## 4 Discussion  
The aim of this study was to evaluate and compare the CoP behavior and the hemodynamics response of the prefrontal cortex during dual-task performances of increasing levels of cognitive difficulty using the smartphone in young adults. The CoP impairments (postural instability) and the activation of the PFC were increased with the more demanding cognitive task during the dual-tasks performances compared to the postural task. In the cognitive-motor dual-task interference analysis by DTC, young adults showed a pattern of cognitive priority trade-off [39] with improvements in cognitive task performance and deterioration in all COP parameters during both dual-task conditions (easy and difficult) compared to postural and cognitive single-tasks. Greater center of pressure sway was observed in more challenging conditions (dual-task) than the postural task, showing that the young adults prioritized the concurrent task (cognitive task) under dual-task conditions. Furthermore, performing a concurrent cognitive task (easy and difficult) in static standing posture negatively affected postural control, and the differences in the CoP variables were significant between the postural task and the easy dual-task and the postural task and the difficult dual-task. However, the increase in cognitive load was not reflected in a significant difference in postural control between the dual-tasks with different demanding levels (easy DT versus difficult DT). Furthermore, the DTC for each CoP variable showed that the difficult cognitive task had slightly more interference in postural stability deterioration than the easy cognitive task; however, this difference was not significant. Another indicator that young adults prioritized the cognitive task over postural control was the percentage of change in cognitive task performance from cognitive single-task to dual-task. The accuracy of correct answers was higher during dual-task than cognitive single-task. However, the increase in the accuracy of correct answers was only significant between the difficult cognitive single-task and difficult dual-task. The present study demonstrated that the oxy-Hb, deoxy-Hb, and total-Hb concentrations during the difficult dual-task performance were higher than in the easy dual-taskand postural task (single-task). However, only the oxy-Hb and deoxy-Hb concentrations differed significantly across the postural task and difficult dual-task. On the other hand, significant differences were found in total-Hb concentrations between the easy dual-task and postural task, and the difficult dual-task and postural task. The hemodynamic response usually reflects an increase in [Oxy-Hb] and a decrease in [deoxy-Hb]. The increase in the oxyhemoglobin concentration is related to increased cerebral blood volume in response to cortical activation; but is more confounded with physiological factors (e.g., heart rate, respiration); and the deoxyhemoglobin is more robust to systemic changes [40]. Our results showed changes in [Oxy-Hb], [deoxy-Hb] and [total-Hb] during the difficult dual-task compared to postural task, demonstrating an increase in neural activity, possibly due to the higher load of the cognitive task. To our knowledge, we did not find other studies that report this difference in all these parameters using the fNIR device, especially during the simple postural task (static standing postural). A study demonstrated an increase in the brain activity in the high working memory span group during dual-task compared to the low working memory span group. The authors suggested that in the low working memory span group, the changes in the brain activity may have been difficult to detect due to low working memory capacity and the postural task to be more challenging (one leg standing) [25]. Another study showed that the frontal brain activation during the dual-task (walking while performing a cognitive task) was associated with the cognitive load during gait and not a response to verbalizing words [41].