The variability of serial fMRI data: correlation between a visual and a motor task.

We investigated whether the intersession variability of serial fMRI studies correlates between two activation modalities, i.e. a standardized visual and a standardized motor task. Six volunteers were scanned in at least weekly intervals. The number of pixels activated as well as the activation amplitude varied widely. The maximal difference of the number of pixels activated was 1150%, of the activation amplitude 250%. In three volunteers, the variability was highly correlated between the two tasks. Three other volunteers showed one outlier each. We conclude that the intersession variability is due to global factors affecting the whole brain, but that due to unpredictable outliers, using a standardized task to normalize the data of interest is of limited value.