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Magnetic resonance imaging enables the parameterisation of various physiological processes by comparison of a baseline set of images with a set of images taken after some perturbation of the nuclear spins. By linking this perturbation to a physiological process, changes in signal intensity become a proxy measure of that physiological process. This thesis is concerned with the accuracy and meaning of the measurements made using the natural self-diffusion of water. The first section deals with diffusion tensor measurements, which are three-dimensional measures of water self-diffusion using linear magnetic field gradients. The thesis looks in turn at the effect of signal noise, and partial volume effects on parameters such as the mean diffusivity, the eigenvalues and the fractional anisotropy of the diffusion tensor. The accuracy of the technique, as indicated by the results of numerical simulations, is compared to those acquired from phantom and brain imaging. These results provide an estimate of the dependence of diffusion measurements on technique. The coefficient of variation of diffusion measurements is shown to depend on the metric being used. The voxel volume is shown to have a large effect on the measured anisotropy but not the mean diffusivity. The elevation in diffusion anisotropy observed in acute stroke was then investigated using the eigenvalues of the diffusion tensor, and isotropic and anisotropic tensors. The second section addresses the relationship between gradient-echo and spin-echo dynamic-susceptibility-contrast measurements of cerebral perfusion. The basic contrast measurement is the diffusion of water through pseudo-random magnetic fields created by the injection of a contrast agent. Numerical simulations were used to elucidate the relationship between signal loss and biophysical variables, such as the vessel geometry, the blood volume, and the rate of water self-diffusion.

Research to date suggests that the extra time taken by people to attend to items they had previously ignored, referred to as Negative Priming (NP), is either missing or impoverished in, among others: young children, people with schizophrenia, people with high schizotypy scores and with poor comprehension. These groups also show deficits in selective attention; and this common ground, combined with the original theoretical view of negative priming as due to efficient inhibition, had encouraged the view that inhibitory difficulties may be associated with attention problems. This research was planned to replicate, develop and clarify some of these findings, with the aim of answering three questions; 1) At what age does negative priming normally develop? 2) Is it a stable and a general mechanism? 3) Is it an inhibitory mechanism? A series of six multi-task experiments were carried out using both a standard Colour name NP task, and a new 'Animal choosing' NP task, developed to engage the interest of children. The new animal task was run with both a manual and a verbal response mode. Other tasks included a schizotypy questionnaire, and stories for comprehension tests NP did not show a developmental course, some children show significant NP at six years old, and many 12 year olds show none on the same task. Test re-test of one task showed stability of priming effects between the tasks, but the NP delay dissipated with practice. Experiments using different tasks and both verbal and manual response modes did not support the idea of a general inhibitory mechanism, as there was no correlation of NP across task and mode. It is suggested that NP is not a stable individual phenomenon, but a task and response-mode specific activation of selective attention. As such, the use of NP tests as diagnostic of inhibitory levels in individuals or groups should be treated with caution. Proposals are made for a theoretical formulation, with associated testable hypotheses.