Stimulus dependence of oxygenation-sensitive MRI responses to sustained visual activation.

Oxygenation-sensitive MRI responses to repetitive and sustained visual activation were compared for stimuli with different temporal, spatial, and luminance contrasts, i.e. reversing checkerboard, flashing diffuse red light, and stationary diffuse gray light with darkness as a control. All paradigms elicited an initial oxygenation 'overshoot' as well as a post-stimulus 'undershoot'. However, whereas flashing and stationary diffuse light resulted in more than a 50% decrease of the initial signal response after 6 min of stimulation, checkerboard responsed remained largely unaffected (less than 20% signal attenuation). The demonstration of a stimulus dependence for sustained visual activation reconciles apparently contradictory reports for stimuli involving checkerboards as opposed to goggles, flickerlight, and movies. It may be caused by stimulus-dependent adjustments of neuronal activity, oxygen consumption, blood flow, or blood volume.