Statistical parametric mapping (SPM) is a statistical technique for examining differences in brain activity recorded during functional neuroimaging experiments. It was created by Karl Friston. It may alternatively refer to software created by the Wellcome Department of Imaging Neuroscience at University College London to carry out such analyses. Approach Unit of measurement Functional neuroimaging is one type of 'brain scanning'. It involves the measurement of brain activity. The measurement technique depends on the imaging technology (e.g., fMRI and PET). The scanner produces a 'map' of the area that is represented as voxels. Each voxel represents the activity of a specific volume in three-dimensional space. The exact size of a voxel varies depending on the technology. fMRI voxels typically represent a volume of 27 mm3 in an equilateral cuboid. Experimental design Researchers examine brain activity linked to a specific mental process or processes. One approach involves asking 'which areas of the brain are significantly more active when doing task A compared to task B?'. Although the tasks might be designed to be identical, except for the behaviour under investigation, the brain is still likely to show changes in activity between tasks due to factors other than task differences (as the brain coordinates many parallel functions unrelated to the task). Further, the signal may contain noise from the imaging process itself. To filter out these random effects, and to highlight the areas of activity linked specifically to the process under investigation, statistics look for the most significant differences. This involves a multi-stage process to prepare the data, and to analyse it using a general linear model. Image pre-processing Images from the scanner may be pre-processed to remove noise or correct for sampling errors. A study usually scans a subject several times. To account for the motion of the head between scans, the images are typically adjusted so voxels in each image correspond (approximately) to the same site in the brain. This is referred to as realignment or motion correction, see image realignment. Functional neuroimaging studies usually involve multiple participants, each of whom have differently shaped brains. All are likely to have the same gross anatomy, saving minor differences in overall brain size, individual variation in topography of the gyri and sulci of the cerebral cortex, and morphological differences in deep structures such as the corpus callosum. To aid comparisons, the 3D image of each brain is transformed so that superficial structures line