HOW TO UNDERSTAND IT Statistical parametric mapping

© 2004 Blackwell Publishing Ltd THE BASIC IDEA Brain responses are recorded using an imaging technique such as fMRI, which allows the repeated measurement of brain activity at hundreds of thousands of points, or voxels, throughout the brain (Fig. 1). Each voxel represents physiological responses from a small anatomical portion of the brain (typically 3 × 3 × 3 mm). The analysis of such data seeks to identify voxel-by-voxel whether any brain regions show a defi nite difference in brain activity. As with any other measurement in clinical science, to reliably establish a difference consideration must be given to the size of any difference relative to its variability. This requires that a statistical test be carried out to assess any differences in brain activity between the two different conditions, rather than just simply subtracting the activity in the two conditions without any consideration of the variability. The computation and visualization of such tests is known as statistical parametric mapping, and the purpose of this article is to explain how it works. HOW TO UNDERSTAND IT