A compartment-based, asymmetric representation of the retina in an induced projection to the olfactory cortex

Displacing the optic nerve into the telencephalon in adult Rana pipiens induces a projection to olfactory cortex. We have examined the topographic organization of this projection anatomically by injecting a mixture of biotin dextran (BDA) with 3H-amino acids into the affected eye immediately after making cuts across defined sectors of the nerve fiber layer to trace the complementary patterns of anterograde migration of BDA and 3H label in the cut and intact retinal axons, respectively. Fibers from the temporal side of the optic disc terminated in an oblique band along the posterior two-thirds or more of the ectopic projection field. In contrast, fibers arising in the nasal retina terminated in a parallel strip occupying the anterior one-third or less of the field. Varying the location of the cuts within each hemiretina did not reveal any further organization along the nasotemporal or dorsoventral axes of the retina. The retinal location of the cells involved in this projection was further studied with injections of wheat germ agglutinin conjugated to horseradish peroxidase into the olfactory cortex. Ganglion cells labeled by retrograde transport were found throughout the retina, but they were much more numerous on the temporal side, having a mean spatial density 3.7-7.4 times greater in the temporal hemiretina, whereas the overall ganglion cell density (labeled plus unlabeled) was roughly the same in the two halves of the retina. These data provide an example of a permanent projection in which the overall representation of the retina, though nontopological, is polarized in one axis (nasotemporal) and, therefore, compartmentally organized.