Laterality: implications for equine management and performance

Laterality: assessment via cerebral hemisphere control

Cerebral hemisphere differentiation is well documented in vertebrates (De Boyer Des Roches et al, 2008; Rogers, 2010) (Table 1), with the left hemisphere considered to be responsible for instruction-driven (top down) processing while the right hemisphere computes stimulus-driven (bottom up) tasks (Rogers, 2010).

While individual hemispheric dominance is acknowledged, it is not a black or white scenario, with control of resultant responses occurring in degrees not absolutes. For example, it has been identified that vocal and visual recognition of conspecifics occurs in left hemispheric control in horses (Austin and Rogers, 2007; Basile et al, 2009), dogs (Guo et al, 2009; Siniscalchi et al, 2010), monkeys and humans (Guo et al, 2009). However, cognitive states are believed to be intrinsically linked to affective states and emotions (Rogers, 2010); thus, when fear is introduced, assessment of vocalization in dogs has been found to transfer to right hemispheric processing (Siniscalchi et al, 2010), illustrating the complementary relationship between the right and left sides of the brain.

Rogers (2010) theorized that individuals governed by right hemisphere dominance (RHD), would be more reactive to novel stimuli and, therefore, easily distracted in everyday environments (Figure 1), and could be more prone to aggression or fearful responses. In contrast, those with left hemisphere dominance (LHD), would project a calmer, less reactive or proactive attitude with a predilection to perform focused tasks.

Stress complicates the assessment of hemisphere control (Paul et al, 2005). Acute stress will initiate fight-or-flight responses attributable to right hemispheric control; however, in chronic stress states a shift towards RHD has also been recorded in previously LHD individuals (Rogers, 2010). This could be applied to a clinical environment where the rehabilitation patient who has previously exhibited a calm temperament develops anxiety and potentially negative behaviour representing the transition from calm (LHD), to reactive (RHD).

Laterality: assessment via motor preference

Laterality assessment of motor preference is usually measured via left and right lead leg/paw/hand preference during task completion or gait (Klar, 1999; McGreevy and Rogers, 2005; Poyser et al, 2006; Tomkins et al, 2010), dominant eye use during assessment of novel objects (Larose et al, 2006; De Boyers Des Roches et al, 2008), nostril choice during olfaction of novel objects (De Boyers Des Roches et al, 2008) and orientation of tail wagging (Siniscalchi et al, 2010).

Research in horses has concentrated to date on lead forelimb preference and visual dominance. De Boyer Des Roches et al (2008) investigated visual laterality in a population of Arabian broodmares in response to the introduction of novel stimuli: positive, familiar food bucket; neutral, unfamiliar plastic cone; and negative, shirt associated with veterinary surgeon. A population bias for left eye investigation, therefore RHD, was observed on presentation of the shirt while a right eye preference was recorded for the cone, LHD, and equal variance was reported between both eyes for the familiar bucket. The study concluded that inert novel stimulus investigation is under left hemisphere control while right hemisphere recruitment is initiated during presentation of a negative stimulus.

Exaggerated fight responses have also been reported to the introduction of a novel object — an umbrella — when approached from the left-hand side (RHD), rather than the right (LHD) (Larose et al, 2006; Austin and Rogers, 2007), supporting this hypothesis.

Applications of laterality

The utilization of lateral preference to ‘read’ body language to enable prediction of behavioural responses in the horse could prove a useful tool for veterinary professionals, riders and trainers. Assessment of motor laterality could indicate hemisphere dominance and therefore predict if a horse was a reactive (RHD) or proactive individual (LHD). Equally, the evaluation of impact of emotional status on cognitive function would be another useful assessment tool within training, management and rehabilitation.

The identification of hemisphere dominance could aid prediction of responses to ambiguous stimuli and predict behavioural responses. For example, an LHD horse would exhibit a positive cognitive bias or an optimistic response, such as investigative behaviour, while an RHD horse would react with a pessimistic or negative cognitive bias, for example to initiate a fight reaction (Rogers, 2010). A simple novel stimulus test could be designed and used for horses, or indeed any patient, on entry to the veterinary practice, and their response used to inform future nursing care plans. A caveat must be considered for animals that have been exposed to states of chronic stress as these individuals may exhibit a transitional shift from LHD to RHD over time.

Motor preference is commonly established via assessment of lead forelimb, however locomotion in the horse is driven from the hind leg. Austin and Rogers (2007) found that horses which displayed a right lead leg preference, and which turned right when presented with a novel threatening stimulus, were more reactive than those that turned to the left. At first glance this does not correlate to hemisphere dominance; however, in quadrupeds the influence of gait patterns requires consideration when interpreting motor response in laterality studies. Turning at walk is initiated by the left hind leg, therefore a right foreleg lead could be argued to represent a left hind leg bias and therefore an RHD individual.

The gallop also has an asymmetrical stride pattern with either left hind, right lead (RHD), or right hind, left lead (LHD) stride characteristics. Williams and Norris (2007) investigated laterality bias within gallop stride patterns in thoroughbred racehorses and found that 90% exhibited a right lead preference, therefore RHD, and 10% a left lead preference, therefore LHD, at the start, during gallop work and within racing performances.

The application of laterality assessment to inform management or training should consider differences in hemisphere control with reference to the nature of motor tasks used in assessment. Therefore, in studies where movement is evaluated to assess lead leg preference, the hind limb that initiated the stride should be used to establish hemisphere dominance. However, in studies assessing lead forelimb dominance during grazing, the author proposes that the horse is static and the dominant foreleg is actively selected to weight bear. In this instance it could be considered that foreleg preference is linked to hemisphere dominance in the same way the dominant eye was in the work of De Boyer Des Roches et al (2008). This could explain the apparent contradiction observed between the work of McGreevy and Rogers (2005) who reported a left lead leg preference (RHD proposed) in thoroughbreds when grazing, and Williams and Norris (2007) who reported right lead leg preference (LHD proposed) during galloping in the same breed; but in reality both populations are RHD.

This information could be used on a practical level to design training regimens, influence management decisions or adapt the approach taken during handling or clinical care thus maximizing the welfare of the horse but also reducing the risk to the handler/rider.

Laterality and influence on injury

Conformation must be considered a contributing factor to motor laterality and is also intrinsically linked to injury prevalence. Differences in lateral development are thought to be determined during embryonic development with individuals who have been exposed to fewer prenatal environmental insults producing greater symmetry postnatally (McManus, 2002).

Increased symmetry has been linked to superior intelligence in humans and enhanced performance in thoroughbreds (Manning and Ockenden, 1994; McManus, 2002). If symmetrical horses have been demonstrated to be superior performers, individuals who exhibit asymmetrical loading due to motor laterality should be at an increased risk of injury or exhibit poor performance. Equestrian athletes are acknowledged to operate within narrow safety margins with regard to mechanical loading during galloping or the strain placed on soft tissues during turning and jumping (Weller et al, 2006). Evaluation of symmetry in equine populations has been conducted; Weller et al (2006) examined various anatomical markers to evaluate the extent of population asymmetry in National Hunt racehorses and found that 25% of variables investigated exhibited significant asymmetry (Figure 2). It is difficult to hypothesize whether laterality, pathology or inherent conformation is the dominant factor in the development of asymmetry, but it is apparent that this does contribute to injury and reduced performance.

Parkin et al (2006) investigated risk factors associated with fatal distal limb fractures during horse racing. Relationships were exposed between leading forelimb at start (66% of cases), most used lead leg (40% of cases), and the lead at the time of fracture (50% of cases), with a significant correlation between leading leg and the side of the fracture recorded. UK racecourses commonly have round or oval circuits with a right or left directional bias. These results suggest that lead-leg preference may be indicated as a risk factor for fracture and track orientation may impact on this variable.

Fatigue can induce biomechanical changes in locomotion patterns and it may be that equine athletes transfer to their ‘natural’ motor preference regardless of training regimens during fatigue or in high arousal environments. Ramzan and Palmer (2011) conducted a 3-year epidemiological review of equine injury prevalence during training within three racing establishments. A consistent level (∼25%) of horses in each yard sustained musculoskeletal injuries per year but variation in specific injury prevalence was recorded between establishments. The study affiliates to epidemiological reviews conducted for injuries recorded on the racetrack (Williams et al, 2001; Stover, 2003). It is interesting that in racing, 45% of thoroughbreds do not reach the racetrack due to injury (Stover, 2003), and the high percentage of musculoskeletal injury recorded is attributed in numerous studies to variation in loading patterns related to extrinsic (for example, going) and intrinsic (for example, hoof pastern axis) risk factors.

If gallop laterality is an endemic population characteristic of thoroughbreds, variation may contribute to gait efficiency exhibited via stride kinematics. To date, no studies have investigated the impact of lateral gallop stride kinematics on subclinical musculoskeletal injury, racecourse performance or the impact of training protocols on their expression (Figure 3). Adaptation of training regimens to consider natural lateral preference may aid in the capacity to produce uniform gallop mechanics, which would represent an inherent advantage to the racing thoroughbred through increased biomechanical efficiency and reduction of injury.

Using laterality in training and rehabilitation

Training and rehabilitation regimens aim to equalize asymmetrical musculoskeletal development. Trainers often comment on a horse's natural ‘side’ or ‘rein’, representing the directional bias where the horse presents in better balance and has enhanced suppleness; training regimens are often designed to equalize the differential between each rein as most equestrian disciplines require a balanced, symmetrical athlete (Figure 4). Interestingly, motor laterality has also been recorded during ridden work in sports horses (Murphy and Arkins, 2008). Studies of this nature are valuable but further consideration of rider handed-ness, competency and the influence this imparts on equine performance are required.

It should also be noted that Rogers (2010) has proposed that hemisphere dominance appears to be relatively resistant to training. Therefore, training can be used to neutralize motor preference and associated musculoskeletal asymmetry via learned motor patterns, but inherent laterality could still be a proximate response in novel situations and may also be harder to ‘train’ in RHD reactive prone horses. The integration of laterality bias into individual training regimens could enhance athletic performance, reduce injury and equine welfare during re-training where a horse may struggle with a particular manoeuvre that goes against its innate bias (McGreevy and Rogers, 2005; Murphy et al, 2005; Warren-Smith and McGreevy, 2010).

Sex-related differences in laterality have also been exposed in dogs (McGreevy et al, 2010), cats (Wells and Millsopp, 2009) and horses (Murphy et al, 2005), with males reported to exhibit left paw/foreleg bias, respectively, in contrast to females who have a right paw/forelimb bias. This could suggest that males would be more reactive (RHD), and therefore sex differentials should be considered during training and management of horses.

Research evaluating breed predilection of motor laterality is, to date, contradictory; McGreevy and Thompson (2006) recorded left forelimb preference during grazing in thoroughbreds and standardbreds but a right lead bias within Quarter horses. If motor actions and cognitive responses are influenced by affective status of the individual, the potential for the development of breed or population directional motor laterality and associated cognitive bias exists.

It is interesting to consider the impact of hemisphere dominance on breed characteristics in horses. Lloyd et al (2008) formulated a horse personality questionnaire and reported thoroughbreds as excitable and reactive. This would suggest a breed bias towards RHD individuals who would exhibit right forelimb motor laterality, in active gaits, as recorded by Williams and Norris (2007) and enhanced reactions to novel environmental stimuli. The thoroughbred is considered to be a supreme equine athlete and its influence can be traced in the breed development of most modern sports horse breeds. Therefore, does this suggest that a ‘south-hoof’, left dominant (RHD), is a superior performer? Schultz (2010) argued that nature loves a southpaw, as these RHD individuals are the population members who are programmed to respond to novel environments and drive evolutionary adaptation.

It could be argued that laterality should be considered when matching individual horses to their role. For example, a novice rider may appreciate a horse that concentrates on the task in hand and is not reactive to extraneous stimuli – characteristics an LHD horse would display. In contrast, an RHD elite equine athlete may be more reactive to its environment, and need an elite rider, but this could confer a performance advantage. On a simpler level, assessment of laterality could influence day-to-day management tasks such as which side the horse is approached from.

Conclusion

Recognition of an individual's lateral preference can be another skill the practitioner or trainer can add to their toolkit to facilitate optimum health, welfare and performance in the horse. Care should be taken in the interpretation of research related to the horse with consideration to the nature of the task being investigated. Recognition of gait patterns, the influence of extrinsic factors, such as rider handedness or the nature of the test used, can complicate analysis of hemisphere and motor laterality. However, the evidence does suggest that identification of laterality could aid the achievement of performance goals and improve management in the horse. Therefore, if a horse is right-hoofed or left-hoofed is perhaps worthy of some thought.

Key Points