Neural connections of the “visual wulst” of the avian telencephalon. Experimental studies in the pigeon (<i>Columba livia</i>) and owl (<i>Speotyto cunicularia</i>)

Abstract Retinal fibers in both the pigeon and owl terminate in a multi‐nucleate complex of the dorsal thalamus, including the nuclei lateralis anterior, dorsolateralis‐anterior, dorsolateralis anterior, pars lateralis et pars magnocellularis, and collectively designated the nucleus opticus principalis thalami (OPT). Efferent projections of OPT were traced with the Fink‐Heimer method into the ipsilateral lateral forebrain bundle, and via the dorsal supraoptic decussation, into the contralateral lateral forebrain bundle. OPT projections terminate within an elevation, or “Wulst” on the dorsum of the telencephalon. The Wulst is a multilaminate structure containing a deep lying layer of large cells the hyper‐striatum dorsale (HD), a dispersed cell layer — the hyperstriatum intercalatus suprema (HISm), a granule cell layer or nucleus intercalatus hyperstriatum accessorium (IHA), an overlying hyperstriatum accessorium (HA) consisting of a broad layer of medium sized neurons, and an overlying fibro‐molecular layer. Each of these laminae are particularly well developed in the owl, where the granule cell layer is divisible into inner and outer bands (IHAex and IHAint). The projections of OPT terminate in the HD, HISm and IHA. A homotopic projection was also found in the contralateral Wulst. The pattern of termination was similar in both the pigeon and the owl, though the pattern of distribution was more apparent in the owl with its massive OPT and Wulst. Medial, nonvisual, thalamic cell groups in the pigeon (nuclei dorsolateralis pars medialis and dorso‐medialis anterior) also project bilaterally upon the Wulst, but terminate in a more medial, nonvisual and cytologically different, portion of HD. The projections of the medial thalamic nuclei did not overlap with those of OPT and appear to be a separate functional system of still undetermined nature. Efferent axons of the “visual Wulst” of the pigeon and owl project upon the ipsilateral lateral hyperstriatum ventrale, neostriatum and upon the peri‐ectostriatal belt (Karten and Hodos, '70). Extratelencephalic projections via the septomesencephalic tract (TSM) terminate in OPT, the internal lamina of the ventral lateral geniculate nucleus (LGv), pretectal nuclei and optic tectum. A small contingent of fibers of the TSM cross to the opposite side in the dorsal supraoptic decussation, to terminate in LGv, and, in the owl, in the contralateral ventromedial tectum. Dense terminal degeneration has also been observed in the deeper layers of the ipsilateral optic tectum of pigeon whereas in the owl the projection of the Wulst also extends to the more superficial layers of the tectum, and appears to be topographically arranged. The numerous similarities between the system described above and the geniculostriate visual pathways of mammals seems apparent. These findings clearly indicate that the geniculo‐striate type of system may attain elaborate degrees of development in nonmammalian as well as mammalian brains.