Discrimination of the local orientation structure of spiral Glass patterns early in human visual cortex

The local orientation structure of a visual image is fundamental to the perception of spatial form. Reports of reliable orientation-selective modulations in the pattern of fMRI activity have demonstrated the potential for investigating the representation of orientation in the human visual cortex. Orientation-selective voxel responses could arise from anisotropies in the preferred orientations of pooled neurons due to the random sampling of the cortical surface. However, it is unclear whether orientation-selective voxel responses reflect biases in the underlying distribution of neuronal orientation preference, such as the demonstrated over-representation of radial orientations (those collinear with fixation). Here, we investigated whether stimuli balanced in their radial components could evoke orientation-selective biases in voxel activity. We attempted to discriminate the sense of spiral Glass patterns (opening anti-clockwise or clockwise), in which the local orientation structure was defined by the placement of paired dots at an orientation offset from the radial. We found that information within the spatial pattern of fMRI responses in each of V1, V2, V3, and V3A/B allowed discrimination of the spiral sense with accuracies significantly above chance. This result demonstrates that orientation-selective voxel responses can arise without the influence of a radial bias. Furthermore, the finding indicates the importance of the early visual areas in representing the local orientation structure for the perception of complex spatial form.