Changing patterns of binocular visual connections in the intertectal system during development of the frog, Xenopus laevis. I. Normal maturational changes in response to changing binocular geometry.

During metamorphic and post-metamorphic life in the frog. Xenopus laevis, growth-related changes in skull shape produce radical alterations in the spatial relationship between the two eyes. These changes in binocular visual geometry were measured using optical techniques. Between the onset of metamorphic climax at stage 60 and adulthood (2 or more years post-metamorphosis) each eye migrates nasally by 55 degrees and dorsally by 50 degrees with respect to the major body axes of the animal. As a result the nasotemporal extent of the binocular visual field increases from 30 degrees to 162 degrees between these ages. Electrophysiological methods were used to determine changes in the neural representation of the binocular visual field at the paired midbrain optic tecta and in the tectal projection of pairs of corresponding retinal loci at various developmental points between these ages. The proportion of each tectal surface devoted to the representation of the binocular visual field increases from 11% at stage 60 to 77% at adulthood. Retinal correspondence, and hence the tectal projection of corresponding retinal loci, undergoes radical alteration during this period. In normal adults an intertectal system of connections selectively links the tectal projection of corresponding retinal loci and thus provides a neuronal mechanism for integrating binocular visual information in the optic tecta. Electrophysiological methods were used to determine how the intertectal system accommodates the developmental challenge posed by the enlarging binocular visual field and changing retinal correspondence. Between stage 60 and adulthood the ipsilateral visuotectal projection which is the product of the intertectal system, increases in size as the binocular visual field and its tectal representation enlarges. Moreover, throughout this period, it provides a mechanism for integrating binocular visual information in the optic tecta by maintaining its spatial registration with the contralateral visuotectal projection from the other eye. Analysis of the pattern of functional intertectal connections reveals that during the course of normal maturation this system undergoes continuous processes of expansion and of orderly and major remodelling.