A METHOD TO DETERMINE ACTIVATION THRESHOLDS IN FMRI PARADIGMS

Determining meaningful activation thresholds in functional magnetic re sonance imaging (fMRI) paradigms is complicated by several factors. Th ese include the time-series nature of the data, the influence of physi ological rhythms (e.g. respiration) and vacillations introduced by the experimental design (e.g. cueing). We present an empirical threshold for each subject and each fMRI experiment that takes these factors int o account. The method requires an additional fMRI data set as similar to the experimental paradigm as possible without dichotomously varying the experimental task of interest. A letter fluency task was used to illustrate this method. This technique differs from classical methods since the Pearson correlation probability values tabulated from statis tical theory are not used. Rather, each subject defines his or her own set of threshold probability values for correlations. It is against t hese empirical thresholds, not Pearson's, that an experimental fMRI co rrelation is assessed. (C) 1997 Elsevier Science Ireland Ltd.