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Biomedical subjects

Neil Burgess

Publications and source records attributed to Neil Burgess.

At least 19 recordsLinked to original sources
PubMed2006

Spatial memory: how egocentric and allocentric combine.

Recent experiments indicate the need for revision of a model of spatial memory consisting of viewpoint-specific representations, egocentric spatial updating and a geometric module for reorientation. Instead, it appears that both egocentric and allocentric representations exist in parallel, and combine to support behavior according to the task. Current research indicates complementary roles for these representations, with increasing dependence on allocentric representations with the amount of movement between presentation and retrieval, the number of objects remembered, and the size, familiarity and intrinsic structure of the environment. Identifying the neuronal mechanisms and functional roles of each type of representation, and of their interactions, promises to provide a framework for investigation of the organization of human memory more generally.

Cognition↗
PubMed2006

Parallel memory systems for talking about location and age in precuneus, caudate and Broca's region.

Language comprehension relies on processing of context. Working memory (WM) evoked by linguistic cues for spatial and nonspatial aspects of a visual scene was investigated by correlating fMRI BOLD signal (or 'activation') with reaction times (RTs). Subjects were asked to indicate either the relative positions or ages of people or objects (referenced by the personal pronouns "he/she/it") in a previously shown image. Good performers of a particular task showed shorter RTs than poor performers. Task-specific activation that is greater in good performers than poor ones is taken to indicate involvement of a given region in performance of the task. Our results indicate that dorsoposterior precuneus supports spatial WM during linguistic processing while a network of areas including the caudate support nonspatial WM in categorization of age. We argue that within-subjects variation of RTs across trials reflects effort. Good performers have higher activity in precuneus as a function of effort compared to poor performers during the spatial task, whereas the opposite is found for the nonspatial task, providing further evidence for specifically spatial WM in dorsoposterior precuneus. Task-independent performance-related modulations of activity were found in Broca's area and amygdala. Broca's area activity increased with effort in both tasks, with a greater increase in good performers than in poor performers, consistent with the region's general role in verbal WM. By contrast, activation in amygdala decreased with effort, with a greater decrease in good performers. We take this deactivation to reflect performance-mediating emotional control. These findings indicate that multiple parallel memory systems are available during language processing, appropriate for different tasks, with performance reflecting which system is selected trial-by-trial and subject-by-subject.

Adult↗
PubMed2006

Development of laparoscopic suturing skills: a prospective trial.

PURPOSE: This multicenter regional trial aimed to evaluate prospectively whether a home-constructed training system comprised of a camcorder, standard television, and laparoscopic box trainer could be used to advance intracorporeal laparoscopic suturing skills. SUBJECTS AND METHODS: Ten urology residents attended a 1-hour teaching session on laparoscopic suturing at the outset of the study. Baseline times and quality assessment were recorded for single-knot intracorporeal laparoscopic sutures, performed on standard laparoscopic stack systems, directly after instruction. Four residents were assigned to train regularly for 4 to 8 weeks on the home-constructed system. All participants were reassessed after 4 weeks on standard laparoscopic stacks, and the times of the study group and controls were compared. RESULTS: Baseline times were not significantly different (p = 0.2764) between the study group and controls. However, the study-group times (111.3 +/- 6.874 [SEM] seconds) were significantly better (p < 0.0001) than those of the control subjects (202.2 +/- 16.96 seconds) after 4 weeks and an average of 303 minutes of total training time on the home-constructed system. Furthermore, subjects training on this system were more likely to throw an acceptable knot (100%) than were the controls (66%). Two subjects who trained for a further 4 weeks achieved greater improvement in facility. CONCLUSIONS: Intracorporeal suturing skills can be learned using a home-constructed system. This could be beneficial for those wishing to develop the advanced skills required for various laparoscopic urologic procedures.

Clinical Competence↗
PubMed2006

Impaired allocentric spatial memory underlying topographical disorientation.

The cognitive processes supporting spatial navigation are considered in the context of a patient (CF) with possible very early Alzheimer's disease who presents with topographical disorientation. Her verbal memory and her recognition memory for unknown buildings, landmarks and outdoor scenes was intact, although she showed an impairment in face processing. By contrast, her navigational ability, quantitatively assessed within a small virtual reality (VR) town, was significantly impaired. Interestingly, she showed a selective impairment in a VR object-location memory test whenever her viewpoint was shifted between presentation and test, but not when tested from the same viewpoint. We suggest that a specific impairment in locating objects relative to the environment rather than relative to the perceived viewpoint (i.e. allocentric rather than egocentric spatial memory) underlies her topographical disorientation. We discuss the likely neural bases of this deficit in the light of related studies in humans and animals, focusing on the hippocampus and related areas. The specificity of our test indicates a new way of assessing topographical disorientation, with possible application to the assessment of progressive dementias such as Alzheimer's disease.

Activities of Daily Living↗
PubMed2006

The boundary vector cell model of place cell firing and spatial memory.

We review evidence for the boundary vector cell model of the environmental determinants of the firing of hippocampal place cells. Preliminary experimental results are presented concerning the effects of addition or removal of environmental boundaries on place cell firing and evidence that boundary vector cells may exist in the subiculum. We review and update computational simulations predicting the location of human search within a virtual environment of variable geometry, assuming that boundary vector cells provide one of the input representations of location used in mammalian spatial memory. Finally, we extend the model to include experience-dependent modification of connection strengths through a BCM-like learning rule - the size and sign of strength change is influenced by historic activity of the postsynaptic cell. Simulations are compared to experimental data on the firing of place cells under geometrical manipulations to their environment. The relationship between neurophysiological results in rats and spatial behaviour in humans is discussed.

Action Potentials↗
PubMed2005

Differential developmental trajectories for egocentric, environmental and intrinsic frames of reference in spatial memory.

We studied the development of spatial frames of reference in children aged 3-6 years, who retrieved hidden toys from an array of identical containers bordered by landmarks under four conditions. By moving the child and/or the array between presentation and test, we varied the consistency of the hidden toy with (i) the body, and (ii) the testing room. The toy's position always remained consistent with (iii) the array and bordering landmarks. We found separate, additive performance advantages for consistency with body and room. These effects were already present at 3 years. A striking finding was that the room effect, which implies allocentric representations of the room and/or egocentric representations updated by self-motion, was much stronger in the youngest children than the body effect, which implies purely egocentric representations. Children as young as 3 years therefore had, and greatly favoured, spatial representations that were not purely egocentric. Viewpoint-independent recall based only on the array and bordering landmarks emerged at 5 years. There was no evidence that this later-developing ability, which implies object-referenced (intrinsic) representations, depended on verbal encodings. These findings indicate that core components of adult spatial competence, including parallel egocentric and nonegocentric representations of space, are present as early as 3 years. These are supplemented by later-developing object-referenced representations.

Child↗
PubMed2005

A metric for the cognitive map: found at last?

A network of brain areas collectively represent location, but the underlying nature of this "cognitive map" has remained elusive. A recent study reports that the activity patterns of some entorhinal cortical neurons form a remarkably regular array of evenly spaced peaks across the surface of the environment. These "grid cells" might be the basis of a metric used for calculating position, and their discovery could greatly advance our understanding of how navigational computations are performed.

Animals↗
PubMed2005

Doing the right thing: a common neural circuit for appropriate violent or compassionate behavior.

Humans have a considerable facility to adapt their behavior in a manner that is appropriate to social or societal context. A failure of this ability can lead to social exclusion and is a feature of disorders such as psychopathy and disruptive behavior disorder. We investigated the neural basis of this ability using a customized video game played by 12 healthy participants in an fMRI scanner. Two conditions involved extreme examples of context-appropriate action: shooting an aggressive humanoid assailant or healing a passive wounded person. Two control conditions involved carefully matched stimuli paired with inappropriate actions: shooting the person or healing the assailant. Surprisingly, the same circuit, including the amygdala and ventromedial prefrontal cortex, was activated when participants acted in a context-appropriate manner, whether being compassionate towards an injured conspecific or aggressive towards a violent assailant. The findings indicate a common system that guides behavioral expression appropriate to social or societal context irrespective of its aggressive or compassionate nature.

Adolescent↗
PubMed2005

Computational models of working memory: putting long-term memory into context.

Detailed computational modeling of human memory has typically been aimed at either short-term (working) memory or long-term memory in isolation. However, recent research highlights the importance of interactions between these systems for both item and order information. At the same time, computational models of both systems are beginning to converge onto a common framework in which items are associated with an evolving "context" signal and subsequently compete with one another at recall. We review some of these models, and discuss a common mechanism capable of modelling working memory and its interaction with long-term memory, focussing on memory for verbal sequences.

Attention↗
PubMed2005

An assessment of the toxicological significance of anthropogenic contaminants in Canadian arctic wildlife.

Anthropogenic contaminants have been a concern in the Canadian arctic for over 30 years due to relatively high concentrations of bioaccumulating and biomagnifying organochlorine contaminants (OCs) and toxic metals found in some arctic biota and humans. However, few studies have addressed the potential effects of these contaminants in Canadian arctic wildlife. Prior to 1997, biological effects data were minimal and insufficient at any level of biological organization. The present review summarizes recent studies on biological effects related to contaminant exposure, and compares new tissue concentration data to threshold effects levels. Weak relationships between cadmium, mercury and selenium burdens and health biomarkers in common eider ducks (Somateria mollissima borealis) in Nunavut were found but it was concluded that metals were not influencing the health of these birds. Black guillemots (Cepphus grylle) examined near PCB-contaminated Saglek Bay, Labrador, had enlarged livers, elevated EROD and liver lipid levels and reduced retinol (vitamin A) and retinyl palmitate levels, which correlated to PCB levels in the birds. Circulating levels of thyroid hormones in polar bears (Ursus maritimus) were correlated to PCB and HO-PCB plasma concentrations, but the impact at the population level is unknown. High PCB and organochlorine pesticide concentrations were found to be strongly associated with impaired humoral and cell-mediated immune responses in polar bears, implying an increased infection risk that could impact the population. In beluga whale (Delphinapterus leucas), cytochromes P450 (phase I) and conjugating (phase II) enzymes have been extensively profiled (immunochemically and catalytically) in liver, demonstrating the importance of contaminants in relation to enzyme induction, metabolism and potential contaminant bioactivation and fate. Concentrations of OCs and metals in arctic terrestrial wildlife, fish and seabirds are generally below effects thresholds, with the possible exception of PCBs in burbot (Lota lota) in some Yukon lakes, Greenland shark (Somniosus microcephalus), glaucous and great black-backed gulls (Larus hyperboreus and L. marinus), and TEQs of dioxin-like chemicals in seabird eggs. PCB and DDT concentrations in several arctic marine mammal species exceed effects thresholds, although evidence of stress in these populations is lacking. There is little evidence that contaminants are having widespread effects on the health of Canadian arctic organisms, with the possible exception of polar bears. However, further research and better understanding of organohalogen exposure in arctic biota is needed considering factors such as tissue levels that exceed effects thresholds, exposure to "new" organohalogen contaminants of concern, contaminated regions, and climate change.

Animals↗
PubMed2005

Attractor dynamics in the hippocampal representation of the local environment.

Memories are thought to be attractor states of neuronal representations, with the hippocampus a likely substrate for context-dependent episodic memories. However, such states have not been directly observed. For example, the hippocampal place cell representation of location was previously found to respond continuously to changes in environmental shape alone. We report that exposure to novel square and circular environments made of different materials creates attractor representations for both shapes: Place cells abruptly and simultaneously switch between representations as environmental shape changes incrementally. This enables study of attractor dynamics in a cognitive representation and may correspond to the formation of distinct contexts in context-dependent memory.

Action Potentials↗
PubMed2005

Characterizing multiple independent behavioral correlates of cell firing in freely moving animals.

The heterogeneous sampling of behavioral states by freely moving animals hinders our ability to relate neuronal firing rates to behavioral variables by introducing dependencies between them. We specifically consider the animal's location and orientation, although our analyses may generalize to other behavioral variables, such as speed of movement. A maximum-likelihood approach is presented for producing estimates of the separate histograms relating firing rate to multiple independent causes. Examples show that the method can be used to avoid the artifactual behavioral correlates of place and head direction-cell firing produced by standard analyses; to characterize the independent influences of both location and orientation in a third cell type (Cacucci et al., 2004); and to demonstrate the location-independence of the directional firing of head-direction cells.

Action Potentials↗
PubMed2005

Dual phase and rate coding in hippocampal place cells: theoretical significance and relationship to entorhinal grid cells.

We review the ideas and data behind the hypothesis that hippocampal pyramidal cells encode information by their phase of firing relative to the theta rhythm of the EEG. Particular focus is given to the further hypothesis that variations in firing rate can encode information independently from that encoded by firing phase. We discuss possible explanation of the phase-precession effect in terms of interference between two independent oscillatory influences on the pyramidal cell membrane potential, and the extent to which firing phase reflects internal dynamics or external (environmental) variables. Finally, we propose a model of the firing of the recently discovered "grid cells" in entorhinal cortex as part of a path-integration system, in combination with place cells and head-direction cells.

Action Potentials↗
PubMed2005

Complementary memory systems: competition, cooperation and compensation.

Spatial navigation depends on dissociable memory systems that have distinct neural bases and employ different forms of representation. One system gradually acquires reliable sequences of responses to given situations (e.g. repeatedly following a fixed route), and depends on the striatum. The other develops flexible representations permitting novel responses (e.g. finding new shortcuts), and depends on the hippocampus. Voermans and colleagues explore the interaction between these two systems using functional neuroimaging and behavioural measures in a clinical population.

Animals↗
PubMed2004

Theta-modulated place-by-direction cells in the hippocampal formation in the rat.

We report the spatial and temporal properties of a class of cells termed theta-modulated place-by-direction (TPD) cells recorded from the presubicular and parasubicular cortices of the rat. The firing characteristics of TPD cells in open-field enclosures were compared with those of the following two other well characterized cell classes in the hippocampal formation: place and head-direction cells. Unlike place cells, which code only for the animal's location, or head-direction cells, which code only for the animal's directional heading, TPD cells code for both the location and the head direction of the animal. Their firing is also strongly theta modulated, firing primarily at the negative-to-positive phase of the locally recorded theta wave. TPD theta modulation is significantly stronger than that of place cells. In contrast, the firing of head-direction cells is not modulated by theta at all. In repeated exposures to the same environment, the locational and directional signals of TPD cells are stable. When recorded in different environments, TPD locational and directional fields can uncouple, with the locational field shifting unpredictably ("remapping"), whereas the directional preference remains similar across environments.

Action Potentials↗
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↗