Simulated Data

The artificial data enabling gold standard comparisons were generated as follows: a high resolution EPI volume (2 x 2 x 2 mm) was duplicated 180 times and each volume was transformed by an affine matrix corresponding to real motion estimates taken from one of two studies where the subject had been asked to move their head appreciably during the scan. Three further groups of images were generated using motion estimates from experiments where the subject had been asked to remain as still as possible. Within these five motion designs, three further groups of data were created corresponding to audiovisual activation at 0%, 2.5% and 5% of the overall voxel intensities by modulating the intensity values according to a mask derived from real fMRI data. Once the activation (if any) had been applied and the volumes transformed by the corresponding parameters, the data was subsampled to 4mm x 4mm x 6mm voxels, and appropriately cropped to avoid introducing any `padding' voxels. The use of a high resolution template image which is then subsampled should minimise the effect of interpolation when applying such transformations to the data. Within our correction scheme, there are a number of stages which can be tuned to optimise the accuracy of the correction. The remainder of this section aims to find a robust set of parameters which give consistently accurate results on all data presented. We begin by examining the comparative accuracy of several cost functions which can be used with our optimisation scheme. Later we proceed to examine the impact made by the choice of interpolation scheme and registration schedule.