Stimulus-dependent BOLD and perfusion dynamics in human V1

Blood oxygenation level-dependent (BOLD) fMRI signals often exhibit pronoun ced over- or undershoot upon changes in stimulation state. Current models p ostulate that this is due to the delayed onset or decay of perfusion-depend ent attenuating responses such as increased cerebral blood volume or oxygen consumption, which are presumed to lag behind the rapid adjustment of bloo d flow rate to a new steady-state level. If this view is correct, then BOLD overshoot amplitudes in a specific tissue volume should be correlated with steady-state increases in perfusion, independent of stimulus type. To test this prediction, we simultaneously recorded BOLD and relative perfusion si gnals in primary visual cortex while inducing graded perfusion increases wi th three types of visual stimulus. Two of these, a diffuse chromatic stimul us with no luminance variation and a very high spatial frequency luminance grating, did not produce detectable BOLD overshoot (or undershoot) when an equal mean luminance baseline was used. Radial checkerboard stimuli, howeve r, caused pronounced over/undershoot of both BOLD and perfusion signals eve n when temporal mean luminance was held constant and stimulus contrast was adjusted to produce the same steady-state blood flow increases evoked by th e other stimuli. Transient amplitudes were relatively invariant in spite of large changes in steady-state response, demonstrating nonlinear BOLD and p erfusion step responses in human V1, These findings suggest that, rather th an a purely tissue-specific biomechanical or metabolic phenomenon, BOLD ove rshoot and undershoot represent transient features in the perfusion signal whose effects may be amplified by slowly evolving blood volume changes. (C) 1999 Academic Press.