Changes in the uncrossed retinotectal projection after removal of the other eye at birth

THE connections between the eyes and the brain are so arranged as to give an orderly representation in the brain, of the visual field. The mechanisms involved in this ordering have been extensively studied, particularly in the retinotectal pathway of submammalian vertebrates1–3. In these species the optic nerves decussate almost completely, retinal ganglion cells in one eye connecting only with the opposite tectum. In mammals, however, there is partial decussation at the optic chiasma, so that the superior colliculus receives optic fibres from both eyes. Binocular representation poses a problem for the ordering of retinotectal projections. If one point on the tectum is to correspond to only one point in visual space, seen through both eyes, the mapping rules onto the tectum must have opposing polarities in the two eyes, along the nasotemporal axis. This is shown in Fig. 1 : a rostral movement in the colliculus corresponds to a temporal movement on the contralateral retina but a nasal movement on the ipsilateral retina. Little is known about the factors determining the topography of the direct ipsilateral projection to the mammalian superior colliculus. Anatomical studies in rodents have shown that neonatal removal of one eye induces an increased projection from the remaining eye to the ipsilateral colliculus4,5. These aberrant uncrossed optic fibres are derived from all regions of the retina and apparently have a distribution appropriate for a normal contralateral projection, that is, the nasotemporal retinal axis is represented caudorostrally6. The only published physiological study investigating the topography (in enucleated rats) confirmed the anatomy as regards the nasotemporal axis but, surprisingly, found the dorsoventral axis reversed over most of the colliculus ipsilateral to the remaining eye7. Here I present results on the enucleated hamster showing that the colliculus ipsilateral to the remaining eye contains a double representation of the visual field displaying mirror-image polarity along the nasotemporal axis.