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Bi-coordinate sound localization in owls
Most owls are nocturnal or crepuscular birds of prey. Because they hunt at night, when light levels often fall below that at which owls can see their prey, they must rely on non-visual senses. However, several experiments have shown that owls are incapable of hunting by infra-red detection or olfaction. In fact, the necessary and sufficient cues by which an owl locates its prey at night are the sounds made by the prey.
ITD and IID
In addition to detecting directions in the azimuth, animals living above ground must be able to determine the necessary angle of descent, i.e. the elevation. This bi-coordinate sound localization is accomplished through two binaural cues: the interaural time difference (ITD) and the interaural intensity difference (IID), also known as the interaural level difference (ILD).
ITD occurs whenever the distance from the source of sound to the two ears is different, resulting in differences in the arrival times of the sound at the two ears. When the sound source is directly in front of the owl, there is no ITD. In sound localization, ITDs are used as cues for location in the azimuth.
IID is a measure of the difference in the intensity of the sound as it reaches each ear. In many owls, IIDs for high-frequency sounds (higher than 4 or 5 kHz) are the principal cues for locating sound elevation.
Parallel processing pathways in the brain
The axons of the auditory nerve emanate from the nerve cell bodies in the inner ear. Different sound frequencies are encoded by different fibers of the auditory nerve, but codes for the timing and intensity of the sound are not segregated within the auditory nerve. Instead, the ITD is encoded by phase locking, i.e. firing at or near a particular phase angle of the sinusoidal stimulus sound wave, and the IID is encoded by spike rate. Both parameters are carried by each fiber of the auditory nerve.
The fibers of the auditory nerve innervate both cochlear nuclei, the cochlear nucleus magnocellularis and thecochlear nucleus angularis. The neurons of the nucleus magnocellularis phase-lock, but are fairly insensitive to variations in sound intensity, while the neurons of the nucleus angularis phase-lock poorly, if at all, but are sensitive to variations in sound intensity. These two nuclei are the starting points of two separate but parallel pathways to the inferior colliculus: the pathway from nucleus magnocellularis processes ITDs, and the pathway from nucleus angularis processes IID.
In the time pathway, the nucleus laminaris is the first site of binaural convergence. It is here that that the ITD is detected and encoded using neuronal delay lines and coincidence detection, as in the Jeffress model; when phase-locked impulses coming from the left and right ears coincide at a laminaris neuron, the cell fires most strongly. Neurons from the nucleus laminaris project to the core of the central nucleus of the inferior colliculus and to the anterior lateral lemniscal nucleus.
In the intensity pathway, the posterior lateral lemniscal nucleus is the site of binaural convergence and where IID is processed. Stimulation of the contralateral ear excites and that of the ipsilateral ear inhibits the neurons of the nuclei in each brain hemisphere independently. The degree of excitation and inhibition depends on sound intensity, and the difference between the strength of the inhibitory input and that of the excitatory input determines the rate at which neurons of the lemniscal nucleus fire. Thus, the response of these neurons is a function of the differences in sound intensity between the two ears.
At the lateral shell of the central nucleus of the inferior colliculus, the time and intensity pathways converge. The lateral shell projects to the external nucleus, where each space-specific neuron responds to acoustic stimuli only if the sound originates from a restricted area in space, i.e. the receptive field of that neuron. These neurons respond exclusively to binaural signals containing the same ITD and IID that would be created by a sound source located in the neuron’s receptive field. Thus, their receptive fields arise from the neurons’ tuning to particular combinations of ITD and IID, simultaneously in a narrow range. These space-specific neurons can thus form a map of auditory space in which the positions of receptive fields in space are isomorphically projected onto the anatomical sites of the neurons.
Significance of asymmetrical ears for localization of elevation
The ears of many species of owls, including the barn owl (Tyto alba), are asymmetrical. For example, in barn owls, the placement of the two ear flaps (operculi) lying directly in front of the openings to the ear canals is different for each ear. This asymmetry is such that the center of the left ear flap is slightly above a horizontal line passing through the eyes and directed downward, while the center of the right ear flap is slightly below the line and directed upward. In two other species of owls with asymmetrical ears, the saw whet and the long-eared owls, the asymmetry is achieved by very different means: in saw whets, the skull is asymmetrical; in the long-eared owl, the skin structures lying near the ear form asymmetrical entrances to the ear canals, which is achieved by a horizontal membrane. Thus, ear asymmetry seems to have evolved on at least three different occasions among owls. Because owls depend on their sense of hearing for hunting, this convergent evolution in owl ears suggests that asymmetry is important for sound localization in the owl.
Ear asymmetry allows for sound originating from below the eye level to sound louder in the left ear, while sound originating from above the eye level to sound louder in the right ear. Asymmetrical ear placement also causes IID for high frequencies (between 4 kHz and 8 kHz) to vary systematically with elevation, converting IID into a map of elevation. Thus, it is essential for an owl to have the ability to hear high frequencies. Many birds have the neurophysiological machinery to process both ITD and IID, but, because they have small heads and relatively low frequency sensitivity, they use both parameters only for localization in the azimuth. Through evolution, the ability to hear frequencies higher than 3 kHz, the highest frequency of owl flight noise, enabled owls to exploit elevational IIDs, produced by small ear asymmetries that arose by chance, and begun the evolution of more elaborate forms of ear asymmetry.
Another demonstration of the importance of ear asymmetry in owls is that, in experiments, owls with symmetrical ears, such as the screech owl (Otus asio) and the great horned owl (Bubo virginianus), could not be trained to located prey in total darkness, whereas owls with asymmetrical ears could be trained.
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