Factors affecting stimulus visibility for horses

Abstract

Eleven Morgan horses ranging from 1 to 14 years of age participated in two experiments assessing the relative contributions of stimulus size, contrast, and environmental illumination to visibility of stimuli on the ground. Natural outdoor illumination was used and the horses were tested unconstrained and freely walking. Repeated measures analyses of variance found that both the size and contrast of stimuli had significant, additive, non-interacting effects on the distance at which a stimulus on the ground was first noticed by a horse approaching at a walk, ranging from 15.2 cm (6 in.) away for a low-contrast, 1.27 cm (0.5 in.) wide stripe to 2.30 m (7.6 ft) for a high-contrast, 10.2 cm (4 in.) wide stripe. Similar effects of stimulus characteristics were found for a time delay measure of the amount of slowing in a horse's walk, with the narrowest, low-contrast stimulus adding only 0.2 s (5% increase) to a 4 s control walk while the widest, high-contrast stimulus added 6 s (150% increase). The possibility of using the more easily obtained time delay measure for clinical assessments and multi-condition, large-sample testing was discussed. A 2.54 cm (1 in.) wide stimulus seen on a overcast day was found to be more visible than a 5.08 cm (2 in.) stripe encountered on a sunny day, suggesting that bright daytime conditions may be less favorable to the equine rod-dominated eye. Stimuli underfoot appeared to be less visible to younger (<4 years) horses, but examination of neck angles on control trials indicated that the younger animals did not spontaneously lower their heads in response to the task to the same extent as did older, more trained animals. Since the equine retina has only a narrow horizontal streak of higher ganglion cell density, the level at which the eye is carried is important in determining the visibility of stimuli at particular locations in the environment. Head/neck angles characteristic of certain ages and breeds, changes in head position and motion with changes in gait, training level, and background illumination are all factors which need to be measured or controlled when evaluating the effect of physical stimulus characteristics on stimulus visibility for horses.