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Harman, A. M., Moore, S., Hoskins, R., & Keller, P. (1999). Horse vision and an explanation for the visual behaviour originally explained by the 'ramp retina'. Equine Vet J, 31(5), 384–390.
Abstract: 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.
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McGreevy, P. D., Harman, A., McLean, A., & Hawson, L. (2010). Over-flexing the horse's neck: A modern equestrian obsession? Journal of Veterinary Behavior: Clinical Applications and Research, 5(4), 180–186.
Abstract: We used an opportunistic review of photographs of different adult and juvenile horses walking, trotting, and cantering (n = 828) to compare the angle of the nasal plane relative to vertical in feral and domestic horses at liberty (n = 450) with ridden horses advertised in a popular Australian horse magazine (n = 378). We assumed that horses in advertisements were shown at, what was perceived by the vendors to be, their best. Of the ridden horses, 68% had their nasal plane behind the vertical. The mean angle of the unridden horses at walk, trot, and canter (30.7 ± 11.5; 27.3 ± 12.0; 25.5 ± 11.0) was significantly greater than those of the ridden horses (1.4 ± 14.1; ?5.1 ± ?11.1; 3.1 ± 15.4, P < 0.001). Surprisingly, unridden domestic horses showed greater angles than feral horses or domestic horses at liberty. We compared adult and juvenile horses in all 3 gaits and found no significant difference. Taken together, these findings demonstrate that the longitudinal neck flexion of the degree desirable by popular opinion in ridden horses is not a common feature of unridden horses moving naturally. Moreover, they suggest that advertised horses in our series are generally being ridden at odds with their natural carriage and contrary to the international rules of dressage (as published by the International Equestrian Federation). These findings are discussed against the backdrop of the established doctrine, which states that carrying a rider necessitates changes in longitudinal flexion, and in the context of the current debate around hyperflexion.
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