Horse vision and an explanation for the visual behaviour originally explained by the 'ramp retina'

Here we provide confirmation that the 'ramp retina' of the horse, once thou ght to result in head rotating visual behaviour, does not exist. We found a 9% variation in axial length of the eye between the streak region and the dorsal periphery. However, the difference was in the opposite direction to that proposed for the 'ramp retina'. Furthermore, acuity in the narrow, int ense visual streak in the inferior retina is 16.5 cycles per degree compare d with 2.7 cycles per degree in the periphery. Therefore, it is improbable that the horse rotates its head to focus onto the peripheral retina. Rather , the horse rotates the nose up high to observe distant objects because bin ocular overlap is oriented down the nose, with a blind area directly in fro nt of the forehead.