Topography of the retinal projection to the superficial pretectal parvicellular nucleus of goldfish: A cobaltous‐lysine study

The retinal projection to the superficial pretectal parvicellular nucleus (SPp) of goldfish was examined by filling select groups of optic axons with cobaltous‐lysine. The tracer was applied intraocularly to peripheral retinal slits in some fish. In other fish, it was applied to optic axons from an intact hemiretina after one‐half of the retina was ablated and the corresponding optic axons had degenerated. The results indicated that SPp is a folded structure, having a dorsal surface innervated by axons from temporal retinal ganglion cells and a ventral surface innervated by axons from nasal retinal ganglion cells. Peripheral retina innervates the anterodorsal and anteroventral edges of SPp, while central retina innervates the posterior genu. Dorsal retina innervates lateral SPp and ventral retina innervates medial SPp. Thus, although SPp is a folded nucleus, the topography of the retino‐SPp projection is similar to the topography of the retinotectal projection. That is, the relative position of optic axons within SPp mirrors the retinal location of the ganglion cells that project to SPp. Retino‐SPp axons occupy the center of the main optic tract before it divides into the two optic brachia. These axons are topographically arranged, with temporal retino‐SPp axons being flanked on both sides by nasal retino‐SPp axons. Retino‐SPp axons arborize within SPp and then continue to enter the superficial tectal retinorecipient lamina. Thus, these axons innervate both SPp and the optic tectum. These findings are discussed with respect to chemospecific and morphogenetic views of visual system topography.