d'Ingeo, S., Quaranta, A., Siniscalchi, M., Stomp, M., Coste, C., Bagnard, C., et al. (2019). Horses associate individual human voices with the valence of past interactions: a behavioural and electrophysiological study. Scientific Reports, 9(1), 11568.
Abstract: Brain lateralization is a phenomenon widely reported in the animal kingdom and sensory laterality has been shown to be an indicator of the appraisal of the stimulus valence by an individual. This can prove a useful tool to investigate how animals perceive intra- or hetero-specific signals. The human-animal relationship provides an interesting framework for testing the impact of the valence of interactions on emotional memories. In the present study, we tested whether horses could associate individual human voices with past positive or negative experiences. Both behavioural and electroencephalographic measures allowed examining laterality patterns in addition to the behavioural reactions. The results show that horses reacted to voices associated with past positive experiences with increased attention/arousal (gamma oscillations in the right hemisphere) and indicators of a positive emotional state (left hemisphere activation and ears held forward), and to those associated with past negative experiences with negative affective states (right hemisphere activation and ears held backwards). The responses were further influenced by the animals' management conditions (e.g. box or pasture). Overall, these results, associating brain and behaviour analysis, clearly demonstrate that horses' representation of human voices is modulated by the valence of prior horse-human interactions.
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Camerlink, I., Menneson, S., Turner, S. P., Farish, M., & Arnott, G. (2018). Lateralization influences contest behaviour in domestic pigs. Scientific Reports, 8(1), 12116.
Abstract: Cerebral lateralization, i.e. hemispheric asymmetries in structure and function, relates in many species to a preference to attack from their left. Lateralization increases cognitive capacity, enabling the simultaneous processing of multiple sources of information. Therefore, lateralization may constitute a component of fighting ability (Resource Holding Potential), and/or influence the efficiency of information-gathering during a contest. We hypothesized that lateralization will affect contest outcome and duration, with an advantage for more strongly lateralized individuals. In 52 dyadic contests between weight-matched pigs (Sus scrofa; n = 104; 10 wk age), the direction of orientation towards the opponent was scan sampled every 10 s. Laterality indexes (LI) were calculated for the direction and strength of lateralization. Up to 12.5% of the individuals showed significant lateralization towards either the right or left but lateralization was absent at the population level. In line with our hypothesis, animals showing strong lateralization (irrespective of direction) had a shorter contest duration than animals showing weak lateralization. Winners did not differ from losers in their strength or direction of lateralization. Overall the results suggest that cerebral lateralization may aid in conflict resolution, but does not directly contribute to fighting ability, and will be of value in the study of animal contests.
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Lansade, L., Colson, V., Parias, C., Trösch, M., Reigner, F., & Calandreau, L. (2020). Female horses spontaneously identify a photograph of their keeper, last seen six months previously. Scientific Reports, 10(1), 6302.
Abstract: Horses are capable of identifying individual conspecifics based on olfactory, auditory or visual cues. However, this raises the questions of their ability to recognize human beings and on the basis of what cues. This study investigated whether horses could differentiate between a familiar and unfamiliar human from photographs of faces. Eleven horses were trained on a discrimination task using a computer-controlled screen, on which two photographs were presented simultaneously (32 trials/session): touching one was rewarded (S+) and the other not (S-). In the training phase, the S+ faces were of four unfamiliar people which gradually became familiar over the trials. The S- faces were novel for each trial. After the training phase, the faces of the horses' keepers were presented opposite novel faces to test whether the horses could identify the former spontaneously. A reward was given whichever face was touched to avoid any possible learning effect. Horses touched the faces of keepers significantly more than chance, whether it was their current keeper or one they had not seen for six months (t = 3.65; p < 0.004 and t = 6.24; p < 0.0001). Overall, these results show that horses have advanced human face-recognition abilities and a long-term memory of those human faces.
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Nakamura, K., Takimoto-Inose, A., & Hasegawa, T. (2018). Cross-modal perception of human emotion in domestic horses (Equus caballus). Scientific Reports, 8(1), 8660.
Abstract: Humans have domesticated many kinds of animals in their history. Dogs and horses have particularly close relationships with humans as cooperative partners. However, fewer scientific studies have been conducted on cognition in horses compared to dogs. Studies have shown that horses cross-modally distinguish human facial expressions and recognize familiar people, which suggests that they also cross-modally distinguish human emotions. In the present study, we used the expectancy violation method to investigate whether horses cross-modally perceive human emotions. Horses were shown a picture of a human facial expression on a screen, and they then heard a human voice from the speaker before the screen. The emotional values of the visual and auditory stimuli were the same in the congruent condition and different in the incongruent condition. Horses looked at the speaker significantly longer in the incongruent condition than in the congruent condition when they heard their caretaker's voices but not when they heard the stranger voice. In addition, they responded significantly more quickly to the voice in the incongruent condition than in the congruent one. To the best of our knowledge, this is the first study to show that horses cross-modally recognized the emotional states of their caretakers and strangers.
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