Cortical magnification underlies differences across but not around the visual field

Human visual performance changes dramatically both across (eccentricity) and around (polar angle) the visual field. It is better at the fovea, decreases with eccentricity, and is better along the horizontal than vertical meridian and along the lower than the upper vertical meridian. However, all neurophysiological and virtually all behavioral studies of cortical magnification have investigated eccentricity effects without considering polar angle. Most performance differences due to eccentricity are eliminated when stimulus size is cortically magnified to equate the size of their cortical representation. But does cortical magnification underlie differences around the visual field? Here, the same human adult observers performed an orientation discrimination task with constant stimulus size at different locations as well as when stimulus size was magnified according to eccentricity and polar angle to equate their cortical representation. We found that although magnifying the stimulus eliminates differences across eccentricity, it does not do so around polar angle of the visual field. This finding indicates that limits in contrast sensitivity across eccentricity and around the visual field emerge from different anatomical and computational constraints.