Removal of retinal input fails to elicit isthmo-tectal sprouting in the frog.

The retina and nucleus isthmi both project in laminar fashion to the superficial layers of the frog's tectum. In order to determine whether isthmo-tectal axons show collateral sprouting after retinotectal input is removed, we injected [3H] proline into the nucleus isthmi and measured the volume of the crossed isthmo-tectal projection. We found no evidence of collateral sprouting.