Spatial heterogeneity of the nonlinear dynamics in the FMRI BOLD response

Recent studies of blood oxygenation level dependent (BOLD) signal responses averaged over a region of interest have demonstrated that the response is nonlinear with respect to stimulus duration. Specifically, shorter duration stimuli produce signal changes larger than expected from a linear system. The focus of this study is to characterize the spatial heterogeneity of thi s nonlinear effect. A series of MR images of the visual and motor cortexes were acquired during visual stimulation and finger tapping, respectively, a t five different stimulus durations (SD). The nonlinearity was assessed by fitting ideal linear responses to the responses at each SD. This amplitude, which is constant for different SD in a linear system, was normalized by t he amplitude of the response to a blocked design, thus describing the amoun t by which the stimulus is larger than predicted from a linear extrapolatio n of the response to the long duration stimulus. The amplitude of the BOLD response showed a nonlinear behavior that varied considerably and consisten tly over space, ranging from almost linear to 10 times larger than a linear prediction at short SD. In the motor cortex different nonlinear behavior w as found in the primary and supplementary motor cortexes.