LATERALIZATION IN THE NERVOUS SYSTEM
Humans and animals prefer to use one side of the body. The most common examples of
motor laterality are right and left-handedness in humans and side or limb preference in animals.
Laterality also refers to specialized tasks of both brain hemispheres. In most humans, the left
hemisphere controls language, and the right hemisphere controls emotional processing. Long
considered unique to humans, increasing evidence points to behavioral asymmetry as a
fundamental principle of organization in vertebrate behavior (Vallortigara and Rogers 2005). At the
most basic level, both sides of vertebrate brains show strong and consistent behavior patterns. In
animals as diverse as chickens, fish, lizards, rodents, and humans, most approach behaviors (finding
food and friends) are left hemisphere tasks, and most avoidance behaviors (responses to predators
and novel stimuli) are right hemisphere tasks (Rogers 2010). Genes and environment influence
lateralization, but the interaction between them is not always clear (Schaafsma et al. 2012).
Laterality in Horses
Horses prefer to work on the left or right side. Whether side preference is a result of genetic
predisposition or if it results from training and use was unknown until recently. Grizmek (1968)
looked into motor asymmetry and found that most horses begin to gallop with the left forelimb
first, and that they also turn left more easily. Heird and Deesing (1998) noticed that horses that are
suddenly startled or frightened focus one eye on novel stimuli and make lateral body movements
around it. Laterality in horses has attracted considerable attention over the last decade (McGreevy
and Rogers 2005; Larouse et al. 2006; Austin and Rogers 2007; Williams and Norris 2007). Austin
and Rogers (2007) found greater fear responses to fear-inducing stimuli presented to horses’ left
eyes, and therefore to the right hemisphere of their brains. This study recorded flight and escapeturning responses in thirty horses exposed to a novel object on three separate occasions. An
experimenter opened an umbrella five meters away from the horse, first to the horse’s left side and
then to the right side, or vice versa. Horses tested first on the left side showed greater reactivity for
left side approach, whereas horses tested first on the right side displayed lessened left side
reactivity. The evidence suggests that stimuli first presented to the right side allowed the horses to
learn that the stimulus posed no threat. This information then transferred to the right hemisphere
of the horses’ brains, leading to a lessened reaction when approached from the left side. In a study
of motor laterality, McGreevy and Thompson (2005) found differences in laterality between breeds
of performance horses. Temperamental Thoroughbreds showed a higher left foreleg preference
during grazing when compared to Standardbred and Quarter Horses. All populations studied
showed varying strengths of lateralization (Rogers 2002; Vallortigara and Bisazza 2002). In
another study of motor laterality, Murphy et al. (2005), found that 52 percent of horses preferred to
start walking or trotting with the right foreleg, while 40 percent preferred the left foreleg and 7.5
percent showed no preference. Findings show strong sex-related directions of laterality, but not
degrees of laterality. Male horses exhibited significantly more left-lateralized responses and female
horses exhibited significantly more right-lateralized responses. In a study of motor laterality in
Thoroughbred horses grazing on pasture, McGreevy and Rogers (2005) noted the relative position
of the horses’ forelegs every sixty seconds for two hours. Results show a population bias of standing
with the left foreleg advanced. Forty-three out of 106 horses demonstrated a significant left leg
preference, ten horses demonstrated a significant right leg preference, and fifty-three horses were
ambidextrous in their forelimb preference. Motor laterality bias increased with age, suggesting
maturation or influence of training. Horses less than four years old presented with olfactory stimuli
(stallion feces), showed a right nostril preference. Older horses showed no nostril bias. In this
study, McGreevy and Rogers (2005) found no significant relationship between sensory and motor
laterality. Larose et al. (2006) studied visual asymmetry during exposure to novel objects in
Trotters and French Saddlebreds. The temperaments of the two breeds were similar, but
expressions of visual laterality differed between individual horses. Horses with higher emotional
scores glanced at a novel object with the left eye (right hemisphere) more than the right eye (left
hemisphere). De Boyer des Roches et al. (2008) found that emotionally laden objects influence
lateral responses. Horses showed a slight tendency to use the left eye (right hemisphere) to explore
objects with negative emotional value (a veterinarian’s white shirt) more than objects with neutral
emotional value (a plastic cone) or positive value (a feed bucket). Basile et al. (2009) reported the
first evidence of socially dependent auditory laterality in horses. Head and ear orientation
responses to whinnies recorded from both familiar and strange horses showed that horses turn
their heads to the right (right ear orientation) toward recorded whinnies from familiar horses, and
to the left (left ear orientation) toward whinnies from strange horses. Farmer et al. (2010) studied
visual laterality in horses interacting with humans. Perception and emotion caused horses to prefer
people standing to the left—not habit and training. In addition, Sankey et al. (2011) found that
yearling horses approached on the left side showed more escape and threat (avoidance) behaviors,
as opposed to more positive (approach) behaviors when approached from the right side. Two-yearold horses, on the other hand, reacted positively to approach and contact from both sides. These
results show the influence of work and training on laterality in horses. Field studies of feral horses
by Austin and Rogers (2012) revealed a left side bias of reactivity and vigilance, and, during
agnostic exchanges, showed plainly that lateralization is a characteristic of horses as a species that
is not caused by human handling. Of particular interest, the absence of population-level forelimb
preference in feral horses suggests that training and work influence forelimb preference in
domestic horses (Austin and Rogers 2012).
These studies clearly show that the right and left sides of horses’ brains are specialized to
handle information in different ways, and to control different classes of behavior. Specialization of
hemispheric control of behavior during stressful and non-stressful conditions is obvious. In a
relaxed state, most horses use the right eye (left hemisphere) to process information. These
findings have important implications for horse welfare (Rogers 2012).
Hemispheric Dominance
Population asymmetries show a consistent direction. Reversed or absent asymmetries
occur in some individuals. Ghirlanda and Vallortigara (2004) note population-level handedness
reversals ranging from about 10 percent in humans to about 35 percent in chimpanzees (in
Corballis 2009). Discussing reversals in horses interacting with humans, Farmer et al. (2010)
writes, “Intriguingly, the right-eyed horses in both groups, although only a very small sample,
showed the same trend with regard to the right eye, that is to say they showed stronger
lateralization under the stanger condition.”
Do right-hemisphere dominant and left-hemisphere dominant horses respond differently to
various similar? Austin and Rogers (2007) remarked that horses that turn right when approached
head-on by a person walking toward them while suddenly opening an umbrella were more reactive
than horses that turned to the left. In chimpanzees, Hopkins and Bennett (1994) found that righthanded chimps interact more with novel objects than left-handed individuals. More evidence
suggests that preferred use of the left paw (and right hemisphere) is characteristic of more fearful
marmosets (Cameron and Rogers 1999). Rogers (2009) also found that left-handed marmosets are
more likely to have higher levels of the stress hormone cortisol when compared with right-handed
individuals. Rogers (2010) also suggests that right-handed marmosets are active, and left-handers
are reactive. Handedness predicts a tendency to adopt a positive or negative cognitive bias.
In horses, no studies exist that specifically address differences in hemisphere dominance
and cognitive bias. However, the first author has noticed possible cognitive bias between several
horses with right and left-side facial hair whorls. To explain this point, his mare Bobbi has a high
left-side hair whorl, and his other mare, Sam, has a high right-side hair whorl. The mares are both
six years old and share the same sire, but different mothers. The first author bred, raised, and
trained both mares himself. Sam and Bobbi are both reactive to sudden novelty and social
separation. Sam vocalizes louder and more often when separated. She also makes more attempts to
return to the herd. Sam also nickers and whinnies more than Bobbi when the first author is
carrying a bucket of grain. The emotional responses to positive and negative stimuli between the
two are clear. The first author recently tested both mares on flight and escape-turning responses
using the method described by Austin and Rogers (2007). Placing an oat bucket in the center of a
large paddock, he released each mare into the paddock and allowed her to eat for one to two
minutes. To make sure that each horse was looking at him using both eyes, the first author
positioned himself in front of each horse while holding an umbrella at waist level and pointing it at
the horse’s face. From about five meters away, he quickly walked toward each horse while suddenly
opening the umbrella. Sam quickly turned and escaped to the left. Bobbi quickly turned and escaped
to the right. Both horses ran about thirty feet before stopping and turning to look back. Moving out
of the paddock, the first author noted the latency of each horse to return to the feed bucket. Bobbi
returned to the bucket and resumed feeding within three minutes. Sam was more reluctant, and
after ten minutes she had still not returned to feed. Sam also snorted constantly for roughly three
minutes after escaping from the umbrella. Horses snort when startled, in pain, or when frightened
(McGreevy 2004). We suggest that the reader interpret with caution the results of one experiment
on two horses. Convincing evidence awaits further investigation.
In both Sam and Bobbi, the escape-turning responses are consistent with their motorturning preferences while under saddle or when lunged in a circle. Sam prefers turning to the left;
Bobbi prefers turning to the right. McGreevy and Rogers (2005) did not find an association between
motor and sensory laterality in horses. However, Tomkins et al. (2012) did find an association
between motor and sensory laterality in dogs (see below). Specifically, study results report
associations between right visual bias, paw preference, and the presence of a hair whorl on the left
side of a dog’s head and chest area. This finding is consistent with anecdotal evidence collected by
the first author that suggests an association between hair whorl side position and motor and
sensory laterality in horses.
Hair Whorl Patterns and Lateralization
Murphy and Arkins (2005) found a significant association between hair whorl orientation
(direction) and motor asymmetry in horses. The study recorded orientation of single hair whorls in
219 horses. Experienced trainers rode each horse to determine motor laterality. Right-lateralized
horses had significantly more clockwise hair whorls. Left-lateralized horses had more counterclockwise hair whorls. In prospective guide dogs for the blind, Tomkins et al. (2012) assessed all
three measures of laterality (hair whorl, motor, and sensory). It takes a significant amount of time
to train guide dogs. The dogs must learn several complex tasks, and it is important to find a quick
and efficient means of discovering traits in the early stages of training that decide whether a dog
will be a successful guide dog. A need to work on the left side of trainers often disqualifies dogs that
are less flexible when turning right. Tomkins et al. (2012) researched associations between hair
whorl characteristics in guide dogs. Dogs’ performances on the Kong™ test, the First-Stepping Test
and the Sensory Jump Test determined motor and sensory laterality. Several associations emerged
between hair whorls and motor and sensory laterality. These results demonstrate a strong
association between behavioral asymmetries and hair whorl characteristics. Most significantly, the
location of a whorl on the left side of a dog’s head and neck was associated with a right visual bias,
and a right visual bias was also probable if the hair on the ventral mandibular (back of the bottom
jaw) was present in a counter-clockwise direction. Dogs with clockwise hair whorls on the chest
were also more right-pawed in the First-stepping Test compared to dogs with counter-clockwise
hair whorls on the chest (Tomkins et al. 2012).
Experience-Dependent Lateralized Learning
In a previous section, the first author discussed his approach to horses that are difficult to
shoe on one side or the other. He found that switching sides and shoeing the “less difficult” side first
was helpful. When he finished the less difficult side, horses were often more cooperative when he
returned to the difficult side. This may be an example of experience-dependent lateralized learning.
Information processed and learned in one hemisphere of the brain transfers to the opposite
hemisphere in an experience-dependent manner. Studies report interhemispheric transfer of
memory in species as diverse as fish, amphibians, reptiles, birds, and mammals (Andrews and
Rogers 2002; Bianki 1988; Vallortigara and Rogers 2005; Robbins and Rogers 2006). In cattle,
Robbins and Phillips (2010) report the first example of experience-dependent lateralized learning
in a mammal. In herd-splitting experiments conducted on dairy and beef cattle, a single
experimenter walked repeatedly through herds, either empty-handed or while carrying various
novel objects. At first, the cattle viewed the experimenter using the left eye (right hemisphere).
After habituating to the experimenter carrying various objects, the cattle then switched to viewing
the experimenter with the right eye (left hemisphere). This directional shift in viewing preference
was experience-dependent. Robbins and Phillips (2010) suggest that the reversal of preference
from the left eye (when the stimuli is novel) to the right eye (when the stimuli becomes familiar)
“frees up” the left eye to simultaneously watch for new threats.
There is no known experimental evidence of experience-dependent lateralized learning in
horses. However, it may be safe to assume that this well-conserved pattern of lateralized learning
exists. During horse training, the first author switches sides when a horse shows a lateralized
response. For example: if a horse shows resistance to having a saddle placed on its back from the
left side, switching to a right-side presentation speeds habituation. Switching sides can also help
when training a horse to lunge in a circle. Even more helpful than switching sides, the first author
also uses hair whorl positions (i.e., left or right side of the forehead) to predict a lateralized
response before it occurs. The non-intrusive neonatal foal handling method used by the first author
may be another example of experience-dependent lateralized learning. Foals look at him several
times from both sides before approaching and touching him for the first time. The first author
observes reduced or almost non-existent lateralized responses to handling.
CHRONIC LATERALIZATION
In his experiences as a horse trainer, the first author has faced several one-sided horses. In
some horses, the turning preference was so strong that any attempt to change it caused the horse to
prance, throw its head, or rear. Any use of force to change a preference was likely to cause paniclike responses. Various explanations of extreme one-sidedness exist in the popular horse training
literature. The most often-cited reasons are pain, conformation, lack of confidence, or reduced
blood and nerve supply to one side of the body. Examining possible physical causes through
veterinary or chiropractic examinations is important, but few industry professionals discuss fear as
a possible cause of extreme one-sidedness. To our knowledge, no scientific research into the cause
of extreme laterality exists. Nevertheless, evidence of brain asymmetries previously discussed point
to one-sided behavior as a possible fear response. Also, anecdotal evidence collected by the first
author suggests a link between the lateral position of hair whorls and extreme one-sidedness. This
observation may provide an easy way to research severe sidedness. One-sided presentation of
emotionally important stimuli improved reproductive performance in dairy cows (Rizhova and
Kokorina 2005). In this study, a hopper pulled by a tractor passing either rightwards or leftwards in
front of the stall delivered feed to dairy cows confined in indoor stalls. Cows fed only from the
rightward-moving hopper became pregnant by artificial insemination more successfully, displayed
higher calving rates, and had more calves throughout their lifespan compared with cows fed only
from the leftward-moving hopper. The rightward-fed cows also produced a 10 percent greater
volume of milk than the leftward-fed cows. These effects reversed when the hopper’s travel
direction reversed.
In conclusion, blinders on a racehorse prevent lateral behavior but do not address the
cause.