Horses tend to prefer one eye to inspect the sudden appearance of objects in their environment, researchers have found, but they were surprised that the left eye did not dominate.
Animals must attend to a diverse array of stimuli in their environments, Paolo Baragli and his fellow researchers noted in the journal PLOS ONE.
The nature of a stimulus can affect how this information is processed in the brain. Many species prefer to assess potentially negative stimuli using the sensory organs on the left side of their body, hence engaging the right hemisphere of their brain.
The researchers devised an experiment using 77 Italian saddle horses to examine whether they had a preference for one eye or the other when faced with the rapid appearance of a stimulus designed to spark an avoidance reaction and a negative emotional state.
Each horse was placed in a familiar stall, where a yellow balloon was rapidly inflated near the horse with a compressed air hose.
The study team had predicted that horses would inspect the novel and unexpected stimulus with their left eye, hence engaging the right hemisphere of their brain. However, this was not the case.
“We found that horses primarily inspected the balloon with one eye, and most horses had a preferred eye to do so. However, we did not find a population level tendency for this to be the left or the right eye,” they reported.
The strength of this preference tended to decrease over time, with the horses using their non-preferred eye to inspect the balloon increasingly as the trial progressed.
“Our results confirm a lateralised eye-use tendency when viewing negatively emotionally valent stimuli in horses, in agreement with previous findings,” they said.
“However, there was not any alignment of lateralisation at the group level in our sample, suggesting that the expression of lateralisation in horses depends on the sample population and testing context.”
Discussing their findings, the researchers said lateralisation of neural function appears to be a universal feature of nervous systems. However, the tendency for the direction of lateralised function to be aligned at the population level is highly variable between species and even populations.
“Our current results are not necessarily at odds with previous studies that have found population-level consistency in the direction of laterality among horses,” they said.
The differing backgrounds of the animals included in the study may have led to greater variation in the direction of lateralisation exhibited. Consistent handling, management, and/or training history could increase consistency in sensory laterality at the population level, they suggested.
“Horses are conventionally trained and handled from their left side including leading, saddling, percutaneous injections, and other common procedures. This prior experience could lead horses to react differently when a stimulus appears on their left side compared to their right side.
“This domestication bias has been suggested as a possible explanation for some population-level asymmetries in horses.”
Recently, however, it has been shown that initial training does not affect sensory lateralisation. “The lack of population-level consistency that we observed supports this contention.”
It has been suggested that sensory and motor laterality may be strengthened with age because of training, while other authors suggest that training may reduce the natural emotional asymmetry to human approach shown by horses.
“The horses tested in our study were all trained from the left side, per conventional stable management practice, and we did not detect any effect of age across a range of 4–24 years, suggesting that training history did not have a major effect in our sample.”
The authors said the individual variation seen in the study suggests that training history is not the predominant cause in determining visual laterality in the horse.
“Our study suggests that in at least in some testing contexts, horses appear to show individual but not population-level laterality.”
They cautioned that stimuli assumed to act as stressors may not always do so for all sampled individuals, and ideally should be verified with physiological data in future work.
The study team comprised Baragli and Martina Felici, with the University of Pisa; Chiara Scopa, with the Italian National Reference Centre for Animal Assisted Interventions; and Adam Reddon, with Liverpool John Moores University in England.
Baragli P, Scopa C, Felici M, Reddon AR (2021) Horses show individual level lateralisation when inspecting an unfamiliar and unexpected stimulus. PLoS ONE 16(8): e0255688. https://doi.org/10.1371/journal.pone.0255688