Method for functional MRI mapping of nonlinear response.

Nonlinear systems analysis combining blood oxygen level dependent (BOLD), functional magnetic resonance imaging (fMRI) and m-sequence stimulation paradigms are proposed as a new method for exploring neuronal responses and interactions. Previous studies of electrical activity in the human visual cortex have observed significant nonlinearities of task-induced activity with temporal dynamics on a timescale of 10-20 ms. Despite the confounding effect of the seconds-long hemodynamic response, it is demonstrated that BOLD fMRI can be used to probe neuronal interactions on a time scale of tens of ms. Visual activation experiments were performed with various stimuli, and amplitude maps of first and second order kernel coefficients were generated using correlation analysis. Second order nonlinearities in BOLD fMRI were observed and attributed to temporal contrast caused by transitions in the stimulus sequence. In addition, the kernel maps showed significant differences between second order nonlinearities of foveal and peripheral vision. By including a reference experiment with a slightly modified stimulus presentation, a distinction could be made between (fast) neuronal nonlinearities and hemodynamic effects on the time scale of the seconds. The results indicate that BOLD fMRI can probe fast neuronal nonlinearities.