**2. Material and Methods**

## **2.1. Vibrotactile stimulation apparatus**

The vibrotactile stimulation apparatus consisted of an oscillating body and a control unit, as shown in Hiraba et al. (2008) and Yamaoka et al. (2007). The oscillating body was composed of the following two parts: (1) a headphone headset equipped with vibrators in the positions of the bilateral microphones and (2) a vibration electric motor (VEM; Rekishin Japan Co., LE12AOG) covered in silicon rubber (polyethyl methacrylate, dental mucosa protective ma‐ terial; Shyofu Co.) for conglobating the stimulation parts and preventing VEM warming due to long periods of vibration (Hiraba et al. 2008).

We examined the amount of salivation during vibrotactile stimulation on the bilateral mass‐ eter muscle belly (on the parotid glands) and on bilateral parts of the submandibular angle (on the submandibular glands). We determined the amount of salivation using a dental cot‐ ton roll (1 cm width, 3 cm length) positioned at the opening of the secretory ducts (i.e., the right and left parotid glands and right and left submandibular and sublingual glands) dur‐ ing vibrotactile stimulation of the bilateral parotid and submandibular glands. The weights of the wet cotton rolls after 3 min of use were compared with their dry weights measured previously (Hiraba et al. 2008).

## **2.2. Stimulating salivation and frontal cortex BBF**

We determined that a 3-min salivation measurement with a 5-min recovery time was suffi‐ cient from a previous experiment (Hiraba et al. 2008, Hiraba et al. 2011). Furthermore, saliva‐ tion is most effectively induced by vibrotactile parotid gland stimulation at 89 Hz-S, which was used in this experiment. We examined adaptation to vibrotactile stimulation by monitor‐ ing changes in salivation during 4 or 5 continuous days using the same time schedule (i.e., 89 Hz-S). Frontal cortex recordings were acquired using a fNIRS OEG16 instrument (Spectra‐ tech, Inc., Shelton, CT, USA) during vibrotactile stimulation. We conducted salivation tests with 19 normal subjects (six males, 13 females; average age: 22 years) and resting-stimula‐ tion examinations for adaptation with 26 normal subjects (11 males and 15 females; average age: 25 years). We also performed fNIRS in eight normal subjects (six males, two females; average age: 22 years) to examine the effects of resting state and classical music (Mozart, *Eine kleine Nachtmusik*). This experiment was performed between 3 and 5 pm in a temperaturecontrolled, quiet room, as described in previous papers (Hiraba et al. 2008, 2011).

## **2.3. HRV analysis during vibrotactile stimulation**

ciples of Neural Science, 2000b). We believe these reflexes are coordinated by a highly inter‐ connected set of structures in the brainstem and forebrain that form a central autonomic

We found that vibrotactile stimulation increased salivation, as reported by Hiraba et al. (2008). Furthermore, Hiraba et al. (2011) reported that increased salivation due to facial vi‐ brotactile stimulation might be due to parasympathetic stimulation based on frontal cortex BBF measurements. Particularly, vibrotactile stimulation at 89 Hz-S using a single motor was most effective in increasing salivation without adaptation following continuous daily use. We know that autonomic activity changes heart rate and pupil diameter. Thus, we be‐ lieve that heart rate and pupil diameter measurements during 89 Hz-S stimulation represent the effects of the autonomic nervous system. In this study, we demonstrated that 89 Hz-S

The vibrotactile stimulation apparatus consisted of an oscillating body and a control unit, as shown in Hiraba et al. (2008) and Yamaoka et al. (2007). The oscillating body was composed of the following two parts: (1) a headphone headset equipped with vibrators in the positions of the bilateral microphones and (2) a vibration electric motor (VEM; Rekishin Japan Co., LE12AOG) covered in silicon rubber (polyethyl methacrylate, dental mucosa protective ma‐ terial; Shyofu Co.) for conglobating the stimulation parts and preventing VEM warming due

We examined the amount of salivation during vibrotactile stimulation on the bilateral mass‐ eter muscle belly (on the parotid glands) and on bilateral parts of the submandibular angle (on the submandibular glands). We determined the amount of salivation using a dental cot‐ ton roll (1 cm width, 3 cm length) positioned at the opening of the secretory ducts (i.e., the right and left parotid glands and right and left submandibular and sublingual glands) dur‐ ing vibrotactile stimulation of the bilateral parotid and submandibular glands. The weights of the wet cotton rolls after 3 min of use were compared with their dry weights measured

We determined that a 3-min salivation measurement with a 5-min recovery time was suffi‐ cient from a previous experiment (Hiraba et al. 2008, Hiraba et al. 2011). Furthermore, saliva‐ tion is most effectively induced by vibrotactile parotid gland stimulation at 89 Hz-S, which was used in this experiment. We examined adaptation to vibrotactile stimulation by monitor‐ ing changes in salivation during 4 or 5 continuous days using the same time schedule (i.e., 89 Hz-S). Frontal cortex recordings were acquired using a fNIRS OEG16 instrument (Spectra‐ tech, Inc., Shelton, CT, USA) during vibrotactile stimulation. We conducted salivation tests

network (Principles of Neural Science, 2000b).

356 Advances in Vibration Engineering and Structural Dynamics

**2. Material and Methods**

previously (Hiraba et al. 2008).

**2.1. Vibrotactile stimulation apparatus**

to long periods of vibration (Hiraba et al. 2008).

**2.2. Stimulating salivation and frontal cortex BBF**

stimulation led to mental stability due to parasympathetic activity.

We recorded changes in power-spectral analysis of heart rates (HRV; Heart Rate Variability module, AD Instruments, Japan, under the following conditions: (1) resting state; (2) 89 Hz-S stimulation on the face (89 Hz-S face); (3) listening to Mozart (Mozart); (4) Mozart + 89 Hz-S on the face; (5) 89 Hz-S on the nape of the neck (89 Hz-S neck); and (6) listening to noise (Noise), as shown in Figure 2E. A power-spectral analysis of HRV module data was con‐ ducted using the period histogram analysis program based on distribution of the length of the RR interval for 3 min, and typical values during various stimuli were analysed in terms of the highest value (i.e., peak value) during the recording period. For example, Figure 1A shows RR intervals (n1, n2, n3, n4 ms, and so on,) on the electrocardiogram (ECG) during vibrotactile stimulation. Figure 1B shows a peak value example (1000 ms) during vibrotac‐ tile stimulation. Heart rates during rest and during various stimuli were recorded for 3 min, and then analyses of 3-min HRV data were performed off-line. When heart rates were com‐ pared among the rest and various stimulation conditions, we used the RR-interval peak val‐ ue (i.e., 1000 ms in this example) obtained from the power-spectra analysis. We conducted these examinations with 16 normal subjects (11 males, five females; average age: 25 years). This experiment was performed at 3 and 5 pm in a quiet, temperature-controlled room.

**Figure 1.** HRV module analysis. Method used to measure RR intervals (n1, n2, n3, n4, etc.) on ECG recordings (A) and frequency spectrum based on RR interval length over 3 min during 89 Hz-S vibrotactile stimulation (B). Horizontal line indicates RR interval (ms), and vertical line indicates number. Note that the peak frequency spectrum was 1000 ms in this experiment.

### **2.4. Pupillography during vibrotactile stimulation**

IRIS (Iriscorder, Hamamatsu Photonics Co., [Japan]) records transverse diameter and veloci‐ ty reactions and can take a picture of the eyes by illuminating visible light (infrared radia‐ tion). The resulting image can record the condition of the iris and eyeball movement on the monitor. For example, when normal subjects are exposed to continuous light stimulation for 1 sec, we can obtain a pupillogram from the IRIS apparatus; constricted pupils indicate para‐ sympathetic activity, and pupil dilation indicates sympathetic activity. Pupil diameter in normal subjects is generally 2-5 mm, which changes under various adaptation conditions. We examined the transverse diameter and velocity of pupil constriction or dilatation after vibrotactile stimulation to explore changes in autonomic activity.

females; average age: 25 years). This experiment was performed at 3 and 5 pm in a quiet, temperature-controlled room. Furthermore, one parameter was explored each experimental day because we obtained information from adaptation to light stimulation, as shown by the

Optimal Vibrotactile Stimulation Activates the Parasympathetic Nervous System

http://dx.doi.org/10.5772/51156

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We examined differences between vibrotactile stimulation of the parotid and submandibu‐ lar glands. We found that the most effective frequency to induce salivation was 89 Hz-S regardless of whether vibrotactile stimulation was delivered to the parotid or submandibu‐

Because patients with hyposalivation often have psychiatric disorders, we conducted an experiment to realistically approximate natural conditions. We examined whether effective salivation occurred continuously when vibrotactile stimulation was performed daily. Specif‐ ically, we used the 89 Hz-S frequency with a single motor from the previous experiment. None of the glands (i.e., right and left parotid glands or right submandibular and sublingual glands) showed a reduced response. Regression curves for each gland showed non-adaptation to continuous stimulation; instead they showed parallel or increasing curves, indicating that continuous use of this apparatus should not be problematic, as shown in a previous paper

The OEG16 spectroscope was used to record BBF haemoglobin concentration from areas in the frontal cortex using 16 channels. We determined the oxyHb concentration schema evoked by 89 Hz-S vibrotactile stimulation by analysing 16 channels. The results showed very weak oxyHb concentrations (i.e., near zero) during 89 Hz-S vibrotactile stimulation. Changes in oxyHb, deoxyhaemoglobin (deoxyHb), and total haemoglobin (totalHb) concentrations dur‐ ing salivation measurements at rest and for each vibrotactile stimulation frequency were measured. As shown in a previous paper (Hiraba et al. 2011), changes during the following six conditions were measured: (1) resting; (2) 89 Hz-S vibrotactile stimulation; (3) 89 Hz-D (89 Hz-D, 89 Hz frequency with double motors, 3.5-μm amplitude); (4) 114 Hz-S; (5) 114 Hz-D (114 Hz-D, 114 Hz frequency with double motors, 3.5-μm amplitude); and (6) "A-" phona‐ tion. Each wave was recorded for 3 min, and each 2-min vibrotactile stimulus is shown between the vertical lines (Fig. 6B and Fig. 4 in the previous paper, 2011). Although each wave meas‐ ured during resting salivation, at 114 Hz-D, and during "A-" phonation showed increased activity, the 89 Hz-D and 114 Hz-S vibrotactile stimuli decreased activity. However, vibrotac‐ tile stimulation at 89 Hz-S showed a value of almost zero. Particularly, when we focused on oxyHb changes based on these results, increased oxyHb occurred during "A-" phonation, the resting condition, and at 114 Hz-D vibrotactile stimulation, whereas a decrease in oxyHb was observed during vibrotactile stimulation at 114 Hz-S and at 89 Hz-D. However, oxyHb con‐

**3.1. Vibrotactile stimulation of the parotid and submandibular glands**

lar glands, as shown in previous paper (Hiraba et al. 2011).

**3.2. Relationship between stimulation and fNIRS activity**

experimental schedule in Figure 2E.

**3. Results**

(Hiraba et al. 2011).

**Figure 2.** IRIS apparatus (A and B) and typical data acquired following light stimulation (C and D). IRIS records the bilateral pupillary reflex simultaneously. IRIS records the constricted and dilated pupil reflex for one second after light stimula‐ tion. Pupillary reflex parameters (e.g., most constricted pupil velocity) can be quantified. Experimental schedule (E).

Figure 2 shows the IRIS experimental apparatus used in Figures 2A and 2B. The pupillary light-reflex test was executed using an infrared pupillometer (Iriscorder C10641, Hamamat‐ su Photonics Co.), and pupil diameter in both the right and left eyes was measured after 3 min of rest or stimulation. Figure 2E shows the timeline of this experiment. Five particular experimental conditions were explored over 5 days; we recorded HRV modulation during the pupillary reflex adaptation test. However, only the right pupil was exposed to light stimulation, as shown in Figure 4. The pupillary test is non-invasive and enables real-time diagnosis. We examined the initial diameter (D1), minimum diameter (D2), constriction ra‐ tio (CR), time to total construction (T3), maximum velocity of constriction (VC), and maxi‐ mum acceleration of constriction (AC) among the parameters obtained from the IRIS. Pupil diameter decreased with parasympathetic activity and increased with sympathetic activity. The IRIS records pupil parameters of the right and left eyes simultaneously. However, we adopted parameters from the right pupil because data from both sides were similar, as shown in Figure 4. We conducted these examinations with eight normal subjects (six males, two females; average age: 25 years). This experiment was performed at 3 and 5 pm in a quiet, temperature-controlled room. Furthermore, one parameter was explored each experimental day because we obtained information from adaptation to light stimulation, as shown by the experimental schedule in Figure 2E.
