## 3 Results
### Hemodynamic Changes in the Prefrontal Cortex  
The changes in hemoglobin concentrations (oxy-Hb, deoxy-Hb and total-Hb) in the prefrontal cortex during the postural task and dual-task (easy and difficult) performance are presented in Figure 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Outcomes** & **Single-Task** & **Easy DT** & **Difficult DT** & \(p\)**-Value 1** \\ \hline TOTEX CoP & 2428.4 (2194.1–2873.0) & 2635.6 (2311.7–3033.2) & 2610.2 (2411.9–3123.8) & \textless{}0.001 * \\ CoP-AP & 1837.5 (1648.6–2186.1) & 1960.6 (1779.8–2309.5) & 2028.2 (1817.7–2338.2) & \textless{}0.001 * \\ CoP-ML & 1221.0 (1075.9–1427.9) & 1319.9 (1170.1–1529.2) & 1282.2 (1204.1–1497.6) & \textless{}0.001 * \\ CEA & 224.6 (150.7–425.6) & 724.1 (236.2–1303.7) & 674.5 (326.2–1786.8) & \textless{}0.001 * \\ MVELO CoP & 485.7 (438.9–574.7) & 527.2 (462.4–606.7) & 522.1 (482.4–624.8) & \textless{}0.001 * \\ MVELO CoP-AP & 367.5 (329.7–437.2) & 392.1 (356.0–461.9) & 405.7 (363.6–467.7) & \textless{}0.001 * \\ MVELO CoP-ML & 244.2 (215.2–285.6) & 264.0 (234.0–305.9) & 256.5 (240.8–299.5) & \textless{}0.001 * \\ \hline \hline \end{tabular}  
* TOTEX CoP, total excursion of the center of pressure (mm); CoP-AP, displacement of the center of pressure in anterior–posterior direction (mm); CoP-ML, displacement medial–lateral direction (mm); CEA, 95% confidence ellipse sway area (mm\({}^{2}\)); MVELO CoP, mean total velocity displacement of CoP (mm/s); MVELO CoP-AP, mean velocity displacement matrix–posterior of CoP (mm/s); MVELO CoP-ML, mean velocity displacement medial–lateral of CoP (mm/s); ST, single-task; DT, dual-task. 1 Friedman test.* \(p<0.05\).  
\end{table}
Table 2: Comparisons of CoP behavior among the postural task (ST), easy and difficult dual-task, median (IQR).  
Figure 3: Changes in hemoglobin concentrations in the prefrontal cortex during the postural task and dual-task (easy and difficult) performance. The y-axis displays relative concentration (median values and standard error (error bars)) changes of hemoglobin (Hb in \(\mu\) mol/L). The x-axis displays tasks performance: postural task (single-task), easy DT (easy dual-task), difficult DT (difficult dual-task). The oxyhemoglobin concentration, [oxy-Hb], is indicated by the red line, the deoxyhemoglobin concentration, [deoxy-Hb], by the blue line, and total hemoglobin concentration, [total-Hb], by the grey line. * \(p<0.05\) in changes [Oxy-Hb], [deoxy-Hb] and [total-Hb] between the difficult dual-task and postural task (Friedman test with Bonferroni correction).  
The oxy-Hb concentration increased from the postural task to both dual-task conditions and from easy dual-task to the difficult dual-task (\(p\) = 0.032), although hemodynamic changes for oxy-Hb values were only observed between the postural task and difficult dual-task (\(p\) = 0.026).  
For deoxy-Hb values, there were significant differences between the postural task and dual-task with two different challenge levels (\(p\) = 0.001). However, the post hoc analyses showed a significant difference only between the postural task and difficult dual-task (\(p\) = 0.001).  
There were significant differences in the total-Hb between the postural task and both dual-tasks (\(p\) < 0.001). The post hoc analyses showed a significant difference between the postural task and easy dual-task (\(p\) = 0.026), and the postural task and difficult dual-task (\(p\) < 0.001). However, no significant differences were found between the easy and difficult dual-task (\(p\) = 0.167) in total-Hb.