A HRP qualitative and quantitative study of the intra-hemispheric connections of kittens

In post-natal kittens which were given injections of HRP in the suprasylvian gyrus (polysensory cortex), the cortico-cortical connections were studied qualitatively and quantitatively. These connections originate essentially from cortical cells localized in cortices of the upper part of the hemisphere. They are formed preferentially with cortices involved in the visual function. Connections between the cingulate, prefrontal and area 19 cortices, have been described here. Layers III and V are quantitatively the most highly involved in these connections. The number of connections is related to axon length: the connections are more numerous the shorter the neuron axons, and thus the nearer they lie to the cortex injected. The HRP+ cells appear first in the cell layers which will subsequently be most marked quantitatively. The cortico-cortical connections appear in the following order: connections of medium length, then short, then long. The system of cortico-cortical connections of the injected cortex forms chronologically during the first 3 weeks following birth in the following order: 1st week: Connections of primary nuclei to primary cortex, 2nd week: Connections of non-primary cortex to non-primary cortex, 3rd week: Connections of primary cortex to non-primary cortex.