Biomarker of Lateralization Index for Stress Calculated from Hemodynamic Responses of fNIRS for Subdividing between Eustress and Distress

While performing various mental stress tasks, we can easily observe differences in brain activations between the stress group and the control group (i.e., non-stress group). Moreover,it is essential to subdivide the stress groups into eustress and distress groups in order to enhance personal cognitive abilities and mental/physical health. However, it is very challenging to subdivide stress group, because of the difficulty to reveal differences in brain activations between two stress groups. In our study, we aim to ascertain the necessity of subdividing two stress groups screened by salivary alpha-amylase (sAA). Furthermore, we develop a potential biomarker to subdivide two stress groups using hemodynamic responses of functional near-infrared spectroscopy (fNIRS). We stimulate two stress groups divided by sAA with an international affective picture system (IAPS) inducing positive or negative emotions and measure hemodynamic responses at the same time. As a result, we have developed a newly designed biomarker, lateralization index for stress (LIS) calculated from hemodynamic responses, resulting in separating the two stress groups. The two stress groups show distinct LIS values in the visually dominant positive session. Therefore, using fNIRS, we, for the first time to the best of our knowledge, figure out that the differences in brain activations among three groups (control vs. eustress vs. distress groups) are distinguishable.