Anisotropic Spread of Cortical Activity in Human Visual Cortex

A visual stimulus not only evokes spiking activity in neurons whose receptive fields (RFs) are overlapped with the stimulus but also promotes subthreshold activity over an extended cortical region outside the directly stimulated region, known as cortical point spread (PS). Optical imaging studies on animal visual cortex demonstrated that PS is biased to cortical sites whose orientation preferences are similar to stimulus orientation (“co-orientation anisotropy”). Anatomical studies reported also “co-axial anisotropy”, predominant horizontal connections between neurons whose RFs are positioned along the retinotopic axis collinear to their orientation preferences. By conducting fMRI experiments, we observed those two types of anisotropy in PS in early visual areas (V1, V2, V3) of human brains. By defining RFs and stimulus orientation preferences for individual voxels, we could show that PS of fMRI activity that were triggered by a small stimulus at the fovea increased with decreasing angular offset of a given voxel's RF from the axis collinear to stimulus orientation and was strong in voxels whose orientation preferences are similar to stimulus orientation. When anisotropy was probed via traveling waves of PS in a large visual field, we found that the “co-axial anisotropy” was more pronounced in the fovea than in the periphery.