SELECTIVE AVERAGING OF RAPIDLY PRESENTED INDIVIDUAL TRIALS USING FMRI

A major limitation in conducting functional neuroimaging studies, part icularly for cognitive experiments, has been the use of blocked task p aradigms. Here we explored whether selective averaging techniques simi lar to those applied in event-related potential (ERP) experiments coul d be used to demonstrate functional magnetic resonance imaging (fMRI) responses to rapidly intermixed trials. In the first two experiments, we observed that for 1-sec trials of full-field visual checkerboard st imulation, the fMRI blood oxygenation level-dependent (BOLD) signal su mmated in a roughly linear fashion across successive trials even at ve ry short (2 sec and 5 sec) intertrial intervals, although subtle depar tures from linearity were observed. In experiments 3 and 4, we observe d that it is possible to obtain robust activation maps for rapidly pre sented randomly mixed trial types (left-and right-hemifield visual che ckerboard stimulation) spaced as little as 2 sec apart. Taken collecti vely, these results suggest that selective averaging may enable fMRI e xperimental designs identical to those used in typical behavioral and ERP studies. The ability to analyze closely spaced single-trial, or ev ent-related, signals provides for a class of experiments which cannot be conducted using blocked designs. Trial types can be randomly interm ixed, and selective averaging based upon trial type and/or subject per formance is possible. (C) 1997 Wiley-Liss, Inc.