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 elevation of blood how and oxygenation and a related increase of the blood oxygenation level-dependent (BOLD) MRI signal. Because most cognitive paradigms involve both switches from a low degree of activity to a high degree of activity 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 multislice gradient-echo EPI sequence (TR = 1 s, mean TE = 54 ms, hip angle 50 degrees) at 2 x 2-mm(2) spatial resolution. Activation and "deactivation" processes were accomplished by reversing the order of stimulus presentations in paradigms using homogeneous gray light and an alternating checkerboard as distinct functional states. For sustained stimulation (greater than or equal to 60 s) the two conditions resulted in markedly different steady-state BOLD 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, whereas negative BOLD effects and undershoot contributions overlap for deactivation processes. Apart from differences in stimulus features (e.g., motion) the used activation and deactivation protocols revealed similar maps of neuronal activity changes. (C) 1999 Academic Press.