The Effect of Inner Speech on Arterial CO2 and Cerebral Hemodynamics and Oxygenation: A Functional NIRS Study

Abstract

The aim of the present study was (i) to investigate the effect of inner speech on cerebral hemodynamics and oxygenation, and (ii) to analyze if these changes could be the result of alternations of the arterial carbon dioxide pressure (PaCO2). To this end, in seven adult volunteers, we measured changes of cerebral absolute [O2Hb], [HHb], [tHb] concentrations and tissue oxygen saturation (StO2) (over the left and right anterior prefrontal cortex (PFC)), as well as changes in end-tidal CO2 (PETCO2), a reliable and accurate estimate of PaCO2. Each subject performed three different tasks (inner recitation of hexameter (IRH) or prose (IRP) verses) and a control task (mental arithmetic (MA)) on different days according to a randomized crossover design. Statistical analysis was applied to the differences between pre-baseline, two tasks, and four post-baseline periods. The two brain hemispheres and three tasks were tested separately. During the tasks, we found (i) PETCO2 decreased significantly (p < 0.05) during the IRH ( 3 mmHg) and MA ( 0.5 mmHg) task. (ii) [O2Hb] and StO2 decreased significantly during IRH ( 1.5 M; 2 %), IRP ( 1 M; 1.5 %), and MA ( 1 M; 1.5 %) tasks. During the post-baseline period, [O2Hb] and [tHb] of the left PFC decreased significantly after the IRP and MA task ( 1 M and 2 M, respectively). In conclusion, the study showed that inner speech affects PaCO2, probably due to changes in respiration. Although a decrease in PaCO2 is causing cerebral vasoconstriction and could potentially explain the decreases of [O2Hb] and StO2 during inner speech, the changes in PaCO2 were significantly different between the three tasks (no change in PaCO2 for MA) but led to very similar changes in [O2Hb] and StO2. Thus, the cerebral changes cannot solely be explained by PaCO2. Abstract: The aim of the present study was (i) to investigate the effect of inner speech on cerebral hemodynamics and oxygenation, and (ii) to analyze if these changes could be the result of alternations of the arterial carbon dioxide pressure (Pa-CO2). To this end, in seven adult volunteers, we measured changes of cerebral absolute [O2Hb], [HHb], [tHb] concentrations and tissue oxygen saturation (StO2) (over the left and right anterior prefrontal cortex (PFC)), as well as changes in end-tidal CO2 (PETCO2), a reliable and accurate estimate of PaCO2. Each subject performed three different tasks (inner recitation of hexameter (IRH) or prose (IRP) verses) and a control task (mental arithmetic (MA)) on different days according to a randomized crossover design. Statistical analysis was applied to the differences between pre-baseline, two tasks, and four postbaseline periods. The two brain hemispheres and three tasks were tested separately. During the tasks, we found (i) PETCO2 decreased significantly (p < 0.05) during the IRH (~3 mmHg) and MA (~0.5 mmHg) task. (ii) [O2Hb] and StO2 decreased significantly during IRH (~1.5 μM; ~2 %), IRP (~1 μM; ~1.5 %), and MA (~1 μM; ~1.5 %) tasks. During the post-baseline period, [O2Hb] and [tHb] of t...

Cerebral hemodynamic and oxygenation changes induced by inner and heard speech: a study combining functional near-infrared spectroscopy and capnography

“…As already indicated in Ref. 9, it is reasonable that the different speech tasks are associated with different characteristics of brain activity. It is, for example, known that mainly stress [35][36][37] and specific types of cognitive processes (particularly memory retrieval and multitasking) 38 are modulating factors for the activity of the PFC.…”

“…In a following study, 9 we demonstrated that even inner speech, i.e., speech not spoken aloud, leads to significant changes in P ET CO 2 as well as cerebral hemodynamics and oxygenation. Table 1 gives an overview of previous study results.…”

“…We demonstrated that inner speech (recitation of hexameter and prose) induces a CO 2 R associated with characteristic changes in cerebral hemodynamics and oxygenation. 9 The nonlinearity between P ET CO 2 changes and changes in the fNIRS parameters was also observed and attributed to a task-dependent intensity of the brain activity and thus NC. In an fNIRS study involving reading aloud as a task, Fallgatter et al 40 observed a decrease in ½O 2 Hb and an increase in [HHb], which is in line with our observation.…”

“…Additionally, data of the present and two previous own studies with equal design 5,9 were combined. To test for significant relations between changes in cerebral hemodynamics and oxygenation and the different speech tasks, linear regression analyses were calculated with a stepwise procedure for each Figure 3(a) shows the changes in P ET CO 2 for all tasks.…”

A New Approach for Automatic Removal of Movement Artifacts in Near-Infrared Spectroscopy Time Series by Means of Acceleration Data

“…Although the light coupling is canceled by the employed self-calibrating algorithm [44], under certain circumstances, light coupling changes still have a small influence on [O2Hb] and [HHb]. On the other hand, such drifts may also reflect real changes in the brain oxygenation, for example caused by transient changes in breathing and/or arterial CO2 [39,68,69], but it is most likely a combination of all these factors. AMARA could have been implemented in a way to eliminate any drifts occurring during a few minutes (e.g., 2 min) after an artifact-but also real signal changes would have been suppressed.…”