160 | mup 4|2011|160-167|© Ernst Reinhardt Verlag München Basel, DOI 10.2378/ mup2011.art11e Konstanze Krüger, Kate Farmer Keywords: Laterality, horse, information processing, training, welfare, human-animal interaction Horses are one-sided, not only on a motor level, but they also prefer to use one eye, ear or nostril over the other under particular circumstances. Horses usually prefer using the left eye to observe novel objects and humans. This preference is more marked in emotional situations and when confronted with unknown persons. Thus the horse’s visual laterality provides a good option for assessing its mental state during training or in human-horse interactions. A strong preference for the left eye may signal that a horse cannot deal with certain training situations or is emotionally affected by a particular person. Laterality in the Horse Krüger, Farmer - Laterality in the Horse mup 4|2011 | 161 Many horses have their special side from which they particularly like being handled by humans, from which they approach objects or humans or like getting longed. Young horses, for instance, have an outright aversion against getting longed from the right hand (Podhajsky 2006). This aversion is said to have its causes in the horses‘ motor one-sidedness. Additionally, horses tend to approach humans (Farmer et al. 2010) and objects (Austin / Rogers 2007; Larose et al. 2006; De Boyer Des Roches et al. 2008) with their left eye and can be handled best from the left. Traditionally, people acted on the assumption that the preference of handling from the left side is based on a merely historical, purely pragmatic background, since soldiers with their sabers hanging down on their left side were only able to get on horseback from the left. Modern equestrians have adopted this tradition (Podhajsky 2006). These two explanations now falter due to recent behavioural and neuro-biological studies on laterality (Austin / Rogers 2007; Larose et al. 2006; De Boyer Des Roches et al. 2008; Farmer et al. 2010). However, since there is currently no applied research dealing with the usage of laterality for training horses and its deployment in the field of therapy, a few study results from the field of behavioural science shall be presented in the following. The following paragraphs will present the status quo of behavioural and neuro-biological literature dealing with laterality of humans, some animals and horses, which shall lead to therapists, its relevance for horse-assisted therapy shall be described, followed by possible prospects on future research. Contradictions to the tradition theory The preference of horses for approaching humans with their left eye does not seem to have its cause primarily in their habituation of left-sided training. Since horses are capable of generalising training situations, in this case you would expect a consistent preference for the left side in any training situations from horses that have only been trained from the left side (Hanggi 1999a; Krueger 2007). As a matter of fact, horses trained from both sides as well as horses trained from the left prefer approaching humans and objects with their left eye. The preference for the left eye is greater when approaching humans they are unfamiliar with than when approaching people they know (Farmer et al. 2010), and this preference is even increased in emotional situations, e.g. when horses are getting scared additionally (Larose et al. 2006). Another hint against the correctness of the tradition theory and in favour of a natural behaviour is provided by two studies dealing with savaged horses, which are currently carried out and prepared for publication. Just recently caught mustangs in the United States of America as well as Australia‘s savaged horses (so-called “Brumbies“) partly display a very dominating visual laterality when approaching conspecifics and humans. The strong laterality of some mustangs gets weaker after both-sided training. Contradictions to motor one-sidedness Also the approach that the preference for one side is only caused by the horses‘ onesidedness, cannot be backed up by scientific literature. An interesting fact is that motor onesidedness with many animal species, also with horses, can have various degrees of intensity (Rogers 2010). Horses do not show absolutely clear motor preferences as for making use of their sense organs (92 % preference of the left eye when approachnig humans) (Farmer et al. 2010). Two third of the thoroughbred prefer left lead (Deuel 1987). Other studies with various parameters such as wallowing, moving along with 162 | mup 4|2011 Krüger, Farmer - Laterality in the Horse another horse, placing one forefoot in front of another while grazing (McGreevy / Rogers 2005), which foot to be used as a forefoot when jumping across an obstable (Grizimek 1949) or with which to start trotting or paceing, the decision concerning the side with which to move past an object (Murphy et al. 2005), hardly shows any side preferences on population level. The most consistent results could be found for placing one forefoot to the front while grazing, when the horse distributes its weight evenly across the remaining three feet. 40.56 % of the horses preferred the left foot, 9.44 % the right one, and 50 % showed no preference at all. The degree of preference of the left forefoot increased with age and training experience (McGreevy / Rogers 2005). Murphy et al. (2005) found that, similar to cats (Wells / Millsopp 2009), male horses are significantly more lateralised in favour of the left side, and female horses significantly more to the right side. Studies dealing with the correlation of motor and sensory one-sidedness still need to be carried out. One study compared the one-sided use of the nose with motor one-sidedness and found no correlation (McGreevy / Rogers 2005). One-sided task sharing between the cerebral hemispheres Current neuro-biological literature suggests that the horse‘s one-sidedness is, in many cases, caused by a one-sided task sharing between cerebral hemispheres, resulting in a sensory onesidedness. Humans, many animals and also horses show a preference for perceiving sensations with a specific ear, eye and nostril (Tommasi 2009; Rogers 2010) and process this information in a specific cerebral hemisphere. Thus, information gathered through the nose or nostrils are forwarded to the cerebral hemisphere on the same side. However, the majority of nerve tracts of eye and ear is connected with the counter-lateral cerebral hemispheres in a crosswise manner. This is why Picture 1: During a behavioural experiment, the mare eats from a bucket while observing the test subject with the left eye. Krüger, Farmer - Laterality in the Horse mup 4|2011 | 163 sensations picked up on the left side are mainly processed in the right cerebral hemisphere and vice versa. For a long time, people acted on the assumption that horses are not able to transfer learnt contexts between the cerebral hemispheres, because horses had no sufficiently developed corpus callosum, i. e. no connection between the cerebral hemispheres. So, the horse would have to perceive each sensation with each eye separately. Today, we know that this assumption was wrong. The majority of information picked up visually is sent to the horse‘s opposite cerebral hemisphere, since about 80 % of the horse‘s visual nerves cross each other in the chiasma opticum (anatomic term for the crossing point of visual nerves) and run directly to the counter-lateral cerebral hemisphere. However, about 20 % of the visual nerves lead to a well-developed corpus callosum. If, for instance, horses learn to spot triangles among other symbols with one eye covered, they are also able to spot triangles with the eye which was covered before (Hanggi 1999b). Tasks of the left cerebral hemisphere Despite the possibility of transferring information between hemispheres, information is sent, depending on its level of quality, preferrably to a specific cerebral hemisphere. Please note that some functions are carried out either in both cerebral hemispheres in an overlapping manner or partially in the left or the right one (Rogers 2010). Thus, dogs and horses prefer the right ear and the left cerebral hemisphere, respectively, to analyse sounds made by their conspecifics. However, if barking sounds of dogs are emotional, e. g. aggressive, or the horse is excited (Basile et al. 2009), the animals make more use of their left ear. Generally speaking, the left cerebral hemisphere can be called the cognitive one. It is in charge of controlling behaviour in every-day situations. It does the most part of controling and processing human language and animal sounds (Basile et al. 2009). Here, humans make rational decisions, and controlled behaviour is processed, based on learning. In scientific terms, the left cerebral hemisphere is the place where so-called “topdown processing“ takes place, i. e. the control of behaviour which is bound to instructions. Tasks of the right cerebral hemisphere Things are different in the right cerebral hemisphere. It is usally called the instinctive, reactive side of the brain. The right cerebral hemisphere responds to unforeseen changes, it spots details and reacts on new objects and situations. Here, “bottom-up processing“ takes place, i. e. spontaneous reactions are born here, triggered by outside stimuli. This is of particular Picture 2: Two savaged stallions from the Pony di Esperia breed are threatening each other from the left side. 164 | mup 4|2011 Krüger, Farmer - Laterality in the Horse importance for reacting in emergency situations. The left cerebral hemisphere controls strong emotions such as fear, aggression and affection, but also endocrinal functions such as the heart frequency are controlled here. Interestingly, humans (Guo et al. 2009) and many mammals prefer the right cerebral hemisphere for recognising conspecifics (sheep: Pierce et al. 2000; monkeys: Hamilton / Vermeire 1988, Guo et al. 2009; dogs: Guo et al. 2009). Apart from that, they use the right cerebral hemisphere in aggressive situations and other types of interaction within the group. The preference of one-sided information processing Man and beast alike benefit from the task sharing between the cerebral hemispheres in that it allows them to handle various and crucial tasks at the same time and to decide faster (Rogers 2010). In the field of view of the left eye and the right cerebral hemisphere, for instance, conspecifics can be observed, while decisions about whether things seen by the right eye are edible or not are made. Animals with a strong laterality are faster in many ways than those with a weak laterality. Cats preferring one specific paw are able to follow a light spot on the ground faster, chimpanzees are quicker in catching termites when preferring one specific hand, and parrots with a strong visual laterality are more successful in learning (Rogers 2010). In addition, the flight animal horse, living in a group, benefits from a both-sided laterality on population level. When all group members pick up information with the same side, synchronise each other with the same side and escape in the same direction, they simply have higher chances of escaping a predator (Tommasi 2009; Rogers 2010). Importance of sensory one-sidedness for man-horse relation When looking at the assumption that sensory one-sidedness is connected with the quality of sensations, the observance of laterality gains more importance for evaluating the well-being of horses, a possible training success and man-horse interactions. One-sidedness suggests well-being When pictures with emotional content are displayed to humans, differences can be measured in the level of released stress hormones (cortisol) in the saliva (Wittling / Pfluger 1990) as well as changes of blood pressure (Wittling et al. 1998) and heart frequency (Wittling 1990) only when the left eye is used (right cerebral hemisphere). Generally, animals are distracted easier when using their left eye (Rogers 2010). Also horses show stronger reactions at first when an object is visible to their left eye - compared with the presentation to their right eye (Austin / Rogers 2007). Based on this, Austin and Rogers (2007) recommend introducing new objects or humans to horses to their right eye first. On the other hand, however, some texts suggests that sheep show less stress and fear when they see pictures of their conspecifics with their left eye (da Costa et al. 2004), but not so when presented to their right eye. Cows show improved milk performance and better reproduction results when they always get their food from the left side (Rizhova / Kokorina 2005). Some even suggest that the left cerebral hemisphere can suppress the stress processing of the right cerebral hemisphere (Sullivan 2004). It has indeed been proven and tested to allow horses to use their left eye, since they need it to put information in the right context (Farmer et al. 2010; Rogers 2010). Krüger, Farmer - Laterality in the Horse mup 4|2011 | 165 One-sidedness during training and contact with humans In this context it is interesting that laterality with humans under stress or frustration, i. e. when a task cannot be carried out, is increased (Schultheiss et al. 2009). According to what we know today increasing stress also increases the horse‘s laterality (Larose et al. 2006). There is no study on the laterality in case of excessive demand. Since the brain structure and laterality of horses has many similarities to the ones of humans and other mammals (Rogers 2010), and excess demand generally triggers stress, we can act on the assumption that also horses respond with increased laterality when confronted with excess demand. Based on this, we suggest cutting back on training requirements with the horse in case of a strong one-sidedness, or to reduce contacts with humans resulting in a strong laterality. When training sessions are repeated and objects and humans are presented to both sides in small steps, laterality with most horses is reduced along with the level of excitement. We would like to point out that there are only few horses with a strong preference for the right eye (Farmer et al. 2010). These horses are considered particularly difficult to handle. It is yet to be clarified whether this has to do with a changed cerebral structure or rather with the experience of that for these horses humans approaching from the classic left side means from the “wrong“ side, since scientific studies concerning this haven‘t been made yet. It is also possible that right-lateralised horses differ from left-lateralised ones. Up to this day, studies show correlations between the level of personality development and the visual laterality with fish (Andrew et al. 2009; Reddon / Hurd 2009) and the motor laterality of dogs (Batt et al. 2009; Branson / Rogers 2006). However, with horses there is nothing known in this respect yet. Our work group is currently carrying out a study on comparing personality and laterality of savaged horses. Humans as social partners? It is a very interesting insight that horses tend to prefer the left eye when lookng at humans, since many mammals use their left eye when looking at conspecifics. Humans (Guo et al. 2009), sheep (Pierce et al. 2000), monkeys (Hamilton / Vermeire 1988; Guo et al. 2009) and dogs (Guo et al. 2009) prefer the left eye when looking at conspecifics. Fully grown Beluga whales look at their social partner, just like Beluga whale and dolphin new borns at their mother, with their left eye (Karenina et al. 2010). We can only assume that the horse considers humans also as a social partner and thus looks at him with its left eye. It is also hard to say whether looking at humans triggers positive or negative emotions within the horse. since objects with a positive and those with a negative evaluation are looked at with the left eye, neutral objects are looked at with the right eye (De Boyer Des Roches et al. 2008). Thus, a weak preference for the left eye when dealing with humans may be considered a normal reaction for the horse. But since the sensory laterality with the horse increases along with the stress level (Larose et al. 2006) and is reduced with two-sided training and the level of familiarity of humans (Farmer et al. 2010), an extremely strong sensory laterality during training, when dealing with humans or during therapeutical deployment, should attract the therapist‘s attention. When in doubt, it is advised to cut back on the requirements for a heavily lateralised horse and to increase them again step by step through both-sided training. In order to clarify this it is urgently required to carry out comparing studies between physiological parameters for measuring emotional 166 | mup 4|2011 Krüger, Farmer - Laterality in the Horse excitement and stress and sensory laterality of horses when dealing with conspecifics and humans as well as when deploying horses in therapeutical activities. Literature ■ Andrew, R. J., Osorio, D., Budaev, S. 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Brain and Cognition 14, 243-265 ■ Wittling, W., Block, A., Schweiger, E., Genzel, S. (1998): Hemisphere Asymmetry in Sympathetic Control of the Human Myocardium. Brain and Cognition 38, 17-35 The authors Dr. Konstanze Krüger studied animal health in Munich, is involved in therapeutic riding and in riding for handicapped people and now occupies herself with cognition research with horses. Kate Farmer studied psychology at St. Andrews university in Scotland. She developed the “thinking horse“ training approach and is involved in horse behavioural and laterality research. Contact details of the authors: Dr. Konstanze Krüger Universität Regensburg · Biologie 1 Universitätsstr. 31 · D-93053 Regensburg, Germany Konstanze.Krueger@biologie.uni-regensburg.de Tel.: +49-(0)-941 / 9433293 · Fax: +49-(0)-941 / 9433304 Kate Farmer Centre for Social Learning & Cognitive Evolution School of Psychology · University of St Andrews St Andrews · Scotland KY16 9JP UK· katefarmer@ utanet.at