title: Effects of single-trial averaging on spatial extent of brain activation detected by fMRI are subject and task dependent.
abstract: OBJECTIVE
  The effects of single-trial averaging on the spatial extent of event-related fMRI activation may vary between subjects and tasks. The purpose of this study was to evaluate this variability using a visual task and a word generation task.
  
  
  METHODS
  Five Chinese right-handed male volunteers participated in the experiment. Experiments were conducted using a 1.5 T clinical MRI scanner with a T2*-weighted single-shot gradient-echo EPI sequence. Each task contained 150 trials that were separated into 5 runs. For each voxel, time courses averaged across different numbers of randomly selected trials, were obtained. They were applied for determining the voxels with significant activations, using a students' t-test (p<0.001, uncorrected).
  
  
  RESULTS
  Consistent with previous findings, the number of the activated voxels increased monotonically with the number of trials combined. The ascending rate and the maximum number of the activated voxels were different, however, between tasks and among subjects.
  
  
  CONCLUSIONS
  The effects of single-trial averaging were found to vary significantly between tasks and subjects. Therefore, we strongly advise to carefully consider such variability when using the spatial extent of activation as a measure in a group or a task comparison.
authors: MRI Center, Chang Gung Memorial Hospital, Taoyuan, Taiwan;
authors: Sy
authors: S
authors: Fang
authors: Jj
authors: J
authors: Wang
authors: Yy
authors: Y
authors: Hsu
authors: Yl
authors: Y
authors: Wan
authors: Yy
authors: Y
authors: Wai
authors: Hl
authors: H
authors: Liu
references: Proc Natl Acad Sci U S A. 1996 Dec 10;93(25):14878-83
references: 8962149
references: Neuroimage. 2004 Jan;21(1):58-68
references: 14741642
references: Neuroimage. 2002 Apr;15(4):823-35
references: 11906223
references: Cortex. 1993 Mar;29(1):115-34
references: 8472549
references: Neuroimage. 2003 May;19(1):132-44
references: 12781733
references: J Cogn Neurosci. 2003 Oct 1;15(7):1019-27
references: 14614812
references: Neuroreport. 2001 Aug 8;12(11):2411-6
references: 11496120
references: Hum Brain Mapp. 2000 May;10(1):16-27
references: 10843515
references: Neuroimage. 2004 Jan;21(1):173-80
references: 14741654
references: J Magn Reson Imaging. 1999 Jul;10(1):33-40
references: 10398975
references: Magn Reson Med. 1997 Sep;38(3):508-17
references: 9339453
references: Neuroimage. 2003 Feb;18(2):468-82
references: 12595200
references: J Int Neuropsychol Soc. 2002 Mar;8(3):349-59
references: 11939694
references: Neuroimage. 1999 Aug;10(2):216-24
references: 10417254
references: Neuroimage. 2003 Jul;19(3):784-96
references: 12880807
references: Neuroimage. 2001 May;13(5):836-46
references: 11304080
references: Hum Brain Mapp. 1994;1(3):210-20
references: 24578041
references: Hum Brain Mapp. 2000 Mar;9(3):156-64
references: 10739366
references: Brain Lang. 2002 Mar;80(3):421-37
references: 11896650
references: Neuroimage. 2000 Jun;11(6 Pt 1):735-59
references: 10860799