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.