Functional neuroanatomy of the insular lobe

The insula is the fifth lobe of the brain and it is the least known. Hidden under the temporal, frontal and parietal opercula, as well as under dense arterial and venous vessels, its accessibility is particularly restricted. Functional data on this region in humans, therefore, are scarce and the existing evidence makes conclusions on its functional and somatotopic organization difficult. 5 patients with intractable epilepsy underwent an invasive presurgical evaluation with implantation of diagnostic invasive-depth electrodes, including insular electrodes that were inserted using a mesiocaudodorsal to laterorostroventral approach. Altogether 113 contacts were found to be in the insula and were stimulated with alternating currents during preoperative monitoring. Different viscerosensitive and somatosensory phenomena were elicited by stimulation of these electrodes. A relatively high density of electrode contacts enabled us to delineate several functionally distinct areas within the insula. We found somatosensory symptoms to be restricted to the posterior insula and a subgroup of warmth or painful sensations in the dorsal posterior insula. Viscerosensory symptoms were elicited by more anterior electrode contacts with a subgroup of gustatory symptoms occurring after stimulation of electrode contacts in the central part of the insula. The anterior insula did not show reproducible responses to stimulation. In line with previous studies, we found evidence for somato- and viscerosensory cortex in the insula. In addition, our results suggest that there is a predominantly posterior and central distribution of these functions in the insular lobe.