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 diffu se red light, and stationary diffuse gray light with darkness as a con trol. All paradigms elicited an initial oxygenation 'overshoot' as wel l as a post-stimulus 'undershoot'. However, whereas flashing and stati onary diffuse light resulted in more than a 50% decrease of the initia l signal response after 6 min of stimulation, checkerboard responses r emained largely unaffected (less than 20% signal attenuation). The dem onstration of a stimulus dependence for sustained visual activation re conciles apparently contradictory reports for stimuli involving checke rboards as opposed to goggles, flickerlight, and movies. It may be cau sed by stimulus-dependent adjustments of neuronal activity, oxygen con sumption, blood flow, or blood volume. (C) 1998 John Wiley & Sons, Ltd .