Brain Research, 204 (1981) 403-409 403 © Elsevier/North-Holland Biomedical Press Short Communications A projection of displaced ganglion cells and giant ganglion cells to the accessory optic nuclei in turtle ANTON REINER Department of Neurobiology and Behavior, S U N Y at Stony Brook, Stony Brook, N. Y. 11794 (U.S.A.) (Accepted August 14th, 1980) Key words: turtles - - displaced ganglion cells - - giant ganglion cells - - accessory optic system - - optomotor function The retinal projection to the accessory optic nuclei (AON) of turtles was found to arise from a distinctive set of giant ganglion cells whose dendrites ramify widely in outer portions of the inner plexiform layer. The majority (80%) of these cells had their perikaryon located in the ganglion cell layer, though displaced ganglion cells (DGCs) were also observed. In contrast, the retinal projection to the avian AON has been reported to arise exclusively from DGCs. The possible involvement of the accessory optic system in optomotor reflexes such as visual field stabilization (nystagmus) 1,8,12,17 has resulted in considerable recent interest in the connections of the accessory optic nuclei (AON) of the rostral midbrain of vertebrates. Karten and his coworkers have demonstrated that the AON of birds, reptiles and fish project directly to the vestibulo-cerebellum, terminating as a mossy fiber system2-4,7,15 A direct projection from the AON to the vestibulocerebellum has also been described in mammals zo. In birds and mammals a climbing fiber input from the AON to the vestibulocerebellum via the inferior olive has been described4,19, In addition to the projections of the AON upon the vestibulocerebellum, a direct projection from the AON to the oculomotor complex has been described in birds and teleostean fish 4,7. The most extensive studies on the source of retinal afferents to the AON have been carried out in birds and show that, at least in birds, the cells of origin for the retinal projection to the AON are a morphologically unique population of retinal ganglion cells, the displaced ganglion cells of Dogiel6,10, ~6. In reptiles, the nucleus of the basal optic root (nBOR) is the major component of the accessory optic nuclei. Located at the basolateral margin of the tegmentum, slightly rostral to the exit of the third cranial nerve, the nBOR receives the terminations of a discrete fascicle of large diameter optic tract axons, the basal optic root (BOR). Reiner and Karten 15 have previously shown that nBOR in the turtle pro- jects to the caudal cerebellum (vest!bulocerebellum). The present research sought to establish the ganglion cell type (or types) that project to nBOR of turtles. Eastern painted turtles (Chrysemys picta picta) and red-eared turtles (Chrysemys scripta) were used as subjects. While under ketamine anesthesia, turtles were secured