Nearly all fMRI analysis methods assume a linear relationship between local neuronal activity and the BOLD signal. This assumption is supported for fMRI at conventional resolutions (>1 mm isotropic). We assess whether linearity of the BOLD signal holds at sub-millimetre resolution, over cortical depth. We acquired functional GE 3D-EPI data at 0.7 mm isotropic resolution (TR/TE = 57/28 ms). Stimuli consisted of moving circular sine gratings at 80%, 20%, and 5% contrast. Our results suggest that response profiles for one contrast are linearly scaled response profiles of any other contrast.