TEMPORAL CHARACTERISTICS OF OXYGENATION-SENSITIVE MRI RESPONSES TO VISUAL ACTIVATION IN HUMANS

Series of single-shot blipped echo-planar images with spin-density wei ghting and T-2 sensitivity (2.0 T, TR = 400 ms, TE = 54 ms, flip angl e = 30 degrees) were used to study the temporal response profiles to r epetitive visual activation (5 Hz, reversing black and white checkerbo ard versus darkness) for protocols comprising multiple cycles of a 1,6 -s stimulus in conjunction with a 8.4-s or 90-s recovery phase and a 1 0-s stimulus with a 20-s or 90-s recovery phase. Analysis of the real- time data from all activated pixels resulted in a strong positive MRI response (mean values 3-6%) as well as a marked poststimulus undershoo t (mean values 1-2%, duration 60-90 s) for all paradigms. Repetitive p rotocols with insufficient recovery periods of 8.4 s or 20 s gave rise to a wraparound effect when analyzing time-locked averages from multi ple activation cycles. This problem may lead to an early signal decrea se that originates from the ongoing undershoot of preceding activation s folded back into the initial latency phase of a subsequent activatio n. When ensuring complete decoupling of responses to successive stimul i by using a 90-s recovery period, the wraparound effect vanished and an initial dip was observed in one of seven subjects for a 10-s/90-s p rotocol.