Mode of termination of retinotectal fibers in macaque monkey: an autoradiographic study

The distribution of retinotectal projections was studied in 4 macaque monkeys by examining the tectum autoradiographically 3-21 days after eye injection with radioactive proline or a proline-fucose mixture. Contrary to previous reports the optic fibers project to all regions of the tectum including a relatively sparse but nevertheless very clear projection to the anterolateral one-third, where the fovea is represented. Here the terminals were distributed within the superficial grey layer of the tectum at a depth extending from about 50 mum to 125 mum and in a patchy fashion, with a tendency to aggregation in clumps 0.1-0.5 mm wide from one or other eye. Further posteromedially, corresponding to more peripheral retinal regions, the input from the contralateral eye became more continuous superficially, with tongues extending more deeply in the superficial grey, apparently enclosing clumps of ipsilateral terminals. These deeper ipsilateral clumps occupied a rather well defined layer extending in depth from about 100 mum to about 175 mum. Still further posteromedially, in the temporal crescent representation, only the continuously distributed label from the contralateral eye was found. Continuous label was also seen in the optic disc region on the ipsilateral side; on the corresponding area contralaterally, label was absent. Both ipsilaterally and contralaterally, the pattern of input was roughly symmetrical about the representation of the horizontal meridian, which ran from anterolateral to posteromedial. The regional aggregations of input from one or other eye were to some extent reflected physiologically in a regional variation in eye dominance, though this was perhaps less than might have been expected from the marked heterogeneity of the inputs.