End-tidal CO2: An important parameter for a correct interpretation in functional brain studies using speech tasks

► Effects of PaCO2 in functional brain studies have as yet been neglected. ► PaCO2 changed significantly and substantially during speech exercises. ► Inverse changes in blood circulation and oxygenation correlated to PaCO2. ► Previously inverse fNIRS/negative BOLD signals had not been attributed to PaCO2. ► PaCO2 is still underestimated as a potential confounder in functional brain studies.