Heart and brain: Change in cardiac entropy is related to lateralised visual inspection in horses.
Authors: Felici Martina, Reddon Adam R, Maglieri Veronica, Lanatà Antonio, Baragli Paolo
Journal: PloS one
Summary
# Editorial Summary: Cardiac Entropy and Visual Lateralisation in Horses When horses encounter something novel, they don't all respond the same way—and this variation may reflect fundamental differences in how their brains are wired and how their bodies react physiologically. Researchers presented 20 horses with an unfamiliar stimulus (an inflated balloon) whilst recording their heart rate variability through electrocardiography, then compared how much time each horse spent viewing the object with their left versus right eye alongside changes in sample entropy (a measure of heart rate complexity that indicates relaxation versus arousal). Horses showing greater reduction in sample entropy during stimulus presentation—meaning their heart rate became more predictable and their physiology shifted towards heightened alertness—were the same individuals who spent significantly more time looking at the balloon with their left eye, implicating their right hemisphere in the response. In contrast, right-eye viewing time bore no relationship to cardiac changes. These findings suggest that right-hemisphere dominance, which is associated with processing emotional and novel stimuli, may drive the observable variation in eye use between horses, offering a physiological explanation for the lateralisation differences we observe in the species. For practitioners, this work implies that lateralised visual responses could reflect genuine differences in emotional reactivity and nervous system regulation, which may have relevance for training approaches, stress assessment, and understanding individual variation in temperament.
Read the full abstract on PubMed
Practical Takeaways
- •Observe which eye a horse preferentially uses when encountering novel or potentially stressful situations—left eye preference may indicate greater emotional reactivity and need for careful handling
- •Horses showing strong left-eye bias to novel stimuli may benefit from gradual desensitization approaches, as they appear to mount stronger physiological stress responses
- •Understanding individual lateralization patterns could help tailor training and handling strategies to match each horse's baseline emotional responsiveness
Key Findings
- •Horses that viewed a novel stimulus longer with their left eye showed greater reduction in sample entropy (heart rate variability complexity), indicating a more reactive physiological state
- •Left eye use (right hemisphere dominance) correlated with increased cardiac entropy reduction, suggesting right hemisphere involvement in emotional processing
- •Right eye viewing time showed no relationship with changes in sample entropy, indicating asymmetric hemispheric control of physiological responses to novel stimuli
- •Individual variation in eye preference during novel object inspection may reflect differences in emotional reactivity and physiological arousal patterns