Isocortex has been classified according to the input it receives from the thalamus. When a sector of isocortex receives fibers from a thalamic “relay” nucleus that, in turn, receives its major afferents from outside the thalamus, the sector is called extrinsic. When a sector of isocortex receives fibers from a thalamic nucleus that receives no such extrathalamic afferents, that cortex is classified as intrinsic. (66 pp.)