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N Burgess

Publications and source records attributed to N Burgess.

At least 19 recordsLinked to original sources
PubMed2005

Anterior prefrontal involvement in episodic retrieval reflects contextual interference.

Different patterns of prefrontal activation are commonly found in studies of episodic and source memory (typically anterior and lateral) compared to those found in studies of autobiographical memory (typically ventromedial). We investigated a proposal that the former pattern reflects contextual interference when retrieving events that occurred in similar contexts. We used virtual reality to simulate contextually varied life-like events, in which subjects received distinct objects from a number of people in a number of locations. We compared fMRI data from two experiments in which the number of events per context varied. The first experiment (Burgess, N., Maguire, E.A., Spiers, H.J., and O'Keefe, J. 2001. A temporoparietal and prefrontal network for retrieving the spatial context of lifelike events. Neuroimage 14, 439-453) involved 16 objects received from one of two people in one of two locations. The second experiment involved 20 objects, each received from a different person in a different location. The first experiment showed extensive bilateral activation of anterior and lateral prefrontal cortex, as well as a medial temporal and parietal network characteristic of both autobiographical and episodic memory. In the second study, the prefrontal activations were largely absent, while the medial parietal and temporal activations remained, and a ventromedial prefrontal area was additionally activated. Direct comparisons revealed large areas of significantly reduced activation in BA10, with lesser reductions in lateral prefrontal regions. We suggest that involvement of these prefrontal regains in episodic and source memory reflects the use of paradigms involving many events and few sources rather than any fundamental processing requirement of contextual retrieval in the absence of interference.

Adolescent↗
PubMed2005

Pseudospirochaetosis of the urinary bladder.

This report describes an elderly patient with urinary symptoms who showed surface colonisation of the transitional mucosa of the bladder by an unusual haematoxophilic microorganism superficially resembling the "blue fuzz" seen in colonic biopsies showing intestinal spirochaetosis. Special stains showed that the organisms were Gram and Giemsa positive, weakly argyrophilic, and Ziehl-Nielsen negative. Immunostains were negative for Helicobacter pylori and electron microscopy revealed curious curved bodies, which were difficult to classify. Therefore, this condition was described as pseudospirochaetosis of the urinary bladder. The urinary symptoms regressed on treatment with ciprofloxacin. The clinicopathological relevance of these findings is discussed in the report.

Aged↗
PubMed2001

Memory for events and their spatial context: models and experiments.

The computational role of the hippocampus in memory has been characterized as: (i) an index to disparate neocortical storage sites; (ii) a time-limited store supporting neocortical long-term memory; and (iii) a content-addressable associative memory. These ideas are reviewed and related to several general aspects of episodic memory, including the differences between episodic, recognition and semantic memory, and whether hippocampal lesions differentially affect recent or remote memories. Some outstanding questions remain, such as: what characterizes episodic retrieval as opposed to other forms of read-out from memory; what triggers the storage of an event memory; and what are the neural mechanisms involved? To address these questions a neural-level model of the medial temporal and parietal roles in retrieval of the spatial context of an event is presented. This model combines the idea that retrieval of the rich context of real-life events is a central characteristic of episodic memory, and the idea that medial temporal allocentric representations are used in long-term storage while parietal egocentric representations are used to imagine, manipulate and re-experience the products of retrieval. The model is consistent with the known neural representation of spatial information in the brain, and provides an explanation for the involvement of Papez's circuit in both the representation of heading direction and in the recollection of episodic information. Two experiments relating to the model are briefly described. A functional neuroimaging study of memory for the spatial context of life-like events in virtual reality provides support for the model's functional localization. A neuropsychological experiment suggests that the hippocampus does store an allocentric representation of spatial locations.

Animals↗
PubMed2001

Methods for developmental studies of fear conditioning circuitry.

Psychophysiologic studies use air puff as an aversive stimulus to document abnormal fear conditioning in children of parents with anxiety disorders. This study used functional magnetic resonance imaging (fMRI) to examine changes in amygdala activity during air-puff conditioning among adults. Blood oxygen level-dependent (BOLD) signal was monitored in seven adults during 16 alternating presentations of two different colored lights (CS+ vs. CS-), one of which was consistently paired with an aversive air puff. A region-of-interest analysis demonstrated differential change in BOLD signal in the right but not left amygdala across CS+ versus CS- viewing. The amygdala is engaged by pairing of a light with an air puff. Given that prior studies relate air-puff conditioning to risk for anxiety in children, these methods may provide an avenue for directly studying the developmental neurobiology of fear conditioning.

Adult↗
PubMed2001

Conservation conflicts across Africa.

There is increasing evidence that areas of outstanding conservation importance may coincide with dense human settlement or impact. We tested the generality of these findings using 1 degree-resolution data for sub-Saharan Africa. We find that human population density is positively correlated with species richness of birds, mammals, snakes, and amphibians. This association holds for widespread, narrowly endemic, and threatened species and looks set to persist in the face of foreseeable population growth. Our results contradict earlier expectations of low conflict based on the idea that species richness decreases and human impact increases with primary productivity. We find that across Africa, both variables instead exhibit unimodal relationships with productivity. Modifying priority-setting to take account of human density shows that, at this scale, conflicts between conservation and development are not easily avoided, because many densely inhabited grid cells contain species found nowhere else.

Africa South of the Sahara↗
PubMed2001

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.

Adult↗
PubMed2001

A temporoparietal and prefrontal network for retrieving the spatial context of lifelike events.

Virtual reality (VR) and event-related functional magnetic resonance imaging were used to study memory for the spatial context of controlled but lifelike events. Subjects received a set of objects from two different people in two different places within a VR environment. Memory for the objects, and for where and from whom they were received was tested by putting the subject back into a place in the company of a person and giving a paired forced choice of objects. In four conditions objects had to be chosen according to different criteria: which was received in that place, which was received from that person, which object was recognized, and which object was widest. Event-related functional magnetic resonance imaging was performed during testing to identify areas involved in retrieval of the spatial context of an event. A network of areas was identified consisting of a temporoparietal pathway running between the precuneus and parahippocampi via retrosplenial cortex and the parieto-occipital sulcus, left hippocampus, bilateral posterior parietal, dorsolateral, ventrolateral and anterior prefrontal cortices, and the anterior cingulate. Of these areas the parahippocampal, right posterior parietal, and posteriodorsal medial parietal areas were specifically involved in retrieval of spatial context compared to retrieval of nonspatial context. The posterior activations are consistent with a model of long-term storage of allocentric representations in medial temporal regions with translation to body-centered and head-centered representations computed in right posterior parietal cortex and buffered in the temporoparietal pathway so as to provide an imageable representation in the precuneus. Prefrontal activations are consistent with strategic retrieval processes, including those required to overcome the interference between the highly similar events.

Adult↗
PubMed2001

Hippocampal amnesia.

This article reviews 147 cases of amnesia following damage including the hippocampus or fornix as reported in 179 publications. The aetiology, mnestic abilities and reference(s) are tabulated for each case. Consistent findings across cases include the association of bilateral hippocampal damage with a deficit in anterograde episodic memory combined with spared procedural and working memory. The limited nature of retrograde amnesia following lesions to the fornix is also noted. Less consistent and thus more controversial findings, include effects of lesion size or laterality, deficits in semantic memory or familiarity-based recognition and the extent of retrograde amnesia. The evidence concerning these issues is reviewed across cases.

Amnesia↗
PubMed2001

Unilateral temporal lobectomy patients show lateralized topographical and episodic memory deficits in a virtual town.

A large-scale virtual reality town was used to test the topographical and episodic memory of patients with unilateral temporal lobe damage. Seventeen right and 13 left temporal lobectomy patients were compared with 16 healthy matched control subjects. After they had explored the town, subjects' topographical memory was tested by requiring them to navigate to specific locations in the town. The ability to recognize scenes from and draw maps of the virtual town was also assessed. Following the topographical memory tests, subjects followed a route around the same town but now collected objects from two different characters in two different locations. Episodic memory for various aspects of these events was then assessed by paired forced-choice recognition tests. The results showed an interaction between laterality and test type such that the right temporal lobectomy (RTL) patients were worse on tests of topographical memory, and the left temporal lobectomy (LTL) patients worse on tests of context-dependent episodic memory. Specifically, the RTL group was impaired on navigation, scene recognition and map drawing relative to control subjects. They were also impaired on recognition of objects in the episodic memory task. The LTL group was impaired relative to control subjects on their memory for contextual aspects of the events, such as who gave them the objects, the order in which objects were received and the locations in which they received them. They were also mildly impaired on topographical memory, but less so than the RTL group. These results suggest that topographical memory is predominately mediated by structures in the right medial temporal lobe, whereas the context-dependent aspects of episodic memory in this non-verbal test are more dependent on the left medial temporal lobe.

Adult↗
PubMed2000

Modeling place fields in terms of the cortical inputs to the hippocampus.

A model of place-cell firing is presented that makes quantitative predictions about specific place cells' spatial receptive fields following changes to the rat's environment. A place cell's firing rate is modeled as a function of the rat's location by the thresholded sum of the firing rates of a number of putative cortical inputs. These inputs are tuned to respond whenever an environmental boundary is at a particular distance and allocentric direction from the rat. The initial behavior of a place cell in any environment is simply determined by its set of inputs and its threshold; learning is not necessary. The model is shown to produce a good fit to the firing of individual place cells, and populations of place cells across environments of differing shape. The cells' behavior can be predicted for novel environments of arbitrary size and shape, or for manipulations such as introducing a barrier. The model can be extended to make behavioral predictions regarding spatial memory.

Afferent Pathways↗
PubMed2000

Predictions derived from modelling the hippocampal role in navigation.

A computational model of the lesion and single unit data from navigation in rats is reviewed. The model uses external (visual) and internal (odometric) information from the environment to drive the firing of simulated hippocampal place cells. Constraints on the functional form of these inputs are drawn from experiments using an environment of modifiable shape. The place cell representation is used to guide navigation via the creation of a representation of goal location via Hebbian modification of synaptic strengths. The model includes consideration of the phase of firing of place cells with respect to the theta rhythm of hippocampal EEG. A series of predictions for behavioural and single-unit data in rats are derived from the input and output representations of the model.

Animals↗
PubMed2000

Recoding, storage, rehearsal and grouping in verbal short-term memory: an fMRI study.

Functional magnetic resonance imaging (fMRI) of healthy volunteers is used to localise the processes involved in verbal short-term memory (VSTM) for sequences of visual stimuli. Specifically, the brain areas underlying (i) recoding, (ii) storage, (iii) rehearsal and (iv) temporal grouping are investigated. Successive subtraction of images obtained from five tasks revealed a network of left-lateralised areas, including posterior temporal regions, supramarginal gyri, Broca's area and dorsolateral premotor cortex. The results are discussed in relation to neuropsychological distinctions between recoding and rehearsal, previous neuroimaging studies of storage and rehearsal, and, in particular, a recent connectionist model of VSTM that makes explicit assumptions about the temporal organisation of rehearsal. The functional modules of this model are tentatively mapped onto the brain in light of the imaging results. Our findings are consistent with the representation of verbal item information in left posterior temporal areas and short-term storage of phonological information in left supramarginal gyrus. They also suggest that left dorsolateral premotor cortex is involved in the maintenance of temporal order, possibly as the location of a timing signal used in the rhythmic organisation of rehearsal, whereas Broca's area supports the articulatory processes required for phonological recoding of visual stimuli.

Adult↗
PubMed1999

Human spatial navigation: cognitive maps, sexual dimorphism, and neural substrates.

Recent research on navigation has been particularly notable for the increased understanding of the factors affecting human navigation and the neural networks supporting it. The use of virtual reality environments has made it possible to explore the effect of environment layout and content on way-finding performance, and it has shown that these effects may interact with the sex and age of subjects. Functional brain imaging, combined with the use of virtual environments, has revealed strong parallels between humans and other animals in the neural basis of navigation.

Aging↗