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Biomedical subjects

N Burgess

Publications and source records attributed to N Burgess.

At least 37 records · Page 2Linked to original sources
PubMed1998

Place cells, navigational accuracy, and the human hippocampus.

The hippocampal formation in both rats and humans is involved in spatial navigation. In the rat, cells coding for places, directions, and speed of movement have been recorded from the hippocampus proper and/or the neighbouring subicular complex. Place fields of a group of the hippocampal pyramidal cells cover the surface of an environment but do not appear to do so in any systematic fashion. That is, there is no topographical relation between the anatomical location of the cells within the hippocampus and the place fields of these cells in an environment. Recent work shows that place cells are responding to the summation of two or more Gaussian curves, each of which is fixed at a given distance to two or more walls in the environment. The walls themselves are probably identified by their allocentric direction relative to the rat and this information may be provided by the head direction cells. The right human hippocampus retains its role in spatial mapping as demonstrated by its activation during accurate navigation in imagined and virtual reality environments. In addition, it may have taken on wider memory functions, perhaps by the incorporation of a linear time tag which allows for the storage of the times of visits to particular locations. This extended system would serve as the basis for a spatio-temporal event or episodic memory system.

Animals↗
PubMed1998

Knowing where and getting there: a human navigation network.

The neural basis of navigation by humans was investigated with functional neuroimaging of brain activity during navigation in a familiar, yet complex virtual reality town. Activation of the right hippocampus was strongly associated with knowing accurately where places were located and navigating accurately between them. Getting to those places quickly was strongly associated with activation of the right caudate nucleus. These two right-side brain structures function in the context of associated activity in right inferior parietal and bilateral medial parietal regions that support egocentric movement through the virtual town, and activity in other left-side regions (hippocampus, frontal cortex) probably involved in nonspatial aspects of navigation. These findings outline a network of brain areas that support navigation in humans and link the functions of these regions to physiological observations in other mammals.

Brain Mapping↗
PubMed1998

Knowing where things are parahippocampal involvement in encoding object locations in virtual large-scale space.

The involvement of the medial temporal-lobe region in allocentric mapping of the environment has been observed in human lesion and functional imaging work. Cognitive models of environmental learning ascribe a key role to salient landmarks in representing large-scale space. In the present experiments we examined the neural substrates of the topographical memory acquisition process when environmental landmarks were more specifically identifiable. Using positron emission tomography (PET), we measured regional cerebral blood flow changes while normal subjects explored and learned in a virtual reality environment. One experiment involved an environment containing salient objects and textures that could be used to discriminate different rooms. Another experiment involved a plain empty environment in which rooms were distinguishable only by their shape. Learning in both cases activated a network of bilateral occipital, medial parietal, and occipitotemporal regions. The presence of salient objects and textures in an environment additionally resulted in increased activity in the right parahippocampal gyrus. This region was not activated during exploration of the empty environment. These findings suggest that encoding of salient objects into a representation of large-scale space is a critical factor in instigating parahippocampal involvement in topographical memory formation in humans and accords with previous studies implicating parahippocampal areas in the encoding of object location.

Adult↗
PubMed1998

Interactive mortality factors in common loons from Maritime Canada.

Between August 1992 and November 1995, 31 moribund or dead common loons (Gavia immer) found in the three Maritime provinces of Canada (New Brunswick, Nova Scotia, Prince Edward Island) were necropsied. Eight of these birds were in good body condition and died acutely from drowning or trauma. The remaining 23 birds were in poor body condition and had either chronic lead poisoning, respiratory mycosis, or oil contamination of their plumage. Loons in poor body condition had significantly higher numbers of intestinal trematodes and significantly higher levels of total renal mercury than loons in good body condition. Therefore, poor body condition in many loons was associated with two or more concurrent potential disease processes, although we could not establish a cause-effect relationship among these processes in individual birds. These results suggest that mortality in chronically ill wild animals can result from synergism among several potentially debilitating agents present in their environment.

Animals↗
PubMed1997

Robotic and neuronal simulation of the hippocampus and rat navigation.

The properties of hippocampal place cells are reviewed, with particular attention to the nature of the internal and external signals that support their firing. A neuronal simulation of the firing of place cells in open-field environments of varying shape is presented. This simulation is coupled with an existing model of how place-cell firing can be used to drive navigation, and is tested by implementation as a miniature mobile robot. The sensors on the robot provide visual, odometric and short-range proximity data, which are combined to estimate the distance of the walls of the enclosure from the robot and the robot's current heading direction. These inputs drive the hippocampal simulation, in which the robot's location is represented as the firing of place cells. If a goal location is encountered, learning occurs in connections from the concurrently active place cells to a set of 'goal cells', which guide subsequent navigation, allowing the robot to return to an unmarked location. The system shows good agreement with actual place-cell firing, and makes predictions regarding the firing of cells in the subiculum, the effect of blocking long-term synaptic changes, and the locus of search of rats after deformation of their environment.

Animals↗
PubMed1997

Directional control of hippocampal place fields.

Pyramidal cells in the rat hippocampus fire whenever the animal is in a particular place, suggesting that the hippocampus maintains a representation of the environment. Receptive fields of place cells (place fields) are largely determined by the distance of the rat from environmental walls. Because these walls are sometimes distinguishable only by their orientation with respect to the outside room, it has been hypothesised that a polarising directional input enables the cells to locate their fields off-centre in an otherwise symmetrical environment. We tested this hypothesis by gaining control of the rat's internal directional sense, independently of other cues, to see whether manipulating this sense could, by itself, produce a corresponding alteration in place field orientation. Place cells were recorded while rats foraged in a rectangular box, in the absence or presence of external room cues. With room cues masked, slow rotation of the rat and the box together caused the fields to rotate accordingly. Rotating the recording box alone by 180 degrees rarely caused corresponding field rotation, while rotating the rat alone 180 degrees outside the environment and then replacing it in the recording box almost always resulted in a corresponding rotation of the fields. This shows that place field orientation can be controlled by controlling the internal direction-sense of the rat, and it opens the door to psycho-physical exploration of the sensory basis of the direction sense. When room cues were present, distal visual cues predominated over internal cues in establishing place field orientation.

Animals↗
PubMed1996

Geometric determinants of the place fields of hippocampal neurons.

The human hippocampus has been implicated in memory, in particular episodic or declarative memory. In rats, hippocampal lesions cause selective spatial deficits, and hippocampal complex spike cells (place cells) exhibit spatially localized firing, suggesting a role in spatial memory, although broader functions have also been suggested. Here we report the identification of the environmental features controlling the location and shape of the receptive fields (place fields) of the place cells. This was done by recording from the same cell in four rectangular boxes that differed solely in the length of one or both sides. Most of our results are explained by a model in which the place field is formed by the summation of gaussian tuning curves, each oriented perpendicular to a box wall and peaked at a fixed distance from it.

Animals↗
PubMed1996

Neuronal computations underlying the firing of place cells and their role in navigation.

Our model of the spatial and temporal aspects of place cell firing and their role in rat navigation is reviewed. The model provides a candidate mechanism, at the level of individual cells, by which place cell information concerning self-localization could be used to guide navigation to previously visited reward sites. The model embodies specific predictions regarding the formation of place fields, the phase coding of place cell firing with respect to the hippocampal theta rhythm, and the formation of neuronal population vectors downstream from the place cells that code for the directions of goals during navigation. Recent experiments regarding the spatial distribution of place cell firing have confirmed our initial modeling hypothesis, that place fields are formed from Gaussian tuning curve inputs coding for the distances from environmental features, and enabled us to further specify the functional form of these inputs. Other recent experiments regarding the temporal distribution of place cell firing in two-dimensional environments have confirmed our predictions based on the temporal aspects of place cell firing on linear tracks. Directions for further experiments and refinements to the model are outlined for the future.

Animals↗
PubMed1995

Cephalosporin utilisation review and evaluation.

The clinical misuse of drugs may result in preventable patient morbidity and mortality, costly remedial care, additional costs for diagnosis and management of iatrogenic disease and unnecessary wastage of healthcare resources. In recognition of this problem, drug utilisation evaluation (DUE) has been recommended as a method for identifying inappropriate or unnecessary drug use and for promoting rational therapy. Growing concern over the widespread misuse of antibiotics, together with the emergence of antimicrobial resistance and escalating expenditures, has resulted in antibiotics being the drugs most frequently chosen for DUE projects. Cephalosporin DUE is well documented as being successful for modifying cephalosporin use and for containing drug expenditure. Studies range from isolated projects to ongoing programmes that comprehensively evaluate cephalosporin use and the impact of corrective strategies. Sensible use of antibiotics requires a clear understanding of the infectious process, the clinical pharmacology of anti-infective agents and an appreciation of clinical and microbiological monitoring and assessment. Audit criteria that incorporate the above principles, and which are described in the studies reviewed in this article, will be useful for other investigators. Through its DUE programme, the Royal Adelaide Hospital has investigated the use of cephalosporins, including ceftriaxone, ceftazidime and cefoxitin. These reviews have resulted in improvements in cephalosporin use and significant cost savings. Alterations to cephalosporin use that were recommended following these reviews have not resulted in adverse changes to post-operative infection rates, clinical outcomes or adverse drug reactions. This experience, combined with that of other investigators, serves as a useful model for the promotion of rational and economical therapy with cephalosporins and other drug groups.

Bacterial Infections↗
PubMed1994

A constructive algorithm that converges for real-valued input patterns.

A constructive algorithm is presented which combines the architecture of Cascade Correlation and the training of perceptron-like hidden units with the specific error-correcting roles of Upstart. Convergence to zero errors is proved for any consistent classification of real-valued pattern vectors. Addition of one extra element to each pattern allows hyper-spherical decision regions and enables convergence on real-valued inputs for existing constructive algorithms. Simulations demonstrate robust convergence and economical construction of hidden units in the benchmark "N-bit parity" and "twin spirals" problems.

Algorithms↗
PubMed1993

The frusemide test: simple screening test for renal acidification defect in urolithiasis.

Urinary pH was first labelled as a risk factor for patients with recurrent calcium stone disease of the renal tract in 1978. The standard diagnostic test for patients with a history of stones, who appear to have urinary acidification defects is the ammonium chloride test. However, the performance of this test results in significant morbidity, particularly nausea and vomiting, and as a result there is little enthusiasm for screening for such defects. We describe a screening test that may be used to determine which patients require more definitive testing. It involves an oral dose of frusemide (40 mg) followed by half-hourly urine sampling for pH. For the detection of a renal tubular acidification defect, the frusemide test had sensitivity = 100%, specificity = 82%, predictive value of a positive result = 40%, predictive value of a negative result = 100%, screening efficiency = 84%, and there was no morbidity. During the ammonium chloride test 80% of the patients vomited or felt very nauseated.

Ammonium Chloride↗
PubMed1991

Earmoulds: some benefits from horn fitting.

The narrow bore of conventional hearing aid earmoulds restricts the transmission of higher auditory frequencies. For subjects with moderate to severe high-frequency hearing loss this is likely to have an adverse effect on the perception and recognition of phonemes with high-frequency components. Twenty-two subjects with predominantly high-frequency hearing loss underwent a series of tests to compare the performance of earmoulds having conventional 1.9 mm through-tubing with earmoulds fitted with a smooth horn with a final internal diameter of 4 mm. The test battery comprised free-field Békésy audiometry, in-the-ear pressure measurement, speech audiometry using AB word lists and semantic differential rating of the relative sound quality of the two systems. Subjectively the horn fitting was rated as clearer, more natural, undistorted and acoustically comfortable. Objectively, by both Békésy audiometry and in-the-ear pressure measurement, the horn gave more gain in the higher auditory frequencies. With the horn there was improved recognition of phonemes, especially fricatives and affricatives.

Adult↗
PubMed1985

Improved intraoperative evaluation of mitral valve operations utilizing two-dimensional contrast echocardiography.

Whenever possible, precise mitral valve repair is preferable to valve replacement. Present methods for intraoperative detection of mitral regurgitation, primarily hemodynamic measurements and direct palpation, may underestimate or not detect the presence and severity of regurgitation. We have investigated two-dimensional contrast echocardiography as a means of improving our intraoperative assessment of mitral valve function both before and after repair or replacement. After exposure of the heart, a baseline two-dimensional echocardiogram (in modified long- and short-axis planes) is performed using a hand-held 5 mHz mechanical transducer. Five milliliters of agitated 5% dextrose in water is injected into the left ventricle through a transseptal needle to generate detectable microbubbles. In the absence of mitral regurgitation, virtually all microbubbles exit through the aorta; in the presence of regurgitation, a mass of microbubbles reflux into the left atrium. After repair of the mitral valve and immediately after bypass, the contrast echocardiogram is repeated and hemodynamic measurements are obtained. Forty-three patients (37 with mitral valve disease and six additional patients without mitral disease) undergoing cardiac operations were evaluated. Experience with intraoperative two-dimensional contrast echocardiography has accurately demonstrated relatively small degrees of mitral regurgitation when conventional techniques failed to do so and has allowed more precise repair of the residual regurgitation. Two commissurotomy and two annuloplasty patients who were thought to have satisfactory repairs underwent immediate second procedures because of significant residual mitral regurgitation demonstrated solely by this echocardiographic microbubble technique. No complications associated with this technique have developed. We conclude that intraoperative two-dimensional contrast echocardiography is a sensitive and safe technique that allows intraoperative detection of even small degrees of mitral regurgitation and provides a basis for precise repair of mitral valve lesions.

Adult↗