Meridional variations and other properties suggesting that acuity and orientation discrimination rely on different neuronal mechanisms.

It has been widely reported that both grating acuity and orientation discrimination show meridional variations: performance is better for targets oriented horizontally or vertically than for those with oblique orientations. In spite of such similarities, we now present both behavioral and psychophysical evidence from cats and humans to show that grating acuity and orientation discrimination depend upon different neuronal mechanisms.