## 4 Discussion
Our results also showed a significant increase in prefrontal activity during difficult dual-task. However, this change was found during a simple postural task (static standing posture), suggesting that the brain activity increase can be independent of the levels of difficulty of the postural task and be more related to cognitive demands. The total excursion, displacements of the CoP in medial-lateral and anterior-posterior, mean total displacement velocity, mean displacement velocity of CoP in medial-lateral and anterior-posterior directions, and 95% confidence ellipse sway area were negatively affected during easy and difficult dual-task performance compared to the postural task. Our study is in line with previous research. For example, a study showed that the center of pressure path length, 90% confidence area, and maximum CoP speed were significantly affected by the use of different smartphone functions (talking, texting, and sending a text message on the smartphone) in young adults [31]. A recent systematic review and meta-analysis about the effect of cognitive task complexity on dual-task postural stability suggest that the cognitive task complexity cannot determine a positive or negative change in postural stability during quiet standing in healthy young adults [42]. The outcomes analyzed in this review included the center of pressure sway area, sway velocity, sway variability, total sway path length, and sway frequency, but not including hemodynamic response in the prefrontal cortex analysis. We also used similar CoP variables to analyze postural stability during dual-task. In the difficult dual-task compared to postural task, our results showed an increase in brain activity in the prefrontal cortex and postural instability, and an increase in cognitive performance, demonstrating an increase in attentional resource competition among cognitive and postural tasks; and that postural control is not an automatic process. Contrarily, most studies included in recent systematic review and meta-analysis referred to non-significant changes in cognitive performance during dual-task in static standing postural and reported that postural instability occurs when postural tasks are more challenging [42]. Another previous study has also shown that young adults under dual-task conditions increased their cognitive performance. However, in oxyhemoglobin concentration and CoP sway path (total, AP, and ML) did not find changes from the single-task (standing) to dual-task in young adults [24]. The differences in cognitive demands during the postural task may explain the inconsistency between these and our results. The bottleneck theory can explain the postural stability deterioration during the difficult dual-task performance due to the need to share the same neural or cognitive resources. On the other hand, the capacity sharing theory can explain the interference between cognitive and postural tasks because there was an increase in cognitive performance and a decline in postural performance, possibly because both require common limited resources [3; 33]. This study's essential strong point was to evaluate the differential effects of cognitive tasks with different difficulty levels while performing postural standing simultaneously by analyzing the center of pressure, hemodynamic response in the prefrontal cortex, and cognitive-motor dual-task interference. Regarding the level of cognitive task difficulty, the choice of tasks proved to be adequate since the young adults had a significantly better cognitive performance in the easy cognitive task than in the difficult cognitive task in both single and dual-task conditions. In our study, the postural task was performed without a smartphone, based on previous studies [31; 43; 44]. However, some studies reported an increase in postural instability due to head position in the frontal plane [45; 46]; for that reason, we recommend postural analysis in following studies and the addition of a single-task in which the participants hold the smartphone when standing.