Use of short intertrial intervals in single-trial experiments: A 3T fMRI-study

We investigated the detectability of task-related changes in the fMRI-signa l in an averaged single trial design under systematic variation of intertri al intervals (ITI) in the range between 4 and 12 s. Investigation of the si gnal timecourses showed a shortening of the baseline period and subsequentl y a reduction in signal amplitude with decreasing ITI. The main finding is that effect size, i.e., the ratio of task-related signal changes and error variance remained approximately constant from ITI of 12 s down to 6 s. At I TI = 4 s, the effect size was reduced by about 50%. The effects of ITI redu ction were comparable in all six cortical ROI which were analyzed. In two s ubcortical ROI, effect size was already reduced at longer ITI. At ITI = 4 s , the rising flank of the BOLD response was delayed compared to longer ITI. When the data were corrected for the temporal overlap of successive BOLD-r esponses, the signal amplitudes at ITI = 4 s were comparable to the amplitu des measured at an interval of 12 s. This indicated that the amplitude redu ction was mainly due to a linear superposition of the contiguous BOLD-respo nses. (C) 1998 Academic Press.