Does stimulus quality affect the physiologic MRI responses to brief visual activation?

We studied the effect of stimulus quality on the basic physiological response characteristics of oxygenation-sensitive MRI signals. Paradigms comprised a contrast-reversing checkerboard vs. darkness or vs. gray light as well as gray light vs. darkness in a 2 s/52 s protocol (nine subjects). MRI was performed at 2.0 T using single-shot gradient-echo EPI (TR/TE = 500/54 ms, flip angle 30 degrees). All paradigms elicited almost identical signal intensity time courses comprising a latency period (1-2s), an activation-induced signal increase (4-4.5% at about 6-7 s after stimulus onset) and a post-stimulus signal undershoot (-1%) that slowly recovered to baseline (about 50 s). Thus, in contrast to findings for sustained stimulation, brief presentations of distinct visual stimuli exhibit similar physiological response characteristics that support the use of a uniform response profile for the evaluation of event related paradigms.