The effects of single-trial averaging upon the spatial extent of fMRI activation

Neuroreport. 2001 Aug 8;12(11):2411-6. doi: 10.1097/00001756-200108080-00025.


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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Artifacts
  • Evoked Potentials, Visual / physiology*
  • Humans
  • Magnetic Resonance Imaging / methods*
  • Male
  • Middle Aged
  • Photic Stimulation
  • Visual Cortex / physiology*