Search for relationships between functional near-infrared spectroscopy indices and electroencephalography indices: experimental study

INTRODUCTION . Originally developed functional near-infrared spectroscopy (fNIRS) technology for clinical monitoring of cortical tissue oxygenation has not been widely used in research practice. The combination of functional near-infrared spectroscopy and electroencephalography (EEG) provides a unique opportunity for multimodal visualization of human brain function on several spatial and time scales. OBJECTIVE . Determine correlation between standardized fNIRS indices and EEG parameters. MATERIALS AND METHODS. The study involved 100 clinically healthy men and women aged 18-25 of the Caucasian race, native residents of the Republic of Crimea, Volgograd and Arkhangelsk regions. Each subject underwent fNIRS- and EEG-testing consecutively. Statistical data analysis was performed by the Pearson correlation coefficient RESULTS . At the stage of taking background rates with eyes open and the ones with eyes closed before photostimulation, multiple direct strong correlations of EEG electrical potential with the index of oxygenated (HbO) and inverse strong correlations with the index of deoxygenated (HbR) hemoglobin were revealed. At the stage of photostimulation with the frequency of 5Hz, an inverse correlation was observed with respect to the stages of taking background rates and the ones with eyes closed before the start of photosimulation. Many inverse strong correlations of EEG electric potential with HbO index and direct strong correlations with HbR index were detected. The stage of photostimulation with a frequency of 10 Hz was characterized by the minimum number of correlations of the analyzed parameters. At the stage of photostimulation with a frequency of 15 Hz, strong inverse correlations of EEG electric potential with HbO and direct strong correlations with HbR were revealed. DISCUSSION . The oxygen content in the blood vessels of the brain at rest directly correlates wirh the amplitude of its electrical activity. Given that a state of rest on the EEG is characterized by the dominance of relatively high amplitude and low-frequency alpha activity, this kind of correlation is quite logical. It can be assumed that the brain transition into the state of quiet wakefulness and the mode of waiting for stimulation, which is accompanied by a slowing down of the general rhythm on the EEG and an increase in its absolute amplitude, is associated with a general decrease in the activity of neuronal ensembles, which is expressed in a decrease in the level of metabolism and oxygen consumption by tissues. In this case, a large proportion of hemoglobin remains in the oxygenated form, and the proportion of HbR is relatively small. Photostimulation at frequencies of 5, 10 and 15 Hz leads to changes in the pattern of correlations between fNIRS and EEG indices. This applies to both the number and the nature of statistically significant correlations detected. The least number of correlations was observed during stimulation at 10 Hz, while the general revealed pattern held true: HbO concentration correlated directly with EEG amplitude, and HbR concentration – inversely. In terms of functioning of the brain rhythmogenic structures, photostimulation at a frequency of 10 Hz is the most neutral as it coincides with the adult’s dominant resting rate (alpha rhythm with a frequency of 10 ± 1 Hz). Thus, in can be assumed that the imposition of an external rhythm with a frequency close to the natural frequencies of rhythmogenic structures leads to a decrease in the rigidity of correlations between the electrical activities of the bran and its oxygen supply. CONCLUSION . The obtained results confirm the prospect of further research on correlations between fNIRS and EEG indices, providing the possibility of multimodal visualization of brain functions under experimental conditions and clinical practice.