Effects of Tongue Pressure on Cerebral Blood Volume Dynamics: A Functional Near-Infrared Spectroscopy Study

Abstract

Tongue pressure measurement (TPM) is an indicator of oral function. However, the association between tongue pressure and cerebral activation remains unclear. We used near-infrared spectroscopy (NIRS) to examine the correlation between cerebral cortex activation and tongue pressure stimulation against the anterior palatal mucosa. We measured voluntary maximum tongue pressure (MTP) using a TPM device; a pressure value of approximately 60% of the MTP was used for the experimental tongue pressure (MTP_60%). We examined the effect of oral functional tongue pressure stimulation against the anterior palatal mucosa on cerebral activation using NIRS in 13 adults. Tongue pressure stimulation caused significant changes in cerebral blood flow in some areas compared with controls (p < 0.05). We performed a correlation analysis (p < 0.05) between MTP_60% and changes in oxygenated hemoglobin in all 47 NIRS channels. MTP_60% triggered activation of the right somatosensory motor area and right dorsolateral prefrontal cortex and deactivation of the anterior prefrontal cortex (APFC). TPM balloon-probe insertion in the oral cavity activated the bilateral somatosensory motor area and deactivated the wide area of the APFC. Moreover, MTP_60% via the TPM balloon probe activated the bilateral somatosensory and motor cortex areas. Tongue pressure stimulation changes cerebral blood flow, and NIRS is useful in investigating the relationship between oral stimulation and brain function.

Keywords: cerebral blood volume; cerebral cortex hemodynamics; near-infrared spectroscopy; tongue pressure measurement.

Figure 1

Task paradigm. The experimental design consisted of four 70 s task blocks. NIRS holder on the head during the four task measurements (top). Schematic depiction of the tongue posture during each task (second line). Images of the control stature, anterior palatal mucosa (white circle), TPM balloon-type probe, and TPM device (third line). The black line indicates sequential 30 s stimulation periods and 40 s rest periods (bottom). Task 1: resting tongue position for control. Task 2: MTP_60% against anterior palatal mucosa; 5 s, three times (open arrow). Task 3: insertion of the TPM probe (gray arrow). Task 4: MTP_60% via the TPM probe; 5 s, three times (solid arrow). NIRS, near-infrared spectroscopy; TPM, tongue pressure measurement, MTP, maximum tongue pressure; MTP_60%, approximately 60% of the MTP for the experimental tongue pressure.

Figure 2

Channel allocation on the cerebral cortex. The encircled number represents near-infrared spectroscopy (NIRS: ETG-7100) 47 channels around the right temporal, frontal, and left temporal regions. PSC, primary sensory cortex; PMC, primary motor cortex; SMG, supramarginal gyrus; STG, superior temporal gyrus; MTG, middle temporal gyrus; DLPFC, dorsolateral prefrontal cortex; APFC, anterior prefrontal cortex; OFC, orbitofrontal cortex. Brain cortex schema was designed based on the report by Tsuzuki, et al. [23].

Figure 3

Typical waveform and topography. The left figures show near-infrared spectroscopy signal-integration waveforms at channel 27 around the left primary motor cortex; oxygenated hemoglobin (Oxy-Hb, red); deoxygenated hemoglobin (Deoxy-Hb, blue); and total hemoglobin (Total-Hb, green). Horizontal axis: time (min). The start of the tasks is indicated by four colors (light green, light yellow, turquoise-blue, and ocher, respectively), while the end of the tasks is indicated by pink. Vertical axis: Oxy-Hb values using arbitrary units (mM–mm). The center topography depict Oxy-Hb activation at the end of the tasks (pink) from overhead view and the range bar (±0.50 mm, mol–mm). (A) Resting tongue position for control. (B) MTP_60% against the anterior palatal mucosa, 5 s, three times (open arrow). (C) Insertion of the TPM probe (gray arrow). (D) MTP_60% via the TPM probe, 5 s, three times (solid arrow). MTP, maximum tongue pressure; MTP_60%, approximately 60% of the MTP for the experimental tongue pressure; TPM, tongue pressure measurement.

Figure 4

Activation analysis. Significant maps were represented by Dunnett’s post hoc test of oxygenated hemoglobin (Oxy-Hb) comparing Tasks 2, 3, and 4 with Task 1 (control task) (n = 13; Dunnett’s test after analysis of variance corrected false discovery rate, p < 0.05). Red-colored circles indicate significant increases compared with Task 1; blue-colored circles indicate significant decreases compared with Task 1. (A) Task 2 vs. Task 1; (B) Task 3 vs. Task 1; (C) Task 4 vs. Task 1.

Figure 5

Correlation analysis. Functional connectivity between MTP_60% values and changes in oxygenated hemoglobin (ΔOxy-Hb). White-encircled numbers represent significant, positively correlated channels (n = 13; p < 0.05). Scatter plots of MTP_60% and ΔOxy-Hg deviation (bottom). MTP_60%, approximately 60% of the MTP for the experimental tongue pressure.