The Critical Period for Experience-dependent Plasticity in a System of Binocular Visual Connections in Xenopus laevis: Its Extension by Dark-rearing

Following surgical rotation of an eye, the Xenopus‘intertectal’ system is capable of a vision‐dependent alteration of its connectivity, that restores spatial registration of binocular maps on the optic tectum. In the preceding paper (Keating and Grant, Eur. J. Neurosci., 4, 27–36, 1992), we reported that this capacity undergoes a progressive, age‐dependent restriction during a critical period around the time of metamorphosis, so that rotations produced in animals aged 3 months postmetamorphosis normally evoke no such alteration of the system. Here we examine whether this restriction is rigidly age‐dependent or whether vision can influence its profile. We report that in animals dark‐reared from embryonic stage 35 through the critical period to 3 months, 1 year or even 2 years after metamorphosis, rotations instituted at those ages now result in intertectal reorganization if a period of normal vision is allowed after the operation. Similarly, intertectal alteration was also seen in animals eye‐rotated at larval stage 58, then dark‐reared just for the duration of the critical period, and subsequently returned, at 3 months of age, to a normal visual environment. We conclude, therefore, that the normal developmental restriction in the plasticity of the Xenopus intertectal system is not strictly age‐dependent, but that vision contributes to the process by activating the underlying plasticity mechanisms.