AskNature

“The ‘ears’ of this long-eared owl are not ears at all, but merely decorative feathers. The owl relies mainly on its hearing when hunting. Its ears are placed asymmetrically on either side of its head, and the sounds received by them are interpreted to give a very accurate three-dimensional sound map of its surroundings.” (Foy and Oxford Scientific Films 1982:165)

“Barn owls are better at tracking sounds that move horizontally than
those that move vertically, researchers have found…The work, published
in PLoS ONE1, relies on a phenomenon noted by Ivan Pavlov, of salivating
dog fame, in the 1920s. Pavlov saw that animals respond to stimuli such
as sudden movements or novel noises with a set of automatic responses,
including muscle tensing and pupil dilation.

“Avinash Bala, a neurologist
at the University of Oregon in Eugene, and his colleagues have used
this response to monitor when barn owls (Tyto alba) recognize a new
sound.

“The researchers played the owls sounds whose positions differed
either horizontally or vertically, and measured the birds’ pupil
dilation using a beam of infrared light bounced off the cornea.

“Owls
were about twice as sensitive to horizontal shifts compared with
vertical changes. The birds could detect a change in location as small
as 3º when the source was moved horizontally, compared with 7.5º when
the source was moved vertically…The researchers also mapped which neurons in the auditory centre
of the owls’ brains fired in response to the sounds.

“The activity
pattern of the neurons matched the location of the sound, the team
found. Sounds from above, for example, cause neurons towards the top of
the auditory centre to fire, whereas sounds from lower down trigger
neurons towards the bottom. ‘The owls basically have a topographic map
of space in their brain,’ says Bala.” (Ledford 2007)