Retinotopic analysis of fiber pathways is amphibians. I. The adult newt Cynops pyrrhogaster.

The possibility that pathways of retinal fibers within the optic tract and the tectum of the adult newt are retinotopic was examined by selective labeling of the retinal fibers with horseradish peroxidase. Within the optic tract fibers from the ventral, temporal and dorsal retinal quadrants were ordered from the dorsal to ventral edges of th optic tract. The nasal retinal fibers exhibited two different pathways. The fibers from the dorsonasal retina ran along the ventral edge of the optic tract, while the fibers from the ventronasal retina ran along the dorsal edge of the optic tract. Segregation of pathways within the optic tract was incomplete between the nasal and other retinal fibers. The dorsonasal retinal fibers were mixed completely with the dorsal retinal fibers, and the ventronasal retinal fibers were mixed partly with the ventral retinal fibers. Both the dorsal and dorsonasal retinal fibers preferentially entered the lateral tract, and finally projected onto the ventrolateral parts of the middle tectum and of the caudal tectum, respectively. The ventral and ventronasal retinal fibers entered the dorsomedial tract, and projected onto the dorsomedial parts of the middle tectum and of the caudal tectum, respectively. The temporal retinal fibers invaded the nasal tectum directly. Most dorsal, ventral, and nasal retinal fibers ran along the sub-tracts as far as to the level of their terminals, then sharply turned in a direction to the tectum.