Right and left eye bands in frogs with unilateral tectal ablations.

Surgical ablation of a single tectal lobe in Rana pipiens can cause regenerating retinal ganglion cell axons to cross to the remaining tectum. These synaptically deprived fibers can obtain termination space in a retinotopic and highly stereotyped manner. Each of the two eyes can share the undisturbed tectum by terminating in mutually exclusive, eye-specific stripes that alternate across the medial-lateral extent of the tectal lobe. Invading axons from the ipsilateral eye must actively displace established synapses from the contralateral eye in order to form these exclusive termination zones because the normal projection to the intact tectum is not severed in these experiments. In animals in which a large proportion of anomalous fibers do not reach the undisturbed tectum, only a few ipsilateral eye bands are observed. Nevertheless, these bands have the same width, periodicity, and orientation as those observed in fully banded preparations. When ipsilateral eye terminal density is extremely low, banding is absent. The completely striped termination pattern of unitectal animals is identical to the pattern previously reported in the dually innervated tecta of three-eyed R. pipiens. We theorize that this pattern results from a compromise between two synaptogenic forces that are active in regeneration as well as in development.