Telencephalic projections of the nucleus rotundus in the pigeon (<i>Columba livia</i>)

Abstract The nucleus rotundus receives a massive ascending projection from the optic tectum. Stereotaxic lesions were placed in the nucleus rotundus and its efferent projections studied by means of the Nauta‐Gygax and Fink‐Heimer methods for degenerating axons and terminals. Efferent rotundal axons form the lateral part of the fasciculus prosencephali lateralis (FPL), pass through the lateral portion of the paleostriatum primitivum (PP) and augmentatum (PA) and enter the overlying ectostriatum. The ectostriatum was found to consist of at least two distinct cytologic regions: a central core (E) containing larger and more scattered cells and a peripheral belt with smaller, more densely packed neurons (Ep). Rotundal efferents terminate on the full extent of the central core of larger neurons. No evidence was found of any significant projection to the overlying periectostriatal belt or to the surrounding neostriatum. All portions of the central core of the ectostriatum receive projections from the nucleus rotundus. This projection is topologically organized on a rostro‐caudal axis relative to the nucleus rotundus. There was no indication of the existence of inter‐rotundal, rotundo‐tectal or rotundus to contralateral ectostriatal projections. These findings demonstrate the existence of a massive ascending visual pathway to the telencephalon, arising from the optic tectum and terminating in a cytologically distinct projection field within the telencephalon. The relationship of this pathway to known visual pathways in mammals and reptiles is discussed.