טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentEfrati Adi
SubjectEffects of Early Auditory Experience on the Development
of the Auditory Map of Space in the Barn
Owl's Brain
DepartmentDepartment of Medicine
Supervisor Professor Yoram Gutfreund
Full Thesis textFull thesis text - English Version


Abstract

The location of a stimulus is one of the key aspects coded by sensory systems. Commonly, the locations are represented in topographically organized neural maps. The optic tectum (OT, superior colliculus in mammals) contains mutually aligned visual and auditory topographical maps of space. The auditory map is a computational map, based on the calculation of auditory localization cues and their association with the appropriate locations in space. In the barn owl, an auditory localization specialist, the main cue for the azimuth of the sound source is the interaural time difference (ITD), and the main cue for the elevation of the sound source is the interaural level difference (ILD). The auditory spatial map in the OT is modified by prolonged manipulations of the normal relationship between the auditory cues values and the position of the sound source.  The goal of this study was to explore whether normal experience with the auditory cues is essential for the formation of the map. For this, young barn owls were raised in continuous omnidirectional broadband noise from before hearing onset to the age of two months, when they normally leave the nest. At the age of two months, the auditory spatial representation in the OT was surveyed using extracellular recordings. Data from these birds were compared with data from age-matched control owls and from normal adult owls (> 200 days posthatch). In the noise-reared owls, the tuning of tectal neurons to both ITD and ILD was significantly broader than in the control owls. Furthermore, the representation of ILD in the noise-reared owls was biased towards sounds louder in the contralateral ear. A similar contralateral bias appeared, but to a much lesser extent, in the age-matched control owls but was absent in the adult owls. To determine whether the effects of noise-exposure were enduring or could recover following the exposure to normal acoustical environment, we continued to survey the neural representations in the OT for an extended period of up to several months after the cessation of noise. All abnormalities in the ITD and ILD representations in the noise-reared owls tended to gradually recover with normal auditory experience, demonstrating that the auditory map is capable of adaptive plasticity beyond the age of two months. The results of this study suggest that the development of the auditory map of space in the OT of the barn owl is experience-dependent and requires normal acoustic experience for its maturation.