Transcranial magnetic stimulation for neuromodulation of the operculo‐insular cortex in humans

Abstract

The insula is a cortical structure internally folded within the lateral sulcus, an anatomical border dividing the frontal, parietal and temporal lobes. Portions of these lobes that hide the insula are named as operculum (lid). Specifically, the part of the parietal lobe that forms the ceiling of the lateral sulcus operates as the secondary somatosensory cortex (S2). There is no clear functional distinction between the S2 neuronal population and the posterior insula, and thus, both are jointly characterized as the operculo-insular cortex. The operculo-insular cortex is an integrative structure that detects the intensity of multimodal spinothalamic and, possibly, interoceptive inputs, painful or not. Functional magnetic resonance imaging studies based on cytoarchitectural maps and intracortical recordings suggest an integrated functional unit with subpopulations of neurons in posterior insula primarily processing nociceptive inputs. In turn, subpopulations of neurons in S2 preferentially discriminate the nature of non-nociceptive inputs (Mazzola et al., 2012). Importantly, the operculo-insular cortex seems to be an epileptic focus in specific types of seizure, and lesions in this region correlated with the occurrence of a distinct central, neuropathic pain syndrome (Garcia-Larrea & Mauguière, 2018). The functional characterization of the human operculo-insular cortex relies on electrophysiological recordings, intraoperative stimulation, pathological conditions assessment, and functional neuroimaging (Garcia-Larrea & Mauguière, 2018). Notwithstanding, the specific functional role of operculo-insular cortex neuronal populations in the processing of each submodality of somesthetic sensation remains elusive. Recently, transcranial magnetic stimulation (TMS) was proposed as a promising technique for neuromodulation and assessment of the operculo-insular cortex (Lenoir et al., 2018).