Plasticity in the chick ventral lateral geniculate nucleus.

The chick ventral lateral geniculate nucleus (GLv) receives topographically corresponding projections from the retina and optic tectum. Tectal lesions produced on the day of hatching removed the tectogeniculate input to the GLv region corresponding to the tectal lesion and also severed some retinotectal axons. Following a survival period of 3 to 10 weeks, a patch of augmented retinogeniculate projection was noted in the GLv segment that corresponds topographically to the damaged area of the tectum. Changing the site of the tectal lesion led to changes in the locus of heavy retinal projection to the GLv predictable from topographic maps. Nuclei which received retinal but not tectal projections did not appear to have regions of augmented retinal termination nor did nuclei which received tectal but not retinal innervation. It is unlikely that the increased retinogeniculate termination is due to rerouting of growing retinotectal axons since the chick retinofugal pathway is well established by the time of hatching. Furthermore, there was no evidence of a projection from the ipsilateral eye to the affected GLv. On the basis of these light microscopic studies, it would appear that retinogeniculate terminals have sprouted in the GLv and that competition for terminal space, conservation of terminal space, proximity, and perhaps other factors are necessary for the augmented projection to occur.