Biomedical subjects

Neil Burgess

Publications and source records attributed to Neil Burgess.

33 records · Page 2Linked to original sources

PubMed2004

Geometric determinants of human spatial memory.

Geometric alterations to the boundaries of a virtual environment were used to investigate the representations underlying human spatial memory. Subjects encountered a cue object in a simple rectangular enclosure, with distant landmarks for orientation. After a brief delay, during which they were removed from the arena, subjects were returned to it at a new location and orientation and asked to mark the place where the cue had been. On some trials the geometry (size, aspect ratio) of the arena was varied between presentation and testing. Responses tended to lie somewhere between a location that maintained fixed distances from nearby walls and a location that maintained fixed ratios of the distances between opposing walls. The former were more common after expansions and for cued locations nearer to the edge while the latter were more common after contractions and for locations nearer to the center. The spatial distributions of responses predicted by various simple geometric models were compared to the data. The best fitting model was one derived from the response properties of 'place cells' in the rat hippocampus, which matches the 'proximities' 1/(d+c) of the cue to the four walls of the arena, where d is the distance to a wall and c is a global constant. Subjects also tended to adopt the same orientation at presentation and testing, although this was not due to using a view matching strategy, which could be ruled out in 50% of responses. Disoriented responses were most often seen where the cued location was near the center of the arena or where the long axis of a rectangular arena was changed between presentation and testing, suggesting that the geometry of the arena acts as a weak cue to orientation. Overall, the results suggest a process of visual landmark matching to determine orientation, combined with an abstract representation of the proximity of the cued location to the walls of the arena consistent with the neural representation of location in the hippocampus.

Adult↗

PubMed2004

Orientational manoeuvres in the dark: dissociating allocentric and egocentric influences on spatial memory.

Subjects in a darkroom saw an array of five phosphorescent objects on a circular table and, after a short delay, indicated which object had been moved. During the delay the subject, the table or a phosphorescent landmark external to the array was moved (a rotation about the centre of the table) either alone or together. The subject then had to indicate which one of the five objects had been moved. A fully factorial design was used to detect the use of three types of representations of object location: (i) visual snapshots; (ii) egocentric representations updated by self-motion; and (iii) representations relative to the external cue. Improved performance was seen whenever the test array was oriented consistently with any of these stored representations. The influence of representations (i) and (ii) replicates previous work. The influence of representation (iii) is a novel finding which implies that allocentric representations play a role in spatial memory, even over short distances and times. The effect of the external cue was greater when initially experienced as stable. Females out-performed males except when the array was consistent with self-motion but not visual snapshots. These results enable a simple egocentric model of spatial memory to be extended to address large-scale navigation, including the effects of allocentric knowledge, landmark stability and gender.

Adult↗

PubMed2004

The hippocampal role in spatial memory and the familiarity--recollection distinction: a case study.

Memory for object locations and for events (comprising the receipt of an object) was tested in a case of developmental amnesia with focal hippocampal damage. Tests used virtual reality environments and forced-choice recognition with foils chosen to equalize the performance of control participants across conditions. Memory for the objects received was unimpaired, but the context of their receipt was forgotten. Memory for short lists of object locations was unimpaired when tested from the same viewpoint as presentation but impaired when tested from a shifted viewpoint. Same-view performance was disrupted by changing the background scene. These results are consistent with Jon having preserved matching to fixed sensory-bound representations but impaired reconstructed or manipulable representations underlying shifted-viewpoint recognition and episodic recollection.

Adult↗

PubMed2004

Endoscopic management of renal candidiasis.

Renal candidial infections have been described in the literature in susceptible populations of all age groups. Systemic antifungal agents have been successful in treating these patients, but in the presence of an obstructed kidney, antegrade (perurethral) or retrograde (percutaneous) drainage of the pelvicaliceal system is mandatory to salvage the kidney. An aggressive percutaneous endoscopic approach for the management of such a case in a young diabetic patient with multiple comorbidities is described here with initial success.

Adult↗

PubMed2003

Independent rate and temporal coding in hippocampal pyramidal cells.

In the brain, hippocampal pyramidal cells use temporal as well as rate coding to signal spatial aspects of the animal's environment or behaviour. The temporal code takes the form of a phase relationship to the concurrent cycle of the hippocampal electroencephalogram theta rhythm. These two codes could each represent a different variable. However, this requires the rate and phase to vary independently, in contrast to recent suggestions that they are tightly coupled, both reflecting the amplitude of the cell's input. Here we show that the time of firing and firing rate are dissociable, and can represent two independent variables: respectively the animal's location within the place field, and its speed of movement through the field. Independent encoding of location together with actions and stimuli occurring there may help to explain the dual roles of the hippocampus in spatial and episodic memory, or may indicate a more general role of the hippocampus in relational/declarative memory.

Action Potentials↗

PubMed2003

The well-worn route and the path less traveled: distinct neural bases of route following and wayfinding in humans.

Finding one's way in a large-scale environment may engage different cognitive processes than following a familiar route. The neural bases of these processes were investigated using functional MRI (fMRI). Subjects found their way in one virtual-reality town and followed a well-learned route in another. In a control condition, subjects followed a visible trail. Within subjects, accurate wayfinding activated the right posterior hippocampus. Between-subjects correlations with performance showed that good navigators (i.e., accurate wayfinders) activated the anterior hippocampus during wayfinding and head of caudate during route following. These results coincide with neurophysiological evidence for distinct response (caudate) and place (hippocampal) representations supporting navigation. We argue that the type of representation used influences both performance and concomitant fMRI activation patterns.

Adult↗

PubMed2003

Navigation expertise and the human hippocampus: a structural brain imaging analysis.

Grey matter volume in the posterior hippocampus of London taxi drivers is greater than in age-matched controls, and the size of this increase correlates positively with time spent taxi driving (E.A. Maguire et al., 2000. Proc Natl Acad Sci USA 97: 4398-4403). This change suggests that increased posterior hippocampal grey matter volume is acquired in response to increased taxi driving experience, perhaps reflecting their detailed representation of the city. However, an alternate hypothesis is that the difference in hippocampal volume is instead associated with innate navigational expertise, leading to an increased likelihood of becoming a taxi driver. To investigate this possibility, we used structural brain imaging and voxel-based morphometry (VBM) to examine a group of subjects who were not taxi drivers. Despite this group showing a wide range of navigational expertise, there was no association between expertise and posterior hippocampal grey matter volume (or, indeed, grey matter volume throughout the brain). This failure to find an association between hippocampal volume and navigational expertise thus suggests that structural differences in the human hippocampus reflect the detail and/or duration of use of the spatial representation acquired, and not innate navigational expertise per se.

Adolescent↗

PubMed2003

Neural representations in human spatial memory.

Ekstrom et al. report the responses of single neurons recorded from the brains of human subjects performing a spatial navigation task in virtual reality. They found cells encoding the subject's current location, view and destination. These data, and related findings in animals, directly reveal some of the representations underlying spatial cognition. They highlight the potential for cognitive psychology and systems neuroscience to combine to provide a neuronal-level understanding of human behaviour.

Journal Article↗

PubMed2003

Selective interference with verbal short-term memory for serial order information: a new paradigm and tests of a timing-signal hypothesis.

Many recent computational models of verbal short-term memory postulate a separation between processes supporting memory for the identity of items and processes supporting memory for their serial order. Furthermore, some of these models assume that memory for serial order is supported by a timing signal. We report an attempt to find evidence for such a timing signal by comparing an item probe task, requiring memory for items, with a list probe task, requiring memory for serial order. Four experiments investigated effects of irrelevant speech, articulatory suppression, temporal grouping, and paced finger tapping on these two tasks. In Experiments 1 and 2, irrelevant speech and articulatory suppression had a greater detrimental effect on the list probe task than on the item probe task. Reaction time data indicated that the list probe task, but not the item probe task, induced serial rehearsal of items. Phonological similarity effects confirmed that both probe tasks induced phonological recoding of visual inputs. Experiment 3 showed that temporal grouping of items during list presentation improved performance on the list probe task more than on the item probe task. In Experiment 4, paced tapping had a greater detrimental effect on the list probe task than on the item probe task. However, there was no differential effect of whether tapping was to a simple or a complex rhythm. Overall, the data illustrate the utility of the item probe/list probe paradigm and provide support for models that assume memory for serial order and memory for items involve separate processes. Results are generally consistent with the timing-signal hypothesis but suggest further factors that need to be explored to distinguish it from other accounts.

Adult↗

PubMed2002

The human hippocampus and spatial and episodic memory.

Finding one's way around an environment and remembering the events that occur within it are crucial cognitive abilities that have been linked to the hippocampus and medial temporal lobes. Our review of neuropsychological, behavioral, and neuroimaging studies of human hippocampal involvement in spatial memory concentrates on three important concepts in this field: spatial frameworks, dimensionality, and orientation and self-motion. We also compare variation in hippocampal structure and function across and within species. We discuss how its spatial role relates to its accepted role in episodic memory. Five related studies use virtual reality to examine these two types of memory in ecologically valid situations. While processing of spatial scenes involves the parahippocampus, the right hippocampus appears particularly involved in memory for locations within an environment, with the left hippocampus more involved in context-dependent episodic or autobiographical memory.

Animals↗

PubMed2002

Long-term plasticity in hippocampal place-cell representation of environmental geometry.

The hippocampus is widely believed to be involved in the storage or consolidation of long-term memories. Several reports have shown short-term changes in single hippocampal unit activity during memory and plasticity experiments, but there has been no experimental demonstration of long-term persistent changes in neuronal activity in any region except primary cortical areas. Here we report that, in rats repeatedly exposed to two differently shaped environments, the hippocampal-place-cell representations of those environments gradually and incrementally diverge; this divergence is specific to environmental shape, occurs independently of explicit reward, persists for periods of at least one month, and transfers to new enclosures of the same shape. These results indicate that place cells may be a neural substrate for long-term incidental learning, and demonstrate the long-term stability of an experience-dependent firing pattern in the hippocampal formation.

Animals↗

PubMed2002

Human hippocampus and viewpoint dependence in spatial memory.

Virtual reality was used to sequentially present objects within a town square and to test recognition of object locations from the same viewpoint as presentation, or from a shifted viewpoint. A developmental amnesic case with focal bilateral hippocampal pathology showed a massive additional impairment when tested from the shifted viewpoint compared with a mild, list length-dependent, impairment when tested from the same viewpoint. While the same-view condition could be solved by visual pattern matching, the shifted-view condition requires a viewpoint independent representation or an equivalent mechanism for translating or rotating viewpoints in memory. The latter mechanism was indicated by control subjects' response latencies in the shifted-view condition, although the amnesic case is not impaired in tests of mental rotation of single objects. These results show that the human hippocampus supports viewpoint independence in spatial memory, and suggest that it does so by providing a mechanism for viewpoint manipulation in memory. In addition, they suggest an extremely sensitive test for human hippocampal damage, and hint at the nature of the hippocampal role in episodic recollection.

Adult↗

PubMed2002

Neurodevelopmental aspects of spatial navigation: a virtual reality fMRI study.

Navigation in spatial contexts has been studied in diverse species, yielding insights into underlying neural mechanisms and their phylogenetic progression. Spatial navigation in humans is marked by age-related changes that may carry important implications for understanding cortical development. The emergence of "allocentric" processing, reflecting that ability to use viewer-independent spatial abstractions, represents an important developmental change. We used fMRI to map brain regions engaged during memory-guided navigation in a virtual reality environment in adolescents and adults. Blood oxygen level-dependent (BOLD) signal was monitored in eight adolescents and eight adults in a 1.5-T MRI scanner during three conditions: (1) memory-guided navigation (NAV); (2) arrow-guided navigation (ARROW); and (3) fixation (FIX). We quantified navigation ability during scanning and allocentric memory after scanning, based on subjects' ability to label a previously unseen, aerial view of the town. Adolescents and adults exhibited similar memory-guided navigation ability, but adults exhibited superior allocentric memory ability. Memory-guided navigation ability during scanning correlated with BOLD change between NAV/ARROWS in various regions, including a right frontal and right-anterior medial temporal lobe region. Age group and allocentric memory together explained significant variance in BOLD change in temporoparietal association cortex and the cerebellum, particularly in the left hemisphere. Consistent with developmental models, these findings relate maturation in the coding of spatial information to functional changes in a distributed, left-lateralized neural network.

Adolescent↗

PubMed2002

What can the hippocampal representation of environmental geometry tell us about Hebbian learning?

The importance of the hippocampus in spatial representation is well established. It is suggested that the rodent hippocampal network should provide an optimal substrate for the study of unsupervised Hebbian learning. We focus on the firing characteristics of hippocampal place cells in morphologically different environments. A hard-wired quantitative geometric model of individual place fields is reviewed and presented as the framework in which to understand the additional effects of synaptic plasticity. Existent models employing Hebbian learning are also reviewed. New information is presented regarding the dynamics of place field plasticity over short and long time scales in experiments using barriers and differently shaped walled environments. It is argued that aspects of the temporal dynamics of stability and plasticity in the hippocampal place cell representation both indicate modifications to, and inform the nature of, the synaptic plasticity in place cell models. Our results identify a potential neural basis for long-term incidental learning of environments and provide strong constraints for the way the unsupervised learning in cell assemblies envisaged by Hebb might occur within the hippocampus.

Action Potentials↗

PubMed2002

The hippocampus, space, and viewpoints in episodic memory.

A computational model of how single neurons in and around the rat hippocampus support spatial navigation is reviewed. The extension of this model, to include the retrieval from human long-term memory of spatial scenes and the spatial context of events is discussed. The model explores the link between spatial and mnemonic functions by supposing that retrieval of spatial information from long-term storage requires the imposition of a particular viewpoint. It is consistent with data relating to representational hemispatial neglect and the involvement of the mammillary bodies, anterior thalamus, and hippocampal formation in supporting both episodic recall and the representation of head direction. Some recent behavioural, neuropsychological, and functional neuroimaging experiments are reviewed, in which virtual reality is used to allow controlled study of navigation and memory for events set within a rich large-scale spatial context. These studies provide convergent evidence that the human hippocampus is involved in both tasks, with some lateralization of function (navigation on the right and episodic memory on the left). A further experiment indicates hippocampal involvement in retrieval of spatial information from a shifted viewpoint. I speculate that the hippocampal role in episodic recollection relates to its ability to represent a viewpoint moving within a spatial framework.

Animals↗