Functional near-infrared spectroscopy reveals heterogeneous patterns of language lateralization over frontopolar cortex

"Functional near-infrared spectroscopy (fNIRS), a non-invasive neuroimaging optical technique which measures the cortical concentration changes in oxygenated and deoxygenated hemoglobin (O2Hb and HHb, respectively), has been extensively utilized in language studies. Most of these studies investigated the ventrolateral\/dorsolateral cortex responses, while no studies on the frontopolar cortex, which is involved in higher level of control, are reported. The aim of this study was to investigate by fNIRS the frontopolar cortex response to a letter verbal fluency task (VFT) in single healthy subjects to better understand the symmetry\/asymmetry of language processing. The O2Hb and HHb changes were measured on 33 right-handed University students by an 8-channel fNIRS system. A significant increase in O2Hb (p<0.001), accompanied by a smaller significant decrease in HHb (p<0.001), was observed in all the measurement points of both hemispheres in response to the VFT. No significant main effect for hemisphere and channel, and hemisphere x channel interaction were found. However, the laterality index of 21 out of the 33 subjects evidenced a hemispheric dominance (right 9, left 12). Although these results have confirmed a bilateral activation over the frontopolar cortex upon VFT, no clear pattern of lateralization was found. Considering the importance to establish a response pattern related to cognitive functions in healthy and clinical populations, the fNIRS monitoring of all areas involved in language and the use of the laterality index are recommended for identifying the hemispheric dominance."