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

Abstract

Summary Here we provide confirmation that the ‘ramp retina’ of the horse, once thought 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, intense visual streak in the inferior retina is 16.5 cycles per degree compared 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 binocular overlap is oriented down the nose, with a blind area directly in front of the forehead.

The impact of visual perception on equine learning

“…Although the horse could detect a perpendicular yellow line 5 millimetres wide from a distance of 3.3.metres (equivalent to 18.46 cycles / degree), a blue line had to be 20 millimetres wide for detection to be possible from the same distance (equivalent to 2.9 cycles / degree), (Grzimek, 1952). These values correspond with acuity values obtained for the visual streak and peripheral retina respectively (Harman et al, 1999). It is likely that this is the result of the relative distribution of the two different cone types, the short wavelength photoreceptors being less prolific than the medium-long wavelength photoreceptors in the visual streak (Sandmann et al, 1996).…”

“…Even in the area of the visual streak the horse has a limited ability to see detail in comparison with the human. Anatomical data provided an estimate of peak visual acuity in the area of the visual streak of about 16.5 cycles / degree, with far lower acuity (3.3 -3.5 cycles / degree) in other retinal regions (Harman et al, 1999). Behavioural evidence of visual acuity in the horse has also been obtained.…”

“…Although more errors were made when the black cloth was placed in either of the new positions, performance was found to be more accurate when the black cloth was in the low position than in the high position. As stated above, Harman et al (1999) found that when the horse lowers its head to observe stimuli on the ground, the image will be projected onto the most sensitive area of the retina. By approaching with the head lowered, the binocular field is directed towards the ground and should result in the stimuli remaining visible to the horse.…”

“…The binocular portion of the visual field is located down the nose of the horse and is limited to between 65° (Crispin et al, 1990) and 80° (Harman et al, 1999). Harman et al (1999) also found that a blind spot existed in front of the forehead. When the horse lowers its head to observe stimuli on the ground, the image will be projected onto the most sensitive area of the retina.…”

“…The density was found to be greatest at the temporal end of this visual streak (Hebel, 1976;Harman et al, 1999;Guo and Sugita, 2000), corresponding with the area responsible for binocular vision. The binocular portion of the visual field is located down the nose of the horse and is limited to between 65° (Crispin et al, 1990) and 80° (Harman et al, 1999). Harman et al (1999) also found that a blind spot existed in front of the forehead.…”