Specificity and plasticity of retinotectal connections: a computational model.

A computational model is presented which simulates the development and regeneration of orderly connections between retinal fibers and tectal cells in frogs and goldfish. The model distinguishes two aspects of retinotectal connectivity: (1) the contact adhesion between retinal fibers and tectal cells as mediated by fixed chemospecific markers and (2) the formation of modifiable synapses between them. Chemospecificity is assumed to be an intrinsic property of both the retina and tectum and is modeled as a graded distribution of a binding determinant or marker. Synapse formation depends upon the timing of neural activity as well as on the intinsic chemospecificity of retinotectal contacts. In addition to the normal development and regeneration of the retinotectal map, the model simulates the compressed, expanded, translocated, and rotated maps that have been found in surgically manipulated contexts. There examples of plasticity in the retinotectal map can be simulated without assuming any changes in the marker distributions. Moreover, the model demonstrates that a very shallow gradient of a single marker suffices to organize retinotectal connections in a variety of contexts.