Laminar patterns of termination of cortico-cortical afferents in the somatosensory system.

Two distinct patterns of terminal labeling were seen after injections of [3H]amino acids into the second somatosensory area (S2) and the retroinsular area (Ri). The first pattern, seen both in S2 after an injection in Ri and in the granular or dysgranular insular fields after an injection in S2, is characterized by heavy labeling in layers IIIb and IV. By contrast, the second pattern, seen in the first somatosensory area and Ri after an injection in S2, is distinguished by heavy labeling of layer I and no labeling in layer IV. A reciprocal relationship between these two laminar patterns has now been seen in the somatosensory, visual and auditory systems.