Reduction of early thalamic input alters adult corticocortical connectivity.

The functional specificity of mammalian isocortex requires that precise connections be established between cortical areas and their targets. While recent studies of cortical development have focused on intrinsic specification, the role of extrinsic factors has received considerably less attention. In the present study, we examined how early removal of thalamic input affects the development of visual corticocortical connections. Hamster pups received ablations of visual thalamic nuclei on the day of birth. At 30 days of age, an injection of horseradish peroxidase (HRP) was placed into the area of cortex deafferented by the early thalamic ablation to retrogradely label adult corticocortical connections. Ablated animals displayed a significant increase in the number of corticocortical connections compared to control animals. The increased connectivity in ablated animals was primarily due to a significant increase in the number of corticocortical projections arising from non-visual areas. These results demonstrate that an intact thalamocortical projection is necessary for the development of normal cortical connectivity.