MRI of functional deactivation: Temporal and spatial characteristics of oxygenation-sensitive responses in human visual cortex

Magnetic resonance imaging (MRI) of neuronal "activation" relies on the ele vation of blood how and oxygenation and a related increase of the blood oxy genation level-dependent (BOLD) MRI signal. Because most cognitive paradigm s involve both switches from a low degree of activity to a high degree of a ctivity and vice versa, we have undertaken a baseline study of the temporal and spatial characteristics of positive and negative BOLD MRI responses in human visual cortex. Experiments were performed at 2.0 T using a multislic e gradient-echo EPI sequence (TR = 1 s, mean TE = 54 ms, hip angle 50 degre es) at 2 x 2-mm(2) spatial resolution. Activation and "deactivation" proces ses were accomplished by reversing the order of stimulus presentations in p aradigms using homogeneous gray light and an alternating checkerboard as di stinct functional states. For sustained stimulation (greater than or equal to 60 s) the two conditions resulted in markedly different steady-state BOL D MRI signal strengths. The transient responses to brief stimulation (less than or equal to 18 s) differed insofar as activation processes temporally separate positive BOLD and negative undershoot effects by about 10 s, where as negative BOLD effects and undershoot contributions overlap for deactivat ion processes. Apart from differences in stimulus features (e.g., motion) t he used activation and deactivation protocols revealed similar maps of neur onal activity changes. (C) 1999 Academic Press.