Ablation behavior experimentation on the hindbrain auditory system has led to the conclusion that the superior olivary complex (or SOC) in the medulla may serve the auditory system as an """"""""acoustic chiasm"""""""". The primary evidence is that unilateral lesions in the auditory pathway anywhere below this structure (that is, in the cochlea, cochlear nucleus, trap]ezoid body) result in sound localization deficits either in both hemifields of auditory space or in the henifield ipsilateral to the lesion. In sharp contrast, unilateral lesions in the pathway anywhere above the SOC (that is, in the lateral lemniscus or beyond) result in sound localization deficits only in the hemifield contralateral to the lesion. Therefore, there is a clear functional parallel between the SOC and the optic chiasm even though there is no obvious anatomical parallel. We propose to study the anatomical basis and functional consequences of this chiasmatic process in a series of tract-tracing and ablation-behavior experiments in cats. Using standard techniques of HRP, HRP-WGA conjugate, fluorescent dye, and tritiated amino-acid autoradiography, we will determine the locations and cell-types originating the afferent supply converging on the binaural nuclei of the SOC and the trajectories and possible collateralization of their axons. We will also determine the distribution, collateralization, and trajectory of the efferents of these nuclei as they ascend to the pontine and midbrain levels. Using rigorous psychophysical techniques for testing sound localization and basic hearing capacities in cats with unilateral lesions in the afferents or efferents of SOC in combination with a cochlear lesion either ipsilateral or contralateral to the CNS lesion, we will study the fractionation of normal hearing capacities as a result of the fractionation of the chiasm in order to express the behavioral consequences of the chiasmatic process both in terms of ears of origin and in terms of hemifields of space. Because the acoustic chiasm is a process that is best defined in ablationn-behavior terms, and because the function of the chiasm obviously depends on its afferent-efferent relationships, we expect both lines of experimentation to aid and guide the other and together, to yield a more coherent understanding of the chiasmatic process than could either line alone.
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