Somatotopic organization of pain responses to direct electrical stimulation of the human insular cortex

The question whether pain encoding in the human insula shows some somatotopic organization is still pending. We studied 142 patients undergoing depth stereotactic EEG (SEEG) exploration of the insular cortex for pre-surgical evaluation of epilepsy. 472 insular electrical stimulations were delivered, of which only 49 (10.5%) elicited a painful sensation in 38 patients (27%). Most sites where low intensity electric stimulation produced pain, without after-discharge or concomitant visually detectable change in EEG activity outside the insula, were located in the posterior two thirds of the insula. Pain was located in a body area restricted to face, upper limb or lower limb for 27 stimulations (55%) and affected more than one of these regions for all others. The insular cortex being oriented parallel to the medial sagittal plane we found no significant difference between body segment representations in the medio-lateral axis. Conversely a somatotopic organization of sites where stimulation produced pain was observed along the rostro-caudal and vertical axis of the insula, showing a face representation rostral to those of upper and lower limbs, with an upper limb representation located above that of the lower limb. These data suggest that, in spite of large and often bilateral receptive fields, pain representation shows some degree of somatotopic organization in the human insula.