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Doctoral thesis2024Open access

Exploring facial expressions in horses : biological and methodological approaches

Lundblad, Johan


Facial expressions can be used as a tool to recognize pain and stress. However, these states are often intertwined, and sedative drugs may also influence pain recognition tools utilizing facial expressions. In order to investigate how the states of stress and sedation could affect facial expressions of pain, the Facial Action Coding System for horses was used in situations where nociception, stress and sedation were experimentally induced, both separately and in combinations, in a cross over design. The frequencies of the Action Units ”upper lid raiser”, “eye white increase”, “inner brow raiser”, “blinking”, “ear movements” and “nostril dilator” all increased during stressful managerial situations (p<0.05), supporting empirical descriptions of a stressed horse. No change in frequency of Action Units during sedation could be found in comparisons to baseline. A Partial Least Squares Discriminant Analysis which created two weighted components containing the frequency of Action Units, (t1 Q2>0.05, t2 Q2<0.05), could successfully discriminate the states of sedation, social isolation stress and nociception (pain) from each other. A combined state of nociception-isolation could not be discriminated from isolation without nociception. The combination of nociception-sedation did not differ from a neutral horse, suggesting that sedation conceals the frequency of Action Units of nociception. Blinking frequency increased in a sedated horse with nociception but not in a sedated horse without nociception (p=0.011). This finding needs further investigation. Lastly, dimensionality reduction methods were compared and their accuracy was measured. Highest accuracy was 76.9 % meaning that certain methods can introduce thresholds for automated evaluations, but methods are suggested to include intensity and temporal information of facial contractions as well as physiological- and behavioural measures for increased performance. In conclusion, the presence of stress, as well as sedation should be carefully considered when evaluating pain using facial expressions.


stress; pain; sedation; facial expression; horse; methodology; EquiFACS; compound; dimensionality reduction; rater agreement

Published in

Acta Universitatis Agriculturae Sueciae
2024, number: 2024:38ISBN: 978-91-8046-340-9, eISBN: 978-91-8046-341-6
Publisher: Swedish University of Agricultural Sciences

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    Medical Bioscience

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