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05_Sound Localization Learning

05_Sound Localization Learning - ARTICLES 2005 Nature...

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© 2005 Nature Publishing Group http://www.nature.com/natureneuroscience NATURE NEUROSCIENCE VOLUME 8 | NUMBER 1 | JANUARY 2005 93 A R T I C L E S Experience that occurs early in life can have long-lasting effects on the brain. For example, early exposure to language determines the capacity of adults to distinguish among speech sounds 1 , and early social interac- tions greatly influence social and emotional responses later in life 2,3 . In barn owls, the capacity to localize sound, an ability that is vital to the success of an owl as a nocturnal hunter, is shaped by early experience. Juvenile barn owls, unlike adults, can learn highly abnormal associa- tions between auditory cues and locations in visual space as a result of abnormal visual experience. Once learned, these associations can be reacquired in adulthood 4 . Associations between auditory cues and locations in visual space are formed in the midbrain localization pathway of the barn owl. Information about auditory cues, such as interaural time difference (ITD), is processed in the central nucleus of the inferior colliculus (ICC). ITD information is relayed topographically to the external nucleus (ICX), where a map of auditory space is created. The auditory space map is conveyed to the optic tectum, where it merges with a visual map of space to create a multimodal map that guides orienting movements of the eyes and head. Individual tectal neurons are tuned to auditory cue values and visual stimuli that arise from the same location in space 5 . Cue-location associations in the optic tectum are shaped by early visual experience 6 . When a juvenile owl (< 200 d) is exposed to pris- matic spectacles (prisms) that produce a large (17 ° or 23 ° ) horizontal displacement of the visual field, the tuning of tectal neurons to ITD (the primary cue for sound source location in azimuth 7 ) shifts over a period of weeks so that the auditory receptive fields become realigned with the prismatically displaced visual receptive fields (VRFs). This shift in ITD tuning across the auditory space map is adaptive because it realigns the auditory and visual maps and causes the owl to orient to sounds so that the sound source is centered in the prismatically displaced visual field 8 . The magnitude of experience-driven shifts in ITD tuning varies dur- ing development. Juvenile owls reared with 23 ° prisms acquire shifts in ITD tuning in the optic tectum that are, on average, ten times as large as the ITD shifts acquired by adult owls, even when the adults experience prisms for many months 6 . Adult owls that are reared with prisms as juve- niles can readapt to prisms in adulthood, however, acquiring ITD shifts that are similar to those that they acquired as juveniles 4 . Readaptation to prisms in adulthood depends on the direction and magnitude of juvenile prism experience, indicating that readaptation does not result from a generalized increase in plasticity, but rather from a specific trace of the early learning that persists in the central nervous system.
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