The distribution of the ipsilaterally projecting population of retinofugal axons has been analyzed following injections of horseradish peroxidase (HRP) into the optic tract of adult hamsters and rats to determine whether the topographical segregation of the cells of origin seen in the retina is maintained by their axons throughout the course of the optic nerve and chiasm.
Axons are limited to a roughly appropriate topographic location within the intraorbital course of the nerve but this organization changes a t levels progressively closer to the optic chiasm. Immediately rostra1 to the chiasm labelled profiles are found dispersed across most of the cross-sectional area of the nerve. This dispersal is maintained within the region of the optic chiasm where a complex rearrangement of ipsilaterally projecting axons takes place.
The results show that axons are not retinotopically organized along the entire length of the optic nerve. The order of axons changes along the course of the nerve and in the optic chiasm. The change seen within the intracranial course may indicate a chronotopic re-sorting of axons prior to the optic tract where the organization of axons has previously been interpretted as a map of time of axon arrival.