Abstract
Most approaches to detecting changes in functional brain images assume activations are focal or very localized. However the brain's response to cognitive or sensorimotor challenge may be spatially or anatomically distributed. In this paper we consider the usefulness of a test based on the mean sum of squares of statistical parametric maps. The performance of this test is evaluated using simulated and real data, and compared to the gamma_2 test (Fox et al., 1989), a test proposed by Friston et al. (1990) on the size of the activated region, and a focal activation test based on the intensity of local maxima (Friston et al., 1991; Worsley et al., 1992). We demonstrate that the mean sum of squares test is more sensitive to non-focal signals and propose that it could be used to complement approaches that are more sensitive to focal activations.