Parametric reverse correlation reveals spatial linearity of retinotopic human V1 BOLD response

Many experiments measuring blood oxygen level dependent (BOLD) signal in functional magnetic resonance imaging (fMRI) data assume that the BOLD signal is predominantly linear in space and time. Previous investigations of temporal linearity have reported that the temporal BOLD response contains both linear and nonlinear components. Here, we used a novel method to investigate spatial linearity of BOLD within area V1. The visual field was divided into regions shaped like wedges, rings, or the intersections of the wedges and rings. The appearance of a flickering checkerboard texture within each region was governed by an independent M-sequence. fMRI data were acquired as the human subjects maintained visual fixation on a central cross. The time series data from each voxel were cross-correlated with every stimulus sequence to estimate each voxel's BOLD responses to all independent regions of the visual field. Linearity by spatial summation was assessed directly by comparing responses to wedges and rings with sums of responses to component patches. The BOLD responses of voxels responding positively to stimuli, measured with independent stimuli subtending several degrees of visual angle, were well predicted by linear spatial summation.