fMRI responses to horizontal and vertical symmetry axes in ascending visual hierarchy

Visual regularities such as reflection and translation are known to engage extrastriate visual cortex, but how regularity type, axis orientation, and task demand jointly shape these responses remains unclear. We used functional Magnetic Resonance Imaging (fMRI) to measure univariate and multivariate responses to reflection and translation patterns with vertical or horizontal axis of orientation while participants (n = 24, 6 male) performed either a regularity discrimination task or an orientation discrimination task. Univariate analysis showed that stimulus-evoked responses increased from V3 onwards, with minimal responses in V1-V2. Contrary to classical salience accounts, early dorsal regions (V3, V3a, V3b) responded more strongly to translation than reflection. Orientation effects dissociated across regions: V3a and V3b showed stronger responses to horizontal than vertical axes, whereas LO2 showed the opposite preference. Task effects were limited, with no overall enhancement of responses during regularity discrimination. Multivariate (searchlight MVPA) analysis revealed reliable decoding of regularity in extrastriate regions and decoding of axis orientation in early visual cortex. Orientation information was reduced in extrastriate regions during regularity discrimination relative to orientation discrimination. These findings show that responses to visual regularities in human visual cortex depend on regularity type, axis orientation, and task demands, with dorsal and ventral regions showing distinct response profiles.