Heart and brain: Change in cardiac entropy is related to lateralised visual inspection in horses

Cerebral lateralisation is the tendency for an individual to preferentially use one side of their brain and is apparent in the biased use of paired sensory organs. Horses vary in eye use when viewing a novel stimulus which may be due to different physiological reactions. To understand the interplay between physiology and lateralisation, we presented a novel object (an inflated balloon) to 20 horses while electrocardiogram traces were collected. We measured the amount of time each horse looked at the balloon with each eye. We calculated 'sample entropy' as a measure of non-linear heart rate variability both prior to and during the stimulus presentation. A smaller drop in sample entropy values between the habituation phase and the sample presentation indicates the maintenance of a more complex signal associated with a relaxed physiological state. Horses that spent longer viewing the balloon with their left eye had a greater reduction in sample entropy, while time spend looking with the right eye was unrelated to the change in sample entropy. Therefore, the horses that exhibited a greater reduction in sample entropy tended to use their right hemisphere more, which may take precedence in emotional reactions. These results may help to explain the variation in lateralisation observed among horses.

Automated summary

Cerebral lateralisation refers to the tendency for individuals to preferentially use one side of their brain, and this is evident in the biased use of paired sensory organs. Horses show variation in eye use when exposed to a new stimulus, which could be attributed to different physiological reactions. To understand the relationship between physiology and lateralisation, researchers presented a novel object to 20 horses while collecting electrocardiogram traces. The study found that horses which exhibited a greater reduction in sample entropy tended to use their right hemisphere more, suggesting that emotional reactions may be prioritized in this hemisphere. These findings provide insight into the observed variation in lateralisation among horses.