Laterality of the horse's brain- are the sides specialised?

Two Brains
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In the world of horse training just about everyone has heard about ‘left or right brained’ horses. The popular natural horsemanship teachings of Parelli describe ‘horsenalities’, with left-brained individuals being dominant, brave and confident, whereas right-brained horses are submissive, fearful and reactive (Parelli Natural Horsemanship, 2007). It is also common practise for some trainers to teach a behaviour on one side and then repeat the teachings on the opposite side under the assumption that information does not pass from one side to the other.
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There is an ever growing body of research investigating how both sides of the brain differ and function in many species. Scientists are beginning to understand how each side may be specialised and therefore create side biases for certain behaviours. The same is true for horses, but does this reflect popular teaching? Do horses have side preferences, and can they transfer learning from one side to the other? I’ve reviewed the current scientific literature to explore these questions in more depth. 
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Horses mutually grooming with right eyes facing outwards. Do they have a side preference while resting or grooming? This question will be the subject of my upcoming work.

​Brain Structure

Let’s start with a basic understanding of the brain’s structure, shall we?

The brain is an incredibly complicated organ, but can be split into basic sections. The most recognisable feature is the cerebrum (see below); the prominent, cauliflower shaped mass seen clearly in most mammals. It makes up the 'thinking brain', processing the senses, voluntary movements and complex cognition. Underneath this structure is the limbic system which is responsible for emotional behaviour, memory, reward, reproduction and feeding. The most primitive part of the brain includes the cerebellum which controls balance, coordination and fine movements, and the brain stem which deals with unconscious functions such as swallowing, breathing, digestion and heartbeat. The cerebrum is separated into two opposing hemispheres, connected by a think bundle of neurons known as the corpus callosum (Carter, 1998; Mills & Nankervis; 2009; Kiley-Worthington, 2007).
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Most senses are processed by the opposing hemisphere. Vison, hearing and touch detected on one side will be dealt with on the other side of the brain, whereas smell is processed on the same side (Carlson, 1979). In people each hemisphere is specialised to some extent for different functions, with the left usually dominant for language and the right for spatial awareness and emotional processing (Burnett et al., 1982; Gregory et al., 1980; Pujol et al., 1999). The extent to which a person uses each hemisphere for certain tasks is also linked to handedness (Burnett et al., 1982) and writing posture (Levy & Reid, 1976).

A comparison of vertebrate brain structure, including a horse and human. Note the structure of the cerebrum and cerebellum. Image from Tweetboard.me

Asymmetric Brain
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Originally the specialisation of hemispheres for different tasks was believed to be a solely human trait. However, there is increasing scientific evidence that it occurs in many animal species (Vallortigara et al., 1999). This includes fish (Bisazza et al., 1998), reptiles (Deckel, 1999), birds (Rogers, 1997, Güntürkün et al., 2000) as well as mammals (Glick & Ross, 1981, Cowell et al., 1997).

Overall for the majority of species the findings suggest that the right hemisphere is dominant for spatial ability and responding to novelty (Vallortigara et al.,1999). It is also important in emotional processing, particularly that involving fear, threatening situations and initiating the fight or flight response (Robins & Phillips, 2012). In contrast the left hemisphere is dominant for cognitive processes involving decisions, discrimination, categorising, previous learning or object manipulation (Vallortigara et al.,1999).

Best Foot Forward

Horses are no exception when it comes to asymmetry. While galloping 90% of racehorses take a right lead stride pattern and land with their left hind leg first. This left leg preference is consistent across breeds and different countries. Individual horses also appear to have a preferred leg, and will only change when forced by injury, fatigue or changing direction (Williams & Norris, 2007). During grazing domestic horses are much more likely to place the left front leg before the right. The strength of this preference was found to increase with age, suggesting it’s influenced by environmental factors (McGreevy & Rogers, 2005).

In feral horses individuals were found to have a preferred leg but this wasn’t the same leg across the population (Austin & Rogers, 2012). However, Przewalski horses showed no front leg preference at all (Austin & Rogers, 2014). These findings suggests that the leg bias in domestic horses is likely a result of training or handling rather than being a natural occurrence. 
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Domestic horses show foreleg preference grazing and tend to place the left foot in front. The same pattern is not seen in feral or wild horses.

The Senses

There is evidence that sensory information is processed asymmetrically in horses. They have been found to use opposite ears when listening to the calls of familiar or unfamiliar horses (Basile et al., 2009). Horses were also shown to preferentially use one nostril to assess different smells. Young horses preferred to use the right nostril to smell stallion faeces (McGreevy & Rogers, 2005). The right nostril was also used more to investigate an item with a negative emotional value than those with a neutral or positive association (De Boyer Des Roches et al., 2008). Smell is processed by the same hemisphere, supporting the right hemisphere’s role in processing emotive, novel and potentially threatening things.
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Visual  

Horses are an ideal species to study how the hemisphere’s process vision as information is processed almost completely by the opposite hemisphere. (Austin & Rogers, 2012). As found in many other species horses are more likely to use their left eye when viewing a potentially threatening object (Austin & Rogers, 2012, 2014; De Boyer Des Roches et al., 2008; Farmer et al, 2010). The emotional association also affects how the hemispheres are used. De Boyer Des Roches et al. (2008) found that horses viewed items with a negative emotional value more with their left eye. In contrast items with a positive association were viewed using both eyes, suggesting positive emotions may be processed by both sides of the brain. Horses preferentially use the left eye to view emotive novel objects (Austin & Rogers, 2014; Larose et al., 2006), for vigilance and for viewing known and unknown people (Farmer et al., 2010).
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The strength of the side preference increases with high levels of aggression or reactivity. In feral populations left eye bias was stronger when the behaviour involved vigilance, reactivity or aggression (Austin & Rogers, 2012). This was true for previously handled and unhandled animals and so was unlikely to be as a result of human interference. An even stronger right-hemisphere preference was found in Przewalski horses during vigilance, attack and aggressive behaviour (Austin & Rogers, 2014). Overall these findings support right hemisphere dominance for modulating aggressive behaviour and responses to potential threats, as seen in other species.

There is a clear left eye preference for reactive behaviours such as vigilance or aggression. Here a mare views a herd mate with her left eye during an agnostic interaction.

Bridging the Gap

The evidence that horses have specialised hemispheres is pretty comprehensive, but how well do these hemispheres communicate? It is widely regarded that horses need to be taught something on both sides before it can be properly learnt. However, the physiology of a horse's brain does not support this. Compared to other species they have a substantial and well developed corpus callosum (Cozzi et al., 2014; Hangii, 1999) which transfers information between the hemispheres.  In rats, rabbits, cats, dogs, horses and cows the corpus callosum was found to increase in size and number of cells as brain size increased (Olivares et al., 2001). Cell structure was very similar across species, and density actually decreased as brain size increased. Overall this suggests that the connectivity of horse's brains is comparable to that of other species for their size.

There is experimental evidence that the horse's hemispheres are indeed well connected. Horses had one eye covered and were taught to discriminate between a set of pictures to choose the correct one. Once they had learnt this task reliably the trained eye was covered and they were asked to repeat the exercise with the untrained eye. The horses were able to repeat the task very quickly using the untrained eye, even when the picture they had to select was changed (Hanggii, 1999).

Conversely, it has been suggested that horses are unable to transfer information about tactile cues from one side to another. Ahrendt et al. (2015) taught horses to move their hindquarters using escalating pressure. They found that a higher force was needed on the right side compared to the left, but there was no difference between first and second side tested. This means that the horses did not immediately transfer the learnt response from one side to the other.

There is very little information on horse brain structure, let alone the workings of the corpus callosum and interconnectivity of the hemispheres (Cozzi, et al., 2014). Currently evidence in the literature is conflicting, and there is a lot of scope to improve our understanding in this area. However, rather than horses being unable to transfer information between hemispheres, it may be that they are more able to transfer visual learning experiences than tactile ones. Regardless, the notion that the horse's brain cannot communicate laterally is incorrect.
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Conclusion

Scientific understanding of laterality and how it affects the animal is constantly improving. Overall the right hemisphere appears specialised for spatial ability, fast responses and emotional processing. In comparison the left is involved in decision making, discrimination and cognitive processes based on learning. Negative emotions and responding to threats are processed by the right hemisphere, which is supported by many studies in horses.

Popular teachings suggest that ‘right brained’ horses are submissive, fearful and reactive. As the right hemisphere processes potentially threatening situations it is possible that more reactive horses use the right hemisphere more often. ‘Left brained horses’ are deemed dominant, brave and confident. From the research on laterality as a whole this claim is a lot less plausible. The hemispheres do not function independently though, in reality both have individual strengths but are used in conjunction. Although specialised the hemispheres communicate constantly via the corpus callosum. In horses the corpus callosum is as well developed and defined as expected for their size, and can be compared to that of a dog or cow. Evidence on transferring information between hemispheres is conflicting, but suggests that at least some types of learning are transferred effectively and quickly.

Conventional teachings may hold some truths, but overall they are a simplistic and inaccurate model of the complexities of the equine brain. 
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References

Ahrendt, L.P. et al., 2015. Development of a standard test to assess negative reinforcement learning in horses. Applied Animal Behaviour Science, 169, pp.38–42.

Andrew, R.J., 1983. Lateralization of Emotional and Cognitive Function in Higher Vertebrates, with Special Reference to the Domestic Chick. In Advances in Vertebrate Neuroethology. Boston, MA: Springer US, pp. 477–509.

Austin, N.P. & Rogers, L.J., 2012. Limb preferences and lateralization of aggression, reactivity and vigilance in feral horses, Animal Behaviour, 83(1), pp.239–247.
 
Austin, N.P. & Rogers, L.J., 2014. Lateralization of agonistic and vigilance responses in Przewalski horses (Equus przewalskii). Applied Animal Behaviour Science, 151, pp.43–50.
 
Basile, M., Boivin, S., Boutin, A., Blois-Heulin, C., Hausberger, M., Lemasson, A., 2009. Socially dependent auditory laterality in domestic horses (Equus caballus). Animal Cognition, 12(4), pp.611–619.
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Bisazza, A., J. Rogers, L. & Vallortigara, G., 1998. The origins of cerebral asymmetry: A review of evidence of behavioural and brain lateralization in fishes, reptiles and amphibians. Neuroscience and Biobehavioral Reviews, 22(3), pp.411–426.
 
Boyd, L. & Bandi, N., 2002. Reintroduction of takhi, Equus ferus przewalskii, to Hustai National Park, Mongolia: time budget and synchrony of activity pre-and post-release. Applied Animal Behaviour Science, 78, pp.87–102.
 
Burnett. S., Lane. D & Dratt. L., 1989. Spatial ability and handedness. Intelligence, 6 (1) pp: 57-68.

Carlson, N., 1979. Physiology of behaviour. Fifth edition. Boston: Allyn and Bacon.

Carter, R., 1998. Mapping the Mind. Second edition. London: Phoenix.

Cowell, P.E., Waters, N.S. & Denenberg, V.H., 1997. The Effects of Early Environment on the Development of Functional Laterality in Morris Maze Performance. Laterality: Asymmetries of Body, Brain and Cognition, 2(3–4), pp.221–232.

Cozzi, B. et al., 2014. The brain of the horse: Weight and cephalization quotients. Brain, Behavior and Evolution, 83(1), pp.9–16.

De Boyer Des Roches, A., Richard-Yris, M. A.,Henry, S., Hausberger, M., 2008. Laterality and emotions: Visual laterality in the domestic horse (Equus caballus) differs with objects’ emotional value. Physiology and Behavior, 94(3), pp.487–490.
 
Farmer, K., Krueger, K. & Byrne, R.W., 2010. Visual laterality in the domestic horse (Equus caballus) interacting with humans. Animal Cognition, 13(2), pp.229–238.
 
Fischhoff, l., Sundaresan, S., Cordingley, J., Rubenstein, D., 2007. Habitat use and movements of plains zebra (Equus burchelli) in response to predation danger from lions. Behavioral Ecology, 18(4), pp.725–729.
 
Glick, S.D. & Ross, D.A., 1981. Right-sided population bias and lateralization of activity in normal rats. Brain Research, 205(1), pp.222–225.
 
Gregory. R. J., Alley. P. & Morris. L., 1980. Left-handedness and spatial reasoning abilities: The deficit hypothesis revisited. Intelligence, 4(2), pp.151–159.
 
Güntürkün, O., Diekamp, B., Manns, M., Nottelmann, F., Prior, H., Schwarz, A., Skiba, M., 2000. Asymmetry pays: visual lateralization improves discrimination success in pigeons. Current Biology, 10(17), pp.1079–1081.

Hanggi, E.B., 1999. Interocular transfer of learning in horses (Equus caballus), Journal of Equine Veterinary Science, 19. 518-523.

Hopkins, W. D., Wesley, M. J., Izard, M. K., Hook, M. & Schapiro, S. J., 2004 Chimpanzees (Pan troglodytes) are predominantly right-handed: replication in three populations of apes. Behaviour Neuroscience. 118, 659–663.

Hunter, L.T.B. & Skinner, J.D., 1998. Vigilance Behaviour in African Ungulates: The Role of Predation Pressure. Behaviour, 135(2), pp.195–211.
 
Karenina, K., Giljov, A., Baranov, V., Osipova, L., Krasnova, V., Malashichev, Y., 2010. Visual Laterality of Calf–Mother Interactions in Wild Whales. PLoS ONE, 5(11), p.e13787.
 
Kiley-Worthington. M., 2007. The behaviour of horses: in relation to management and training. JA Allen, London.
 
Larose, C., Richard-Yris, M. A., Hausberger, M., Rogers, L. J., 2006. Laterality of horses associated with emotionality in novel situations. Laterality: Asymmetries of Body, Brain, and Cognition, 11(4), pp.355–367.            
Levy. J. & Reid. M., 1976. Variations in writing posture and cerebral organization. Science, 194(4262), pp.337–9.

Lippolis, G. Westman, W., McAllan, B. M., Rogers, L. J., 2005. Lateralisation of escape responses in the stripe-faced dunnart, Sminthopsis macroura (Dasyuridae: Marsupialia). Laterality: Asymmetries of Body, Brain and Cognition, 10(5), pp.457–470.

McGreevy, P.D. & Rogers, L.J., 2005. Motor and sensory laterality in thoroughbred horses. Applied Animal Behaviour Science, 92(4), pp.337–352.

Mills. D.S., Nankervis. K.J., 2009. Equine Behaviour: Principles and Practice.
 
Olivares. R., Montiel. J., Aboitiz. F., 2001. Species Differences and Similarities in the Fine Structure of the Mammalian Corpus callosum. Brain Behaviour and Evolution, 57:98-105
 
Parelli. P., 2007. Parelli Horsenality Profile (pdf). Available at www.parelli.com. Accessed 04/07/2018.
 
Penzhorn, B.L., 2010. A Long-term Study of Social Organisation and Behaviour of Cape Mountain Zebras Equus zebra zebra. Zeitschrift für Tierpsychologie, 64(2), pp.97–146.
 
Pujol, J., Deus. J., Losilla. J. M., Capdevila. A., 1999. Cerebral lateralization of language in normal left-handed people studied by functional MRI. Neurology, 52(5), pp.1038–43.

Robins, A. & Phillips, C., 2010. Lateralised visual processing in domestic cattle herds responding to novel and familiar stimuli. Laterality: Asymmetries of Body, Brain and Cognition, 15(5), pp.514–534.

Rogers, L.J., 1997. Early Experiential Effects on Laterality: Research on Chicks has Relevance to Other Species. Laterality: Asymmetries of Body, Brain and Cognition, 2(3–4), pp.199–219.

Thieltges, H., Lemasson, A., Kuczaj, S., Böye, M., Blois-Heulin, C., 2011. Visual laterality in dolphins when looking at (un)familiar humans. Animal Cognition, 14(2), pp.303–308.

Tweetboard . Accessed online, 01/08/2018. http://tweetboard.me/anatomy-of-vertebrate-brain/anatomy-of-vertebrate-brain-nervous-system-chapter-pages-ppt-downl-on/

Vallortigara, G., Rogers, L., & Bisazza, A., 1999. Possible evolutionary origins of cognitive brain lateralization. Brain Research Reviews, 30(2), pp.164–175.
 
Williams, D.E. & Norris, B.J., 2007. Laterality in stride pattern preferences in racehorses. Animal Behaviour, 74(4), pp.941–950.


 
Some citations have been removed from the article during editing but all references were consulted for this study.