PURPOSE: We report the results of 75 intracortical electrical stimulations of the insular cortex performed in 14 patients during stereo-electroencephalography (SEEG) investigation of drug-resistant partial epilepsy. The insular cortex was investigated on electroclinical arguments suggesting the possibility of a perisylvian spread or a rapid multilobar diffusion of the discharges during video EEG. METHODS: In these 14 patients, 27 stereotactically implanted transopercular electrodes reached the insular cortex (11 the right insula, 16 the left insula). Square pulses of current were applied between the two deepest adjacent contacts of each transopercular electrode using low (1 Hz) or high-frequency (50 Hz) stimulation. Only symptoms evoked in the absence of afterdischarges were analyzed. RESULTS: Clinical responses were evoked in 10 of the 14 patients (in 20 of the 27 insular sites) and showed a clear topographic specificity inside the insular cortex. Viscerosensitive and visceromotor responses, similar to those evoked by temporomesial stimulation, were evoked by anterior insular stimulation and somesthetic sensation, similar to those evoked by opercular cortex stimulation, by posterior insular stimulation. CONCLUSIONS: The topographic organization of the induced responses within the insular cortex suggest that two different cortical networks, a visceral network extending to the temporomesial structures and a somesthetic network reaching the opercular cortex, are disturbed with stimulation of the anterior or the posterior insula, respectively. Thus ictal symptoms associated with the spread of the epileptic discharges to the insular cortex might be difficult to distinguish from those usually reported during temporomesial or opercular discharges.