2.9. Measuring HbO/HbR coupling

To investigate significant frequency‐domain correlation between HbO and HbR, we computed their MSC and CPSD during the baseline and task conditions. In this case, we were only interested in the SS and CS signals because we assume that DS is nothing more than the linear combination of the first two. MSC and CPSD data, as well as their averages and significance thresholds, were obtained by the same procedure previously detailed. In addition, from the complex‐valued CPSD we extracted the phase data (Müller et al., ²⁰⁰³; Reinhard et al., ²⁰⁰⁶) at the task‐frequency to assess the temporal relation (time‐shift) between both chromophores. As usual, this procedure was applied for every single participant. Next, we computed the circular mean of phase angles at the group level and then, to assess whether it was significantly oriented in a preferred direction, we applied a bootstrapped Rayleigh test (Oden, ¹⁹⁸³) through 2000 resamples. The 95% CI was also computed by bootstrapping.