The influence of global form on local orientation anisotropies in human visual cortex

Perception of the spatial structure of the environment results from visual system processes which integrate local information to produce global percepts. Here, we investigated whether particular global spatial arrangements evoke greater responses in the human visual system, and how such anisotropies relate to those evident in the responses to the local elements that comprise the global form. We presented observers with Glass patterns; images composed of randomly positioned dot pairings (dipoles) spatially arranged to produce a percept of translational or polar global form. We used functional magnetic resonance imaging (fMRI) to infer the magnitude of neural activity within early retinotopic regions of visual cortex (V1, V2, V3, V3A/B, and hV4) while the angular arrangement of the dipoles was modulated over time to sample the range of orientations. For both translational and polar Glass patterns, V1 showed an increased response to vertical dipole orientations and all visual areas showed a bias towards dipole orientations that were radial to the point of fixation. However, areas V1, V2, V3, and hV4 also demonstrated a bias, only present for polar Glass patterns, towards dipole orientations that were tangential to the point of fixation. This enhanced response to tangential orientations within polar form indicates sensitivity to curvature or more global form characteristics as early as primary visual cortex.