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Auditory and Olfactory Laterality
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, whereas no bias was found during calls from unfamiliar horses.
McGreevy and Rogers (2005)
investigated olfactory laterality in
Thoroughbred horses (32 males, 125 females) less than 4 years old. Horses
were presented with olfactory stimuli (stallion feces) and a right nostril pref-
erence was found in younger, but not in the older horses.
These studies clearly show that the right and left sides of horses' brains
are specialized to handle information in different ways, and to control differ-
ent classes of behavior. In addition, varying strengths of lateralization have
been found between breeds, and in individuals within a breed (
De Boyer des
Roches et al., 2008; McGreevy and Rogers, 2005
;
McGreevy and
Thompson, 2006
;
Sankey et al., 2011
). Specialization of hemispheric control
of behavior during stressful and non-stressful conditions is obvious, and has
important implications for horse welfare (
Rogers, 2010
). In a relaxed state,
most horses use the right eye (left hemisphere) to process information. In a
stressed state, dominant use of the left eye (right hemisphere) may compro-
mise welfare. This is especially important considering that the right hemi-
sphere also controls endocrine function, heart rate, and blood pressure
(
Conrad et al., 2011; Wittling and Pfluger, 1990
; Wittling et al., 1998).
Recent data also suggest that the right and left cerebral hemispheres differ in
their ability to regulate autonomic processes, and direct unilateral stimulation
of the brain provokes side-dependent endocrine, immune and other visceral
reactions. The study of laterality and fearfulness can be used as a new
approach to investigate the processes that underlie the inter-individual vari-
ability of the vulnerability to stress and stress related diseases.
Hemispheric Dominance
Population asymmetries, such as right-handedness in humans, show consis-
tent directions. Reversed or absent asymmetries occur in some individuals
(
Corballis, 2009
).
Ghirlanda et al. (2009)
note population-level handedness
reversals ranging from about 10% in humans to about 35% in chimpanzees
(
Corballis, 2009
). In 326 normal humans,
Knecht et al. (2000)
measured lat-
eralization directly by functional transcranial Doppler sonography and found
the incidence of right-hemisphere language ranged from 4% in strong right-
handers to 15% in ambidextrous individuals and 27% in strong left-handers.
Knecht et al. (2000)
suggest that these results demonstrate that the relation-
ship between handedness and language dominance is not an artifact of cere-
bral pathology but a natural phenomenon. Discussing reversals in horses
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