Temporal and spatial nonlinearities in FMRI BOLD response in human primary visual cortex

It is generally assumed that fMRI BOLD response (i.e., functional magnetic resonance imaging blood-oxygen-level-dependent signal) is linear and time-invariant. In this study, we investigated spatial and temporal nonlinearities in fMRI BOLD response at different flickering durations (nearly instantaneous and 1 s). We presented participants two successive flickering checkerboard wedge stimuli with stimulus onset asynchrony (SOA) of 0 s, 0.5 s, 1.5 s, and 6 s. The spatial locations of the stimuli were categorized as the same location in the same hemifield, adjacent locations in the same hemifield, and between hemifields. We demonstrated that fMRI BOLD response behaves nonlinearly when the successive stimuli spatially and temporally were close to each other for both flickering durations. Nonlinearity between the successive stimuli was the highest for the same location in the same hemifield, higher in adjacent locations in the same hemifield, and the lowest in between hemifields. In addition, nonlinearity at the shorter SOAs (0 s, 0.5 s, and 1.5 s) was found to be higher than nonlinearity at SOA 6.