Normal maturation involves systematic changes in binocular visual connections in Xenopus laevis

Systematic changes in neuronal connections have been observed during the development of many vertebrate neuronal systems. These changes have usually involved a refinement from an initial exuberance of connections or a response to some experimental perturbation. Here we report on a system of neuronal connections, which, during a protracted developmental period, undergo ordered changes in response to normally occurring changes in functional requirements. In the frog Xenopus laevis, interocular alignment changes markedly during late larval and post-metamorphic life, producing a progressive enlargement of the binocular portion of the visual field. An intertectal system links the two mid-brain optic tecta and is concerned with the neural representation of binocular visual space. In the adult animal, connections in this system link corresponding points (points receiving information from one locus of binocular visual space) on the two tecta. Changes in eye position with development, however, change the set of corresponding points. Therefore, if the intertectal connections link corresponding tectal points throughout development, they must undergo an ordered change with time. We present electrophysiological evidence that the intertectal connections do, indeed, undergo such changes in response to changes in eye alignment, and that the changes are major.