Rearrangements of the retinotectal projection in Rana pipiens after unilateral caudal half-tectum ablation.

When the caudal half-tectum is ablated and the optic nerve is cut in adult Rana pipiens, recordings from optic nerve terminals show that the entire visual field comes to be represented in retinotopic order on the remaining rostral half-tectum (compression). In contrast, if the caudal half-tectum is ablated but the optic nerve is left intact, no compression of the retinotectal projection results. Instead, the terminals displaced from the ablated tissue form a permanently disorganized projection to the rostral half-tectum superimposed on the unaltered representation of those parts of the retina which normally project there. The receptive field locations of tectal neurons in both groups of animals show that the altered retinotectal projections make functional synapses. This conclusion is further supported by behavioral data, which show that the accuracy of prey-catching movements is altered in both groups of frogs. The anomalous retinotectal maps can be interpreted by means of a model which includes polarity cues derived from the tectum and ordering cues derived from repelling interactions among fiber terminals.