The ascending auditory pathway in the pigeon (Columba livia). II. Telencephalic projections of the nucleus ovoidalis thalami.

Summary 1. The nucleus ovoidalis of the pigeon is a major diencephalic terminus of the brachium of the inferior colliculus (nucleus mesencephali lateralis, pars dorsalis). 2. The fiber degenerations following stereotaxic lesions of the nucleus ovoidalis were studied with the Nauta-Gygax and Fink-Heimer methods for degenerating axons and boutons terminaux. 3. Efferent axons of the nucleus ovoidalis form the medial portion of the fasciculus prosencephali lateralis and pass through the medial part of the paleostriatum augmentatum to enter the overlying capsula occipitalis interna (CIO). 4. Some scattered ovoidalis axons terminate on the intercalated neurons of the CIO, and in the medial caudal neostriatum. By far the major distribution of the ovoidal projection, however, takes place in a sharply demarcated region of the medial caudal neostriatum, consisting of small, densely packed neurons. This region corresponds to Field L of Rose. 5. This study, taken in conjunction with an earlier report on the projections of the avian inferior colliculus, indicates the existence of a discrete and sharply segregated auditory projection pathway leading from the inferior colliculus over the thalamic nucleus ovoidalis to the forebrain. 6. The relationships between avian and mammalian ascending auditory pathways are discussed.

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