Temporal and spatial MRI responses to subsecond visual activation.

The temporal and spatial characteristics of oxygenation-sensitive MRI responses to very brief visual stimuli (five Hz reversing black and white checkerboard pattern versus darkness) were investigated (nine subjects) by means of serial single-shot gradient-echo echo-planar imaging (2.0 T, TR=400 ms, mean TE=54 ms, flip angle 30 degrees). The use of a 0.2-s stimulus and a 90-s control phase resulted in an initial latency phase (about 2 s, no signal change), a positive MRI response (2.5% signal increase peaking at 5 s after stimulus onset), and a post-stimulus undershoot (1% signal decrease peaking at 15 s after stimulus onset) lasting for about 50-60 s. The finding that a subsecond visual stimulus elicits both a strong positive MRI response and a long-lasting undershoot provides further evidence for the neuronal origin of slow signal fluctuations seen in the absence of functional challenge and their utility for mapping functional connectivity. The additional observation that a reduction of the inter-stimulus control phase from 90 s to 9.8 s does not seem to affect the spatial extent of cortical activation in pertinent maps is of major relevance for the design and analysis of "event-related" MRI studies.