods on various measures of retrograde autobiographical memory and demonstrated intact knowledge regarding famous faces and vocabulary terms that were acquired in the past. In contrast, memory for remote public events was impaired. The current findings indicate that the mesial temporal and/or retrosplenial regions have little role to play in memory for remotely acquired spatial maps, autobiographical memories, famous faces or vocabulary terms. However, the findings for landmark naming, directional calculations between landmarks and knowledge of public events suggest that the MTL and/or retrosplenial cortices remain important for accessing these types of memories indefinitely.

Topographical disorientation: Towards an integrated framework for assessment

Topographical disorientation, the inability to find one's way in large-scale environments, is a relatively common disorder. However, there are relatively few cognitive neuropsychological studies that investigate the nature of topographical cognition. Theoretical progress has been hindered by a number of factors including: terminological confusion; lack of theoretically driven assessment; the use of broad classifications for the nature of underlying impairments; and an ongoing failure to examine topographical skills in real-life settings. As a result, there is currently no well-established or widely accepted theoretical framework encompassing all aspects of this multifaceted area of cognition. In addition, there is a relative paucity of published case studies that include a comprehensive, theoretically based assessment of topographical disorientation, and treatment of the disorder has received virtually no formal investigation (with the exception of Davis & Coltheart, 1999). Thus, the current paper focuses on the development of a broad framework for understanding topographical cognition that integrates a number of recent theories of topographical orientation and mental imagery (Farah, 1984; Kosslyn, 1980; Riddoch & Humphreys, 1989). The aim of the paper is to present a preliminary framework that can be used as a basis for further refinement and development of theoretical proposals, and be employed by clinicians as a starting point for assessment planning.

Neural correlates of human wayfinding in stroke patients

Wayfinding is a complex cognitive function involving different types of information, such as knowledge about landmarks and direction information. This variety of processes suggest that multiple neural mechanisms are involved, e.g., the hippocampal system, the posterior parietal and temporal cortical areas. Although patient studies and imaging studies have given important insights in the exact neural circuitry underlying wayfinding, many controversies remain. Therefore, the current study sets out to further examine the neuroanatomical correlates of wayfinding in a sample of 31 stroke patients with unilateral lesions, tested with a series of different wayfinding tasks, including landmark recognition, landmark ordering, route reversal and route drawing. For all patients, the exact location of their lesion was determined using CT or MRI scans. Based on existing literature, a number of relevant brain areas were demarcated, after which the extent of damage to these areas was determined for each patient separately. Performance on the landmark recognition task was impaired by damage to the right hippocampal formation, whereas a weak correlation was found between damage to the dorsolateral prefrontal cortex and processing the order of the landmarks. Several brain areas were found to be involved in retracing a route from the end to the beginning, including the right hippocampal formation, the right posterior parietal cortex, the right dorsolateral prefrontal cortex and the right temporal lobe. Finally, damage to the right temporal lobe impaired the ability to draw the route.

Wayfinding in familiar and unfamiliar environments in a case of progressive topographical agnosia

A 71-year-old right-handed man (F.G.) presents with prosopagnosia and with an inability to recognize famous and familiar buildings. Despite his deficit, F.G. obtained normal scores on neuropsychological tests of executive functions, language, praxis and primary visuoperceptual skills. Brain MRI showed atrophy predominantly in the right temporal lobe, particularly in the fusiform gyrus and the parahippocampal cortex. The present study investigated F.G.'s ability to orient himself in familiar and new environments. His wayfinding abilities in a familiar environment (i.e., his hometown) were preserved despite an inability to recognize familiar and famous buildings, monuments and landmarks in this environment. Wayfinding was achieved through a heavy reliance on written indications (e.g., names of restaurants and streets), preservation of a pre-existing cognitive map of this familiar environment, and normal executive functions necessary to plan the execution of a given trajectory. In an unfamiliar environment, F.G.'s topographical agnosia resulted in severe wayfinding difficulties and in the inability to build an adequate cognitive spatial representation. F.G.'s topographical agnosia results from a high-level visuoperceptual deficit, characterized by an inability to access a global configuration of complex visual stimuli such as familiar and famous monuments, and an over-reliance on the processing of local features.

Cytology and functionally correlated circuits of human posterior cingulate areas

Human posterior cingulate cortex (PCC) and retrosplenial cortex (RSC) form the posterior cingulate gyrus, however, monkey connection and human imaging studies suggest that PCC area 23 is not uniform and atlases mislocate RSC. We histologically assessed these regions in 6 postmortem cases, plotted a flat map, and characterized differences in dorsal (d) and ventral (v) area 23. Subsequently, functional connectivity of histologically guided regions of interest (ROI) were assessed in 163 [(18)F]fluorodeoxyglucose human cases with PET. Compared to area d23, area v23 had a higher density and larger pyramids in layers II, IIIc, and Vb and more intermediate neurofilament-expressing neurons in layer Va. Coregisrtration of each case to standard coordinates showed that the ventral branch of the splenial sulci coincided with the border between d/v PCC at -5.4 +/- 0.17 cm from the vertical plane and +1.97 +/- 0.08 cm from the bi-commissural line. Correlation analysis of glucose metabolism using histologically guided ROIs suggested important circuit differences including dorsal and ventral visual stream inputs, interactions between the vPCC and subgenual cingulate cortex, and preferential relations between dPCC and the cingulate motor region. The RSC, in contrast, had restricted correlated activity with pericallosal cortex and thalamus. Visual information may be processed with an orbitofrontal link for synthesis of signals to drive premotor activity through dPCC. Review of the literature in terms of a PCC duality suggests that interactions of dPCC, including area 23d, orient the body in space via the cingulate motor areas, while vPCC interacts with subgenual cortex to process self-relevant emotional and non-emotional information and objects and self-reflection.

Cognitive- and motor-related regions in Parkinson's disease: FDOPA and FDG PET studies

OBJECTIVE

Using 6-[(18)F]fluoro-L-dopa (FDOPA) and [(18)F]fluorodeoxyglucoce (FDG) positron emission tomography (PET), multiple regression analyses were performed to determine the specific brain regions that are related to cognitive and motor symptoms in nondemented patients with Parkinson's disease.

METHODS

Spatially normalized images of FDOPA influx rate constant (Ki) values and relative regional cerebral metabolic rates for glucose (rrCMRglc) were created. Raven's Coloured Progressive Matrices (RCPM) scores and the Unified Parkinson's Disease Rating Scale (UPDRS) motor scores were used to determine the patients' cognitive and motor functions, respectively. Multiple correlation analyses between the FDOPA and FDG images and the cognitive and motor scores were performed for each voxel.

RESULTS

RCPM score was significantly positively correlated with the FDOPA Ki in the left hippocampus and with the rrCMRglc in the left middle frontal gyrus and right retrosplenial cortex. Motor function was significantly positively correlated with the FDOPA Ki in the bilateral striatum and with the rrCMRglc in association areas and primary visual cortex. The level of motor function was significantly inversely correlated with the FDOPA Ki in the anterior cingulate gyrus and with the rrCMRglc in bilateral primary motor cortex and right putamen.

CONCLUSIONS

Changes of striatal FDOPA uptake and rrCMRglc in the primary motor cortex likely represent dysfunction in the motor system involving the corticobasal ganglia-thalamocortical loop. Change of FDOPA uptake in the anterior cingulate gyrus may be related to up-regulation of dopamine synthesis in surviving dopamine neurons. The regions where correlation with cognitive function was observed belong to a cognitive frontoparietal-hippocampal network.

Macaque monkey retrosplenial cortex: II. Cortical afferents

We investigated the cortical afferents of the retrosplenial cortex and the adjacent posterior cingulate cortex (area 23) in the macaque monkey by using the retrograde tracers Fast blue and Diamidino yellow. We quantitatively analyzed the distribution of labeled neurons throughout the cortical mantle. Injections involving the retrosplenial cortex resulted in labeled neurons within the retrosplenial cortex and in areas 23 and 31 (approximately 78% of the total labeled cells). In the remainder of the cortex, the heaviest projections originated in the hippocampal formation, including the entorhinal cortex, subiculum, presubiculum, and parasubiculum. The parahippocampal and perirhinal cortices also contained many labeled neurons, as did the prefrontal cortex, mainly in areas 46, 9, 10, and 11, and the occipital cortex, mainly area V2. Injections in area 23 also resulted in numerous labeled cells in the posterior cingulate and retrosplenial regions (approximately 67% of total labeled cells). As in the retrosplenial cortex, injections of area 23 led to many labeled neurons in the frontal cortex, although most of these cells were in areas 9 and 46. Larger numbers of retrogradely labeled cells were also distributed more widely in the posterior parietal cortex, including areas 7a, 7m, LIP, and DP. There were some labeled cells in the parahippocampal cortex. These connections are consistent with the retrosplenial cortex acting as an interface between the working memory functions in the prefrontal areas and the long-term memory encoding in the medial temporal lobe. The posterior cingulate cortex, in contrast, may be more highly associated with visuospatial functions.

Agnosia for scenes in topographagnosia

Topographagnosia is most commonly attributed to an agnosia for landmarks. In order to define the nature of this agnosia, we studied a patient with isolated topographic disorientation (TD) after a stroke in the right medial occipitotemporal region. The patient got lost in familiar environments but could readily read and draw maps, describe familiar routes, and provide correct directions. He had normal perceptual test performance and met criteria for topographagnosia rather than for other forms of topographic disorientation. Two ecologically valid route tests assessed the nature of his agnosia. On a familiar route, he could recognize major landmarks. He could not, however, recognize route configurations made up of combinations of visual features each lacking individual distinctiveness. On a test of route learning, he learned landmarks that differed in minor details and could use them to orient himself along a route. He had difficulty, however, recognizing and learning scenes lacking salient landmarks. This agnosia for scenes was worse for semantically-related environments, but improved with semantic knowledge such as street names. In addition, the patient lacked overt prosopagnosia but tended toward semantic errors in the recognition of famous faces. Together these findings suggest that this patient's inability to recognize a route resulted from an inability of intact perceptual units for scenes, composed of specific visual configurations of individually indefinite features, from accessing stored representations.

Loss of spatial learning in a patient with topographical disorientation in new environments

The case is described of a patient who, following cerebral hypoxia, developed severe difficulty in orienting himself in new environments in the context of a mild global amnesic syndrome. Some episodes he related suggested that his main difficulty was remembering the spatial/directional value of landmarks he recognised. A neuroradiological examination documented severe bilateral atrophy of the hippocampi associated with atrophic changes in the cerebral hemispheres, most marked in the dorsal regions. Neuropsychological and experimental evaluation showed a severe deficit of spatial learning with substantially preserved ability to learn verbal and visual-object information. He was also virtually unable to learn a route in a maze task based exclusively on spatial data, but the availability of visual cues substantially improved his learning. Finally, he performed within normal limits on various tests investigating knowledge acquired premorbidly regarding famous buildings, routes in the town he had been living in since childhood, and geography. Topographical disorientation may be subtended by a specific difficulty in storing the spatial/directional value of visual landmarks in novel environments. The hippocampus appears to be involved in the acquisition of new topographical spatial knowledge.

Bilateral hippocampal pathology impairs topographical and episodic memory but not visual pattern matching

A virtual reality environment was used to test memory performance for simulated "real-world" spatial and episodic information in a 22-year-old male, Jon, who has selective bilateral hippocampal pathology caused by perinatal anoxia. He was allowed to explore a large-scale virtual reality town and was then tested on his memory for spatial layout and for episodes experienced. Topographical memory was tested by assessing his ability to navigate, recognize previously visited locations, and draw maps of the town. Episodic memory was assessed by testing the retrieval of simulated events which consisted of collecting objects from characters while following a route through the virtual town. Memory for the identity of objects, as well as for where they were collected, from whom, and in what order, was also tested. While the first task tapped simple recognition memory, the latter three tested memory for context. Jon was impaired on all topographical tasks and on his recall of the context-dependent questions. However, his recognition of objects from the virtual town, and of "topographical" scenes (as evaluated by standard neuropsychological tests), was not impaired. These findings are consistent with the view that the hippocampus is involved in navigation, recall of long term allocentric spatial information and context-dependent episodic memory, but not visual pattern matching.

Topographical disorientation consequent to amnesia of spatial location in a patient with right parahippocampal damage

We describe a patient who selectively lost the ability to orient himself in the environment after a stroke involving the right parahippocampal gyrus. The neuropsychological assessment showed a specific pattern of impairment of topographical memory; the patient recognised and recalled environmental landmarks but was unable to recall their spatial location. This study provides evidence that different forms of topographical disorientation may be related to distinct mechanisms of cognitive dysfunction. Furthermore, neuroimaging data suggest that a lesion of the right parahippocampal gyrus is critically related to pure topographical disorientation.

The neuroanatomical correlates of route learning impairment

Recent functional imaging studies of topographical learning point to the participation of a large network of cortical and subcortical regions. Nevertheless, areas which are crucial remain poorly specified due to the absence of group studies of subjects with focal lesions distributed throughout the brain. We assessed the ability of 127 subjects with stable, focal lesions to learn a complex real-life route, a critical aspect of topographical functioning. Results indicated that impairment in route learning was highly associated with damage to medial occipital and posterior parahippocampal cortices in either hemisphere, the right hippocampus, and the right inferotemporal region. Impairment was seen among 86% of the subjects with damage to any these regions, in contrast to impairment among 31% of subjects with lesions in other regions. The importance of medial occipitotemporal cortices bilaterally and right inferotemporal cortex likely reflects the critical role of the ability to quickly and accurately perceive and learn multiple topographical scenes. The importance of the right (and probably left) posterior parahippocampal gyrus and of the right hippocampus likely reflects their critical, distinctive roles forming an integrated representation of the extended topographical environment (i.e., the appearance of places and spatial relationships between specific places), and consolidating that representation into multifaceted contextual knowledge of the environment.

Memory for places learned long ago is intact after hippocampal damage

The hippocampus is part of a system of structures in the medial temporal lobe that are essential for memory. One influential view of hippocampal function emphasizes its role in the acquisition and retrieval of spatial knowledge. By this view, the hippocampus constructs and stores spatial maps and is therefore essential for learning and remembering places, including those learned about long ago. We tested a profoundly amnesic patient (E.P.), who has virtually complete bilateral damage to the hippocampus and extensive damage to adjacent structures in the medial temporal lobe. We asked him to recall the spatial layout of the region where he grew up, from which he moved away more than 50 years ago. E.P. performed as well as or better than age-matched control subjects who grew up in the same region and also moved away. In contrast, E.P. has no knowledge of his current neighbourhood, to which he moved after he became amnesic. Our results show that the medial temporal lobe is not the permanent repository of spatial maps, and support the view that the hippocampus and other structures in the medial temporal lobe are essential for the formation of long-term declarative memories, both spatial and non-spatial, but not for the retrieval of very remote memories, either spatial or non-spatial.

Environmental knowledge is subserved by separable dorsal/ventral neural areas

Environmental psychology models propose that knowledge of large-scale space is stored as distinct landmark (place appearance) and survey (place position) information. Studies of brain-damaged patients suffering from "topographical disorientation" tentatively support this proposal. In order to determine if the components of psychologically derived models of environmental representation are realized as distinct functional, neuroanatomical regions, a functional magnetic resonance imaging (fMRI) study of environmental knowledge was performed. During scanning, subjects made judgments regarding the appearance and position of familiar locations within a virtual reality environment. The fMRI data were analyzed in a manner that has been empirically demonstrated to rigorously control type I error and provide optimum sensitivity, allowing meaningful results in the single subject. A direct comparison of the survey position and landmark appearance conditions revealed a dorsal/ventral dissociation in three of four subjects. These results are discussed in the context of the observed forms of topographical disorientation and are found to be in good agreement with the human lesion studies. This experiment confirms that environmental knowledge is not represented by a unitary system but is instead functionally distributed across the neocortex.

Selective sparing of topographical memory

The case of a 61 year old patient with Pick's disease involving predominantly the left temporal lobe, who has been studied over a 5 year period, is reported. She presented with a grave impairment of both verbal and non-verbal memory functions. Her non-verbal memory deficits included profound impairments on the recognition of unfamiliar faces and the recall of abstract designs. Remarkably, her visual recognition memory performance for unknown buildings, landscapes, and outdoor scenes was preserved. Strikingly, her ability to recall familiar routes and learn new ones through a complex virtual reality town was also entirely normal. This seems to be the first case documenting the selective preservation of topographical memory in the context of severe non-verbal and verbal memory impairments. These findings imply that topographical memory and non-verbal memory are subserved by separable neural systems.

Pure topographical disorientation related to dysfunction of the viewpoint dependent visual system

A 70-year-old woman presented with pure topographical disorientation following haemorrhage in the right medial parietal lobe. She could not navigate in the real world despite good ability to draw maps, describe routes, and identify objects and buildings. Her performance on mental rotation, visual memory, and spatial learning tests also was normal. In contrast, she failed totally in a locomotor map test and in a task in which she was requested to judge viewpoints of buildings. Her highly selective topographical disorientation was probably caused by the inability to identify a viewpoint of a particular building. The lesion may have disconnected the association between the spatial information processed in the lateral parietal lobe and the visual memory mediated by the limbic system, which seems to be important for viewpoint dependent analysis.

Head direction cells and the neurophysiological basis for a sense of direction

Animals require two types of fundamental information for accurate navigation: location and directional heading. Current theories hypothesize that animals maintain a neural representation, or cognitive map, of external space in the brain. Whereas cells in the rat hippocampus and parahippocampal regions encode information about location, a second type of allocentric spatial cell encodes information about the animal's directional heading, independent of the animal's on-going behaviors. These head direction (HD) cells are found in several areas of the classic Papez circuit. This review focuses on experimental studies conducted on HD cells and describes their discharge properties, functional significance, role in path integration, and responses to different environmental manipulations. The anterior dorsal thalamic nucleus appears critical for the generation of the directional signal. Both motor and vestibular cues also play important roles in the signal's processing. The neural network models proposed to account for HD cell firing are compared with known empirical findings. Examples from clinical cases of patients with topographical disorientation are also discussed. It is concluded that studying the neural mechanisms underlying the HD signal provides an excellent opportunity for understanding how the mammalian nervous system processes a high level cognitive signal.

Hippocampal involvement in human topographical memory: evidence from functional imaging

Functional brain imaging in humans is beginning to reveal a network of brain regions that subserve topographical learning: the medial parietal lobe, the posterior cingulate gyrus, occipitotemporal areas, the parahippocampal gyrus and the right hippocampus. These findings illuminate the patient lesion literature where all of these brain regions have been implicated at one time or another in cases of topographical disorientation. Once topographical information is acquired, the neuroanatomy that supports its use from either episodic or semantic memory is similar to that activated during encoding. The specific contributions of extrahippocampal regions within the topographical memory system are being revealed, such as the role of the right parahippocampal gyrus in object-in-place encoding. The right hippocampus is clearly involved in processing spatial layouts over long as well as short time-courses, and participates in both the encoding and the retrieval of topographical memory. The ventromedial orbitofrontal cortex is recruited when information in the topographical memory system is not sufficient to produce direct navigation to a goal place.

Topographical disorientation: selective impairment of locomotor space?

We report a single-case study of a patient who suddenly lost her ability to orient herself in her neighbourhood. On formal testing she demonstrated remarkably selective deficits in episodic and semantic memory of topographical items and rouse. Her general intellectual abilities were unaffected as were basic perceptual processes and episodic and semantic memory other than topography. We interpret her deficit as a further example of category-specific processing impairment affecting knowledge acquired through selective channels-in this case visuo-locomotor information.

Topographical disorientation following unilateral temporal lobe lesions in humans

Studies of the non-human temporal lobe, particularly the hippocampus, confirm its significant role in learning and memory, particularly allocentric spatial mapping of the environment. The role of the human temporal lobes in topographical orientation was investigated by examining the formation of representations of a large-scale real-world environment after unilateral left and right temporal lobe surgery. Patients and normal control subjects viewed videotape presentations of overlapping routes through a novel urban area. Topographical orientation was then assessed across a range of parameters. Right temporal lobe lesions alone gave rise to deficits in making proximity judgements. However, on all other topographical orientation tasks both right and left temporal lobe lesion groups were impaired relative to the normal control group, but the two patient groups did not differ significantly from each other. These findings suggest that such is the nature of remembering and way-finding in the environment that the integrity of both human temporal lobes is required.