Local and global patterns during morphogenesis of the retinotectal topographical mapping in the vertebrate brain

The highly ordered neuronal projections from the retina to the tectum mesen cephali (optic tectum) in several vertebrate groups have been intensively s tudied. Several hypotheses so far have been proposed, suggesting mechanisms to explain the topographical and biochemical specificity of the retinotect al projections during ontogeny. In the present paper we compare the main hy potheses of retinotectal development with respect to the nature of specific ity envisaged, the activity-dependence versus inheritance criterium and the strategy of argument, in casu the descriptive versus interferential type o f argument. Matching of the current developmental hypotheses and mechanisms for elimination or persistance of contralateral, respectively ipsilateral connections, is attempted with respect to the known neuroanatomical connect ivity data in the amphibian optic tectum and with respect to the symmetry r elations between ipsilateral and contralateral connection patterns describe d in amphibians and other vertebrate groups. Local mechanisms influencing t he survival potential of synaptic contacts between retinal afferents and te ctal neurons are probably essential for generating global symmetry patterns . Finally, a global topological approach is discussed with respect to its a pplicability in the amphibian retinotectal projection system. Basic assumpt ions of topological modeling appear to rely on anatomical and functional pr operties of the visual system like left-right symmetry, dichotomy, (absence of) convergence and segregation of fibers and neurons into columnar inform ation units. It is concluded however, that more neuroanatomical, physiologi cal and ultrastructural data are needed to establish a formalized operation model of the amphibian retinotectal system.