Regarding the results obtained from the present study, the questions arises whether one of the two effects mentioned was prevailing or whether they were caused by a combination of both. The decrease generally found in , , and [tHb] as well as the increase in [HHb] during all six interventions may at first glance be interpreted as if the NC was overpowered by the , i.e., a hyperventilation induced hypocapnia, which causes a cerebral vasoconstriction as the main effect. However, our results show that the relationships between changes in and hemodynamics and oxygenation are not linear [see Fig. 3(b)]. If the hypocapnia was the sole relevant effect, we would expect linear relationships (at least in the first approximation) as demonstrated by other investigations.33 But in the present study, a task evoking a significantly higher change in compared to another task is not accompanied by a larger change in tHb [see Fig. 3(b)]. In addition, the response of the cerebral blood vessels to changes in is known to be robust and much stronger than other physiological parameters, such as oxygenation or blood pressure. Thus, even a small change in , which does not reach significance in our results, may have a relevant and significant effect on cerebral hemodynamics.34 For both reasons, the observed effects in hemodynamics and oxygenation cannot solely be explained by a hypercapnia. Thus, we consider a combination of NC and . The NC characteristics are task-dependent, i.e., they counteract the to different degrees leading to an apparent nonlinearity between and hemodynamics/oxygenation. As already indicated in 9, it is reasonable that the different speech tasks are associated with different characteristics of brain activity. It is, for example, known that mainly stress35–37 and specific types of cognitive processes (particularly memory retrieval and multitasking)38 are modulating factors for the activity of the PFC. Thus, we deem it likely that two overlapping and counterbalancing effects (NC and ) are causal in our study, where the strength of each effect appears to be task-dependent. For inner speech and heard speech (person) of a hexameter, the was quite different, yet the changes in were the same. This indicates that the NC during inner speech was stronger and counteracted the stronger, which therefore resulted in similar changes in . We also would expect that inner speech, which includes hearing and reciting the verses, necessitates a more pronounced effort than simply hearing the verses and, consequently, leads to larger brain activation. This is in line with the results of the measurements, and thus, it appears to be reasonable to assume that the effects elicited by speech cannot be explained by the alone.