The evolution of the retinotectal map during development in Xenopus

The retinotectal projection was mapped eleetrophysiologically in tadpoles of Xenopus laevis. Recording was performed with the animals immersed in saline inside a transparent hemisphere. Visual responses could be recorded from the optic tectum from about stage 43 onwards. The visual map on the tectum of the tadpole was found to differ in several respects from that in the adult. The earliest responses recorded showed very large multi-unit receptive fields and no discernible retinotopic organization. From about stage 47 the map showed the adult type of order; nasal field projected rostrally, temporal field caudally, superior field medially and inferior field laterally. However, in tadpoles up to stage 63/64 the projection was markedly distorted in that nasal field was confined to the most rostral region of the tectum and theie was an expanded representation of the temporal pole of the field. In tadpoles the entire visual projection covers only the rostral one-half to two-thirds of the tectum. These results, in conjunction with the results of previous studies on the mode of growth of the retina and tectum, indicate a progressive shift of the retinotectal projection with development which may involve changing synaptic relations between retinal fibres and tectal cells.

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