Abstract
The aim of this study was to investigate the effects of inner and heard speech on cerebral hemodynamics and oxygenation in the anterior prefrontal cortex (PFC) using functional near-infrared spectroscopy and to test whether potential effects were caused by alterations in the arterial carbon dioxide pressure (PaCO2). Twenty-nine healthy adult volunteers performed six different tasks of inner and heard speech according to a randomized crossover design. During the tasks, we generally found a decrease in PaCO2 (only for inner speech), tissue oxygen saturation (StO2), oxyhemoglobin ([O2Hb]), total hemoglobin ([tHb]) concentration and an increase in deoxyhemoglobin concentration ([HHb]). Furthermore, we found significant relations between changes in [O2Hb], [HHb], [tHb], or StO2 and the participants' age, the baseline PETCO2, or certain speech tasks. We conclude that changes in breathing during the tasks led to lower PaCO2 (hypocapnia) for inner speech. During heard speech, no significant changes in PaCO2 occurred, but the decreases in StO2, [O2Hb], and [tHb] suggest that changes in PaCO2 were also involved here. Different verse types (hexameter and alliteration) led to different changes in [tHb], implying different brain activations. In conclusion, StO2, [O2Hb], [HHb], and [tHb] are affected by interplay of both PaCO2 reactivity and functional brain activity.
Highlights
Previous studies of guided rhythmic speech exercises in the context of arts speech therapy (AST) on human physiology showed that AST tasks, i.e., recitations, are associated with characteristic changes in variations in the heart rate[1,2] and cardiorespiratory interaction
In order to explain the observed cerebral hemodynamic and oxygenation changes obtained from the studies involving speech tasks, two major physiological processes should be considered:[5] neurovascular coupling (NC) and CO2 reactivity (CO2R)
NC occurs due to increased neuronal activity leading to an increase in the cerebral metabolic rate of O2, resulting in an increase in cerebral blood flow (CBF) and volume (CBV).[29,30]
Summary
Previous studies of guided rhythmic speech exercises in the context of arts speech therapy (AST) on human physiology showed that AST tasks, i.e., recitations, are associated with characteristic changes in variations in the heart rate[1,2] and cardiorespiratory interaction. Effects on cerebral and systemic changes of hemodynamics and oxygenation were investigated by our research group using functional near-infrared spectroscopy (fNIRS).[3,4] A decrease in cerebral hemodynamics and oxygenation was found to occur during speech exercises, which was hypothesized to be a result of a decrease in the partial pressure of carbon dioxide (CO2) in the arterial blood (PaCO2) during speaking This hypothesis was confirmed in a subsequent study combining fNIRS and capnography,[5] where we observed changes in end-tidal CO2 (PETCO2), a reliable and accurate estimate of PaCO2,6–8 during all speech tasks and even during the control task (mental arithmetic). This led us to conclude that the changes in hemodynamics and oxygenation are a combination of two factors: (1) a hypercapnia mediated by changes in breathing (hyperventilation) during the tasks and (2) changes in brain activity (neurovascular coupling), where previously the first factor was probably stronger than the second one.
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