We have previously shown that event-related functional magnetic resonance imaging (ER-fMRI) may be used to record responses to the rapid, interleaved presentation of stimuli in the three-stimulus oddball task. The present study examined the sensitivity of ER-fMRI responses to variations in the range of inter-stimulus intervals (ISIs, calculated as the time from the offset of one stimulus to the onset of the next stimulus) and the type of behavioral response task used. ISIs were varied between a wide ISI range (550-2,050 msec) and a narrow ISI range (800-1,200 msec), while maintaining a similar mean ISI (approximately 1 stimulus per sec) between experiments. The response task was varied between button press and subvocal target counting. Gradient echo, echo planar images were acquired for each of three experiments (wide ISI with button press, narrow ISI with button press, and wide-ISI with counting) in five subjects. Target stimuli generated increased fMRI signal in a wide range of brain regions. The use of a narrow ISI range generated a greater volume of subcortical activity and a reduced volume of cortical activity relative to a wide ISI range. The counting task generated a larger amplitude and longer lasting evoked response in brain regions that responded during all three experiments. Rare distractor stimuli evoked fMRI signal change primarily in orbitofrontal, ventral-medial prefrontal and superior parietal cortex. These results illustrate that although ER-fMRI is relatively insensitive as a technique to small variations in the timing of stimulus-evoked responses, it is remarkably sensitive to consequences such variations have for the topographic location and amplitude of neural responses to stimuli.
Copyright 2001 Wiley-Liss, Inc.