Biomedical subjects

David Gasston

Publications and source records attributed to David Gasston.

3 recordsLinked to original sources

PubMed2005

Effects of ketamine on prefrontal and striatal regions in an overt verbal fluency task: a functional magnetic resonance imaging study.

RATIONALE: Glutamatergic dysfunction at N-methyl-D: -aspartate (NMDA) receptors has been proposed as a neurochemical model for schizophrenia. A key feature of this disorder is impairments in cognitive function. OBJECTIVE: The present study sought to investigate the effects of ketamine, an NMDA antagonist, on the performance and neural correlates of verbal fluency, a task that engages executive function. METHODS: Ten healthy dextral male volunteers received intravenous placebo normal saline or ketamine (bolus of 0.23 mg/kg and infusion of 0.65 mg/kg), administered in a double-blind, randomized order, during two functional magnetic resonance imaging sessions. During scanning, subjects performed a verbal fluency task. Two levels of cognitive load were examined in the task, and overt responses were acquired in order to measure subject performance on-line. RESULTS: Ketamine induced symptoms in the healthy individuals comparable to an acute psychotic state. Although ketamine did not significantly impair task performance relative to placebo, an interaction of task demand with ketamine was observed in the anterior cingulate, prefrontal, and striatal regions. CONCLUSIONS: The behavioural and functional effects of ketamine during verbal fluency in healthy individuals were comparable to those evident in patients with schizophrenia. The findings support a role for glutamatergic dysfunction in the pathophysiology of schizophrenia.

Corpus Striatum↗

PubMed2004

Allocentric spatial memory activation of the hippocampal formation measured with fMRI.

Hippocampal activation was investigated, comparing allocentric and egocentric spatial memory. Healthy participants were immersed in a virtual reality circular arena, with pattern-rendered walls. In a viewpoint-independent task, they moved toward a pole, which was then removed. They were relocated to another position and had to move to the prior location of the pole. For viewpoint-dependent memory, the participants were not moved to a new starting point, but the patterns were rotated to prevent them from indicating the final position. Hippocampal and parahippocampal activation were found in the viewpoint-independent memory encoding phase. Viewpoint-dependent memory did not result in such activation. These results suggest differential activation of the hippocampal formation during allocentric encoding, in partial support of the spatial mapping hypothesis as applied to humans.

Adult↗

PubMed2002

Isotropic resolution diffusion tensor imaging with whole brain acquisition in a clinically acceptable time.

Our objective was to develop a diffusion tensor MR imaging pulse sequence that allows whole brain coverage with isotropic resolution within a clinically acceptable time. A single-shot, cardiac-gated MR pulse sequence, optimized for measuring the diffusion tensor in human brain, was developed to provide whole-brain coverage with isotropic (2.5 x 2.5 x 2.5 mm) spatial resolution, within a total imaging time of approximately 15 min. The diffusion tensor was computed for each voxel in the whole volume and the data processed for visualization in three orthogonal planes. Anisotropy data were further visualized using a maximum-intensity projection algorithm. Finally, reconstruction of fiber-tract trajectories i.e., "tractography" was performed. Images obtained with this pulse sequence provide clear delineation of individual white matter tracts, from the most superior cortical regions down to the cerebellum and brain stem. Because the data are acquired with isotropic resolution, they can be reformatted in any plane and the sequence can therefore be used, in general, for macroscopic neurological or psychiatric neuroimaging investigations. The 3D visualization afforded by maximum intensity projection imaging and tractography provided easy visualization of individual white matter fasciculi, which may be important sites of neuropathological degeneration or abnormal brain development. This study has shown that it is possible to obtain robust, high quality diffusion tensor MR data at 1.5 Tesla with isotropic resolution (2.5 x 2.5 x 2.5 mm) from the whole brain within a sufficiently short imaging time that it may be incorporated into clinical imaging protocols.

Adult↗