• Marr, I.; Farmer, K.; Krueger, K. (2018) Evidence for Right-Sided Horses Being More Optimistic than Left-Sided Horses; Animals 8 219  Abstract  Abstract link to fulltext
  • Farmer, K.; Krüger, K.; Byrne, R.W.; Marr, I. (2018) Sensory laterality in affiliative interactions in domestic horses and ponies (Equus caballus); Anim. Cogn. 21 631-637  Abstract  Abstract link to fulltext
  • Krueger, K; Farmer, K. (2011) Laterality in the Horse [Lateralität beim Pferd ]; mup 4 160-167  Abstract  Abstract
  • Krueger K., Farmer K., Byrne R. (2011) The use of sensory laterality for indicating emotional and cognitive reactions on environmental stimuli in animals [Die sensorische Lateralität als Indikator für emotionale und kognitive Reaktionen auf Umweltreize beim Tier].; in: Current research in applied ethology [Aktuelle Arbeiten zur artgemäßen Tierhaltung 13-23Abstract
  • Farmer, K.; Krueger, K.; Byrne, R. (2010) Visual laterality in the domestic horse (Equus caballus) interacting with humans; Anim. Cogn. 13 229-238  Abstract  Abstract link to fulltext


horse Sensory Laterality Horses are one-sided, in that they usually prefer to use one eye, ear or nostril over the other under particular circumstances. Science attributes this sensory laterality to the processing of different types of information in one or other of the two brain hemispheres. Generally, horses prefer using the left eye to observe novel objects and humans, and this preference is more marked in emotional situations and when confronted with unknown persons. Therefore, the horse’s visual laterality provides a good means of assessing whether horses are making emotional or rational decisions during training or in human-horse interactions. A preference for the left eye may indicate that a horse will prefer a certain eye when making either emotional or rational decisions. A particularly strong laterality can indicate that a horse cannot deal with certain training situations or is emotionally affected by a particular human or the training situation itself.

Visual Laterality in the Domestic Horse (Equus Caballus) in Passive and Active Interaction with Humans

Most horses have a side on which they are easier to handle, and a direction they favour when working on a circle, and recent studies have suggested a correlation between emotion and visual laterality when horses observe inanimate objects. We investigated whether laterality also varies in association with people. We gave horses the choice of entering a chute to left or right, with and without the passive presence of a person unknown to them. The left eye was preferred for scanning under both conditions, but significantly more so when a person was present. Traditionally, riders handle horses from the left, so we repeated the experiment with horses specifically trained on both sides. Again, there was a consistent preference for left eye scanning in the presence of a person, whether known to the horses or not. We also examined horses interacting with a person, using both traditionally and bilaterally trained horses. Both groups showed left eye preference for viewing the person, with no significant effect of training. For those horses tested under both passive and interactive conditions, the left eye was preferred significantly more during interaction. We suggest that most horses prefer to use their left eye for assessment and evaluation, and that there is an emotional aspect to the choice which may be positive or negative, depending on the circumstances. We believe these results have important implications both for welfare and efficient training: emotional laterality effects need to be taken into account in development of training methods.


Sensory laterality in affiliative interactions in domestic horses and ponies (Equus caballus)

Many studies have been carried out into both motor and sensory laterality of horses in agonistic and stressful situations. Here we examine sensory laterality in affiliative interactions within four groups of domestic horses and ponies (N = 31), living in stable social groups, housed at a single complex close to Vienna, Austria, and demonstrate for the first time a significant population preference for the left side in affiliative approaches and interactions. No effects were observed for gender, rank, sociability, phenotype, group, or age. Our results suggest that right hemisphere specialization in horses is not limited to the processing of stressful or agonistic situations, but rather appears to be the norm for processing in all social interactions, as has been demonstrated in other species including chicks and a range of vertebrates. In domestic horses, hemispheric specialization for sensory input appears not to be based on a designation of positive versus negative, but more on the perceived need to respond quickly and appropriately in any given situation.

Evidence for right sided horses being more optimistic than left sided hoses

An individual's positive or negative perspective when judging an ambiguous stimulus (cognitive bias) can be helpful when assessing animal welfare. Emotionality, as expressed in approach or withdrawal behaviour, is linked to brain asymmetry. The predisposition to process information in the left or right brain hemisphere is displayed in motor laterality. The quality of the information being processed is indicated by the sensory laterality. Consequently, it would be quicker and more repeatable to use motor or sensory laterality to evaluate cognitive bias than to perform the conventional judgment bias test. Therefore, the relationship between cognitive bias and motor or sensory laterality was tested. The horses (n = 17) were trained in a discrimination task involving a box that was placed in either a “positive” or “negative” location. To test for cognitive bias, the box was then placed in the middle, between the trained positive and negative location, in an ambiguous location, and the latency to approach the box was evaluated. Results indicated that horses that were more likely to use the right forelimb when moving off from a standing position were more likely to approach the ambiguous box with a shorter latency (generalized linear mixed model, p < 0.01), and therefore displayed a positive cognitive bias (optimistic).