Effects of neonatal enucleation on the functional organization of the superior colliculus in the golden hamster.

Abstract

The responses of visual, auditory and somatosensory superior collicular neurons were investigated using extracellular single unit recording techniques in hamsters which were subjected to the removal of 1 eye on the day of birth. Neonatal enucleation resulted in a marked increase in the region of the colliculus where visual neurons activated by stimulation of the ipsilateral eye could be recorded. In most cases the visuotopic representation in the colliculus ipsilateral to the remaining eye mirrored the contralateral tectum along the rostrocaudal and mediolateral axes: in the colliculi temporal retina projected rostrally and inferior retina projected medially. In some animals a dual mapping of the remaining eye onto the ipsilateral tectum appeared. In these hamsters the central portion of the visual field was represented 2 .times. along the rostrocaudal axis of colliculus. No changes in the topography of the somatosensory and auditory representations in the tectum were observed following neonatal enucleation. The laminar distribution of visual neurons in the ipsilateral colliculus was markedly altered in the neonatally enucleated hamsters. Very few exclusively visual units were encountered in the layers ventral to the stratum opticum and almost all of the visual cells recorded in the ipsilateral colliculus were isolated within 150 .mu.m of the tectal surface. In the posterior half of the ipsilateral tectum a large number of extravisually responsive cells were encountered in the stratum griseum superficial and stratum opticum. In the colliculus contralateral to the remaining eye and in normal hamsters the cells are not encountered. Recordings from animals subjected to neonatal enucleation and acute bilateral removal of somatosensory and auditory cortex indicated the projections from these areas to the colliculus were not essential to the changes in laminar organization. Recordings from normally reared hamsters subjected to removal of 1 eye at the time of the recording experiment suggested the isolation of extravisual cells in the superficial tectal lamina of the neonatal enucleates was probably not the result of the unmasking of extravisual influences in the superficial layers. These influences are present, but ineffective, in the normal case.