Horses have the ability to generate a remarkable repertoire of facial expressions, some which have been linked to certain emotional states, for example pain. Studies suggest that facial expressions may be a more ‘honest’ expression of emotional state in horses than behavioral or physiological parameters. This study sought to describe the facial expressions during stress of healthy horses free of pain, using a standardized method of recording facial expressions in video. Stress was induced in 28 horses by subjecting them to road transport and 10 of these horses were also subjected to social isolation. The horses served as their own control. A body-mounted, remote controlled heart rate monitor provided continuous heart rate measurements during the interventions. The horses’ facial expressions were video-recorded during the interventions. Frequency and duration of each facial expression were then determined, according to the Equine Facial Action Coding System. Heart rate increased during the stressful interventions (p=0.01), confirming that the interventions were stressful. Using both the human investigation- and the co-occurrence methods, the following facial traits could be observed during stress: eye white increase (p<0.001), nostril dilator (p<0.001), upper eyelid raiser (p<0.001), inner brow raiser (p=0.042), tongue show (p<0.001) along with an increase in ‘ear flicker’ (p<0.001) and blink frequency (p<0.001). The facial actions were successfully used to train a machine-learning classifier to discriminate between stressed and calm horses, with an accuracy of 74.2 %. Most of the facial features identified correspond well with previous research on the subject, for example flared nostrils, repetitive mouth behaviors, increased eye white, tongue show and ear movements. Some features selected as indicative of emotional pain-free stress are used in face-based pain assessment tools, such as dilated nostrils, eye white increase or inner brow raiser. The relation between facial expressions of stress and pain should therefore further be studied.

中文翻译：

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马情绪紧张的面部表情

马具有产生令人瞩目的面部表情的能力，其中一些面部表情与某些情绪状态（例如疼痛）有关。研究表明，面部表情可能是马的情感状态比行为或生理参数更“诚实”的表达。这项研究试图使用录制视频中面部表情的标准化方法来描述健康的马匹在无痛苦的压力下的面部表情。通过对28匹马进行公路运输，他们引起了压力，其中10匹马也受到了社会隔离。马匹充当他们自己的控制。在干预过程中，一个安装在身上的遥控心率监测器提供了连续的心率测量。在干预过程中，对马的面部表情进行了录像。然后根据马面部动作编码系统确定每个面部表情的频率和持续时间。压力干预期间心率升高（p = 0.01），证实了干预措施具有压力。使用人类调查和同时发生的方法，在压力下可以观察到以下面部特征：眼白增加（p <0.001），鼻孔扩张器（p <0.001），上眼睑抬高者（p <0.001），内部眉毛提升器（p = 0.042），舌头显示（p <0.001）以及“耳闪烁”（p <0.001）和眨眼频率（p <0.001）的增加。面部动作已成功用于训练机器学习分类器，以区分压力马和镇静马，准确率达到74.2％。识别出的大多数面部特征都与先前对该主题的研究相吻合，例如喇叭形的鼻孔，反复的嘴部行为，眼白增加，舌头发红和耳朵运动。在基于面部的疼痛评估工具中使用了一些选择来表示无情感压力的特征，例如扩张的鼻孔，眼白增加或眉毛内侧抬高。因此，应进一步研究压力面部表情与疼痛之间的关系。