page_title: Sound localization in owls
section_title: Parallel processing pathways in the brain
hierarchical_section_title: Sound localization in owls / Parallel processing pathways in the brain
caption_attribution_description: Parallel processing pathways in the brain for time and intensity for sound localization in the owl.
context_page_description: Most owls are nocturnal or crepuscular birds of prey. Because they hunt at night, they must rely on non-visual senses. Experiments by Roger Payne have shown that owls are sensitive to the sounds made by their prey, not the heat or the smell. In fact, the sound cues are both necessary and sufficient for localization of mice from a distant location where they are perched. For this to work, the owls must be able to accurately localize both the azimuth and the elevation of the sound source.
context_section_description: The axons of the auditory nerve originate from the hair cells of the cochlea in the inner ear. Different sound frequencies are encoded by different fibers of the auditory nerve, arranged along the length of the auditory nerve, but codes for the timing and level 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 in the brainstem, the cochlear nucleus magnocellularis (mammalian anteroventral cochlear nucleus) and the cochlear nucleus angularis (see figure; mammalian posteroventral and dorsal cochlear nuclei). The neurons of the nucleus magnocellularis phase-lock, but are fairly insensitive to variations in sound pressure, while the neurons of the nucleus angularis phase-lock poorly, if at all, but are sensitive to variations in sound pressure. 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 (mammalian medial superior olive) is the first site of binaural convergence. It is here that 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. Thus, the nucleus laminaris acts as a delay-line coincidence detector, converting distance traveled to time delay and generating a map of interaural time difference. 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 sound level pathway, the posterior lateral lemniscal nucleus (mammalian lateral superior olive) is the site of binaural convergence and where IID is processed. Stimulation of the contralateral ear inhibits and that of the ipsilateral ear excites the neurons of the nuclei in each brain hemisphere independently. The degree of excitation and inhibition depends on sound pressure, 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 difference in sound pressure between the two ears.
  The time and sound-pressure pathways converge at the lateral shell of the central nucleus of the inferior colliculus. 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.
caption: Parallel processing pathways in the brain for time and level for sound localization in the owl
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