Further study of the aberrant optic nerve projection to olfactory cortex

When implanted into the cerebral hemisphere, the regenerating optic nerve of the adult frog (Rana pipiens) forms a well-defined terminal field in the pars ventralis of the lateral (olfactory) cortex, and sometimes expands medially into the postolfactory eminence. These adjacent areas receive their normal input from the main olfactory bulb. The aberrant projection extends caudally toward the core neuropil of the medial amygdaloid nucleus, which receives its normal input from the accessory olfactory bulb, but does not enter this vomeronasal sector of the amygdala. The present study tests whether: 1) optic fibers would innervate the vomeronasal amygdala after surgical ablation of the accessory olfactory bulb, 2) the projection would transpose into adjacent cortex after olfactory cortex lesions, and 3) the projection would overflow into adjacent areas after being amplified by hemisection at the di-telencephalic junction (to minimize escape of fibers into the diencephalon). The retinal projection always terminated in the olfactory cortex when this area was intact, or in spared fragments of it after radical cortical lesions, but never entered the vomeronasal amygdala in any specimen, as studied by autoradiographic and horseradish peroxidase tracing techniques. With forebrain hemisection, the cortical terminal field increased in thickness but remained confined to the olfactory area. However, the interruption of the lateral forebrain bundle induced a new projection to the striatum in a region neighboring but separate from the olfactory cortical field. These findings support the hypothesis that retinal fibers have a specific affinity for primary olfactory cortex that is not normally allowed expression in development. Retinal fibers may also have a latent affinity for the striatum that is unmasked after deafferentation.