###### Abstract  
Previous studies have indicated that changes in oxygenated hemoglobin concentration (O\({}_{2}\)Hb) in the prefrontal cortex (PFC) are associated with changes in cognitive function. Therefore, the present study aimed to explore the effect of differences in O\({}_{2}\)Hb levels in the left and right PFC (L-PFC and R-PFC, respectively) on cognitive function after exercise. This study included 12 healthy male college students. The exercise regimen consisted of 4 min of warm-up and rest each, followed by 20 min of moderate-intensity exercise and 20 min of post-exercise rest. Participants underwent the 2-back cognitive test thrice (pre-exercise, post-exercise, and after the 20 min post-exercise rest period), and their reaction times were recorded. O\({}_{2}\)Hb levels in the PFC were monitored using functional near-infrared spectroscopy. We analyzed the correlations between changes in post-exercise reaction times and differences in peak O\({}_{2}\)Hb levels (L-PFC minus R-PFC), area under the curve for O\({}_{2}\)Hb changes, and increases in the O\({}_{2}\)Hb slope during exercise. Peak O\({}_{2}\)Hb, area under the curve (AUC) for O\({}_{2}\)Hb change, and increase in the slope of O\({}_{2}\)Hb were significantly correlated with changes in reaction time. These findings provide insight into the mechanism by which O\({}_{2}\)Hb differences between the L-PFC and R-PFC affect cognitive function. a +
Footnote †: journal: Accepted: 6 February 2021  
energy stores. During exercise, the cardiovascular system is challenged by the need to increase the blood supply to the muscles due to a surge in their activity while maintaining adequate blood supply to vital organs such as the heart, lungs and, most importantly, the brain. Therefore, it is necessary to understand changes in cerebral blood flow during exercise to ensure normal brain function and sustain life. Functional near-infrared spectroscopy (fNIRS) facilitates continuous and noninvasive monitoring of changes in blood flow in various areas of the brain via near-infrared reflection [16]. When the nerve is excited, the capillaries expand to increase cerebral hemoglobin content, in order to meet the energy consumption requirements during excitation [17]. Therefore, changes in neural excitability can be understood by observing changes in cerebral hemoglobin content in the relevant brain areas [18]. In addition, fNIRS is more convenient and efficient than traditional functional magnetic resonance imaging with respect to imaging time and patient position, as the latter requires the patient to remain in the supine position for an extended period of time [19]. Numerous sports medicine and brain science studies have utilized fNIRS because it allows for continuous monitoring in multiple positions [20; 21; 22]. The research in 2010 showed that the cognitive function in acute moderate-intensity exercise (50% peak VO\({}_{2}\)) significantly improved after 15 min of exercise compared with that before exercise [23]. However, this study did not confirm when the improvement of cognitive function began after exercise. Therefore, we set up cognitive tasks before exercise, immediately after exercise, and after exercise rest. Moreover, most studies focus on the changes in cognitive function before and after exercise, and the changes in cerebral blood flow during exercise [24; 25; 26]. However, little attention has been paid to the changes in cognitive function after exercise, which is very important for the treatment of the cognitive disorder. Therefore, we use the difference in cognitive tasks performance immediately after exercise and after exercise rest to express the changes in cognitive function during the rest period after exercise. To explore the relationship between cerebral blood flow changes during exercise and cognitive function changes after exercise rest. Several studies have used PFC activation to represent cognitive function [27]. However, other studies have demonstrated differences in the functions of the left and right PFC (L-PFC and R-PFC, respectively).