Table 1.
Sex, age and breed of the 16 horses used in this study.
Fig 1.
Measures for the assessment of eye wrinkle expression.
(a) Qualitative assessment: Categories reflect the first subjective impression of the expression of eye wrinkles based on the number of wrinkles, their markedness, and the angle they are forming. ‘No wrinkle’: no wrinkle visible. ‘Weak’: the overall impression of the wrinkles is weak with, e.g., some weakly marked wrinkles forming a narrow angle. ‘Strong’: the overall impression of the wrinkles is strong with, e.g., several marked wrinkles forming a wide angle above the eyeball. (b) Eyelid shape: ‘Round’: smooth curve without any sign of the lid being pulled in the dorso-medial direction. ‘Weakly pulled’: curve is continuous but slightly pulled in the dorso-medial direction. The eye looks more angled. ‘Strongly pulled’: the lateral part of the lid is an almost straight line. (c) Markedness: the depth and width of the wrinkles is assessed. If the markedness differs between wrinkles, the most prominent wrinkle is assessed. ‘No wrinkle’: no wrinkle visible. ‘Weak’: wrinkles are flat and narrow lines. ‘Strong’: wrinkles are pronounced in depth and width. (d) Eye white: The sclera (eye white, also when brownish due to pigments) is assessed as visible (‘yes’) or not visible (‘no’). (e) Number: Only wrinkles above the eyelid and those of a minimum length of one third of the eyeball’s diameter are considered. A deep indent, often seen in older horses, is not considered as a wrinkle (as it is not caused by muscle contraction of the inner eyebrow raiser). Moreover, wrinkles originating on the eyelid (mostly one or two) are not counted. (f) Angle: The degree of the angle is measured on the intersection of the extension of a line drawn through the eyeball and the extension of the highest wrinkle. The line through the eyeball extends from the medial to the lateral corner of the eyeball. If the medial corner is not clearly defined, the line goes through the middle of the tear duct.
Fig 2.
Effect of the four conditions on eye wrinkle expression.
The effect of the four conditions (Grooming (G), Food anticipation (FA), Food competition (FC), Plastic bag (PB)) on the six outcome measures is shown with respect to phase (Control (C), Treatment (T)). (a, b) ‘angle’, ‘number’: boxplots with median (black line), interquartile range (box), 1.5 x interquartile range (whiskers). (c) ‘eye white’: bar chart±binomial confidence interval. (d, e, f) ‘qualitative assessment’, ‘eyelid shape’, ‘markedness’: stacked bar charts with the three colours representing the three different scores. (d) ‘qualitative assessment’: ‘no wrinkle’ (black), ‘weak’ (dark grey), ‘strong’ (light grey). (e) ‘eyelid shape’: ‘round’ (black), ‘weakly pulled’ (dark grey), ‘strongly pulled’ (light grey). (f) ‘markedness’: ‘no wrinkle’ (black), ‘weak’ (dark grey), ‘strong’ (light grey).
Table 2.
Statistical models and results for the six outcome measures.