Evidence for plasticity of intertectal neuronal connections in adult Xenopus.

Each optic tectum in Xenopus receives two visuotectal projections of the binocular portion of the visual world, one from each eye. These two visuotectal projections at each optic tectum are superimposed and in register, an arrangement made possible by the functional organization of a topographically ordered system of intertectal neuronal connections. Following surgical rotation of a larval eye in Xenopus a rearrangement of intertectal connections may take place during metamorphosis. The nature of the intertectal reorganization is such that the two visuotectal projections at each optic tectum remain in register. This synaptic reorganization requires visual experience. The capacity of the intertectal system to respond to such changes in interocular geometry reduces with age but even in adult life a residual plasticity of intertectal connections exists. In the adult animal an acute rotation of one eye by 90 $^\circ$ evokes no reorganization of intertectal connections. An equivalent cumulative change in interocular relationship in adult life which is achieved very gradually rather than acutely can, however, lead to an appropriate reorganization of intertectal connections. It is suggested that the normal role of this residual intertectal plasticity is to compensate for the normal changes in interocular geometry that occur with growth in the post-metamorphic animal.