Organization of the visual pathways in the newborn kitten.

We have used retrograde and anterograde transport to examine the major visual pathways in newborn kittens. Retinal projections from both eyes to the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) are present and topographically organized. The dLGN projects topographically to areas 17 and 18 and receives reciprocal projections from cells in layer VI of areas 17, 18, 19 and suprasylvian cortex on the same side of the brain. Area 19 also has a sparse thalamic input but probably not from the dLGN. The laminar distribution of [3H]proline transported from dLGN to area 17 was quantified: label was spread through all layers, with a minimum at the border of layers I/II. Layer I was always labelled less heavily than IV. These results are critically compared with those based on other tracing techniques. Cells of layer V in areas 17, 18, 19 and the suprasylvian cortex project topographically to the superficial layers of the ipsilateral SC. Area 19 and the lateral suprasylvian cortex also send a crossed projection to restricted parts of the opposite SC. Thus these visual projections are not only present and topographically ordered on the day of birth, but, unlike certain highly exuberant interhemispheric and cortico-cortical projections, they are qualitatively remarkably mature, some days before the onset of visual activity. The major subcortical projections to and from the visual cortex appear to be constructed without the benefit of visual experience and much of the activity-dependent plasticity of cortical cells may well involve only local modulation of synaptic input.