title: The effects of single-trial averaging upon the spatial extent of fMRI activation.
abstract: We examined effects of trial averaging upon spatial extent, spatial topography, and temporal properties of fMRI activation. Two subjects participated in an event-related visual stimulation design. There was an exponential relation between number of trials and spatial extent, such that additional trials identified, on average, a constant proportion of the remaining voxels. At values typical of fMRI experimentation (e.g. 50 trials) only about 50% of eventually active voxels were significant; asymptotic values were approached by 150 trials. The variability of the estimated hemodynamic response decreased with signal averaging, becoming stable across samples of > or = 25 trials. Therefore, group or condition differences may result from differences in voxelwise noise exacerbated by averaging small numbers of trials.
authors.affiliation: Brain Imaging and Analysis Center, Duke University Medical Center, Box 3918, Durham, NC 27710, USA.
authors.forename: S A
authors.initials: SA
authors.lastname: Huettel
authors.forename: G
authors.initials: G
authors.lastname: McCarthy
mesh_terms: D000328:Adult; D016477:Artifacts; D005074:Evoked Potentials, Visual / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D010775:Photic Stimulation; D014793:Visual Cortex / Q000502:physiology*