Behavior Genetics of the Horse (Equus caballus) - Genetics and the Behavior of Domestic Animals - page 269

disunited canter. A disunited canter was when a horse began a canter on the
correct lead, but then started to canter with an incorrect sequence of leads. In
both the ridden, and un-ridden horses there was no correlation between graz-
ing stance (relaxed) and any of the lunging indicators (challenged).
One possible reason for the conflicting results of motor bias is that differ-
ent tests measure different characteristics of motor bias. Grazing stance, initi-
ating a walk or trot, and rolling on the ground are biases exhibited while
horses are in a relaxed state. Lunging and galloping biases are exhibited dur-
ing a stressed (physical or psychological) state. Different conclusions about
motor bias may be explained by test conditions, breed and temperamental
characteristics, and selective pressures for a particular type of work.
McGreevy and Thompson (2006) suggest that some breeds may have inad-
vertently been selected for laterality, or ambilaterality, due to the nature of
the work they perform. It is not known if this is possible in horses, but selec-
tive breeding for laterality is routinely done in laboratory mice ( Collins,
1975 ).

Sensory Laterality
Sensory laterality refers to the preferred use of one eye, ear, or nostril over
the other under particular circumstances. Science attributes sensory laterality
to the processing of different types of information in one or other of the two
brain hemispheres ( Andrew, 2002; Rogers, 2009, 2010; Villortigara and
Rogers, 2005 ). Rogers (2009) and Proops and McComb (2012) suggest that
sensory laterality evolved before motor laterality. Evidence to support this is
found in fish without limbs that show lateralized eye preferences to view
certain scenes ( Biazza et al., 1997 ), and prey catching on their right side
( Takeuchi and Hori, 2008 ). Scientists that once believed that hemispheric
asymmetry is unique in humans now embrace the evidence for lateralization
in a wide variety of

species (summarized by Rogers, 2009, 2010 ;

Vallortigara and Rogers, 2005 ).
Until recently, there was no evidence of sensory laterality in horses.
Heird and Deesing (1998) were the first to discuss sensory laterality and
noticed when horses are suddenly startled or frightened by novel stimulus
they focus one eye on the object and make lateral body movements around
it. Over the last decade, sensory laterality in horses has attracted consider-
able attention ( Austin and Rogers, 2007; Laroseet al., 2006; McGreevy and
Rogers, 2005; Williams and Norris, 2007 ). Austin and Rogers (2007) and
Murphy et al. (2005) recorded flight and escape-turning responses in 30
horses of various breeds exposed to a novel object and recorded greater
fear responses to fear-inducing stimuli presented to horses' left eyes, and
therefore the brains' right hemispheres. In this study, an experimenter
opened an umbrella 5 m away from the horse, first to the horse's left side
andthentotherightside,orvice versa. Horses tested first on the left side