The insula is the region at the lateral aspect of the hemisphere that lags behind during development and becomes covered by the more rapidly growing adjacent regions of the hemisphere. The parts of the hemisphere overlapping the insula are called opercula. They are named according to thecerebral lobe they belong to: the frontaloperculum (A1), the parietal operculum (A2),and the temporal operculum (A3). In diagram A, the opercula have been moved apart toexpose the insula. They normally leave only a cleft, the lateral cerebral sulcus (fissure ofSylvius, p. 10, A4), which widens over the in-sula into the lateral fossa. The insula has roughly the shape of a triangle and is bordered at its three sides by the circular sulcus of the insula (A4). Thecentralsulcus of the insula (A5) divides the insulainto a rostral and a caudal part. At its lower pole, the limen of insula (A6), the insular re-gion merges into the olfactory area, the paleocortex.
The insular cortex represents a transitionalregion between paleocortex and neocortex.The lower pole of the insula is occupied by the prepiriform area (B7) (blue) which belongs to the paleocortex. The upper part of the insula is covered by the isocortex (neo-cortex; see p. 244) (B8) (yellow) with the fa-miliar six layers. Between both parts lies a transitional region, the mesocor-tex (proisocortex, see p. 244) (B9) (hatchedarea). Unlike the paleocortex, it has six lay-ers; however, these are only poorly developed as compared to the neocortex. The fifth layer (C10) is characteristic for the mesocortex by standing out as a distinct narrow, dark stripe in the cortical band. It contains small pyramidal cells that are densely packed like palisades, a feature otherwise found only in the cortex of the cingulate gyrus.
Stimulation responses (D).Stimulation ofthe insular cortex is difficult because of the hidden position of the region; it has been carried out in humans during surgical treat-ment of some specific forms of epilepsy. Itcaused an increase (+) or decrease ( – ) in the peristaltic movement of the stomach. Nausea and vomiting (ø) were induced at some stimulation sites, while sensations in the upper abdomen or stomach region (x) or in the lower abdomen (o) were produced at other sites. At several stimulation sites, taste sensations were induced (∆). Al-though the stimulation chart does not show a topical organization of these effects, the results do indicate viscerosensory andvisceromotor functions of the insular cortex.Experiments with monkeys yielded not only salivation but also motor responses in the muscles of the face and the limbs. In humans, surgical removal of the insular re-gion does not lead to any functional losses.
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