Lateral view of the deep insular region.  Located deep to the lateral fissure of Sylvius and insular cortex is a trio of thin structures.  From lateral to medial, these are the extreme capsule, the claustrum, and the external capsule.  Fibers in the extreme capsule course longitudinally and connects mainly the superior temporal gyrus with the lateral frontal cortices. The claustrum is a thin sheet of gray matter with obscure functions; it is sandwiched in between the extreme capsule laterally and the external capsule medially.  The external capsule lies medial to the claustrum and lateral to the putamen; it contains corticocortical association fibers that interconnect the frontoparietal operculum with other parietal cortical areas.  The ventral region of the external capsule also contains fibers of the uncinate fasciculus and the inferior fronto-occipital fasciculus (Image courtesy of AL Rhoton, Jr.)