**Optimal Vibrotactile Stimulation Activates the Parasympathetic Nervous System**

Nelcy Hisao Hiraba, Motoharu Inoue, Takako Sato, Satoshi Nishimura, Masaru Yamaoka, Takaya Shimano, Ryuichi Sampei, Katuko Ebihara, Hisako Ishii and Koichiro Ueda

Additional information is available at the end of the chapter

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

**1. Introduction**

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We currently live in an environment that has become more and more stressful. Escaping from the stress in society through various activities (e.g., acupuncture, massage, listening to classic music or natural sounds, etc.) is important for our mental health. We previously re‐ ported that optimal facial vibrotactile stimulation (i.e., 89 Hz frequency and 1.9 μm ampli‐ tude [89 Hz-S]) might activate the parasympathetic nervous system (Hiraba et al. 2008, 2011). Specifically, we showed that 89 Hz-S stimulation of the face led to increased saliva‐ tion and a feeling of mental well-being through parasympathetic activity based on function‐ al near-infrared spectroscopy (fNIRS) oxyhaemoglobin (oxyHb) activity. Namely, brain blood flow (BBF) oxyHb in the frontal cortex was near zero (Hiraba et al. 2011). We investi‐ gated adaptation to the continuous use of vibrotactile stimuli for 4 or 5 days in the same subjects to determine whether this resulted in decreased salivation (Despopoulos and Silber‐ nagel, 2003; Principles of Neural Science, 2000a). Then, we compared resting and stimulated salivation and investigated the most effective frequency for increasing salivary secretion. In‐ creased salivation in normal subjects was defined as a difference between resting and stimu‐ lated salivation (Hiraba et al. 2011).

Furthermore, to study the mechanism of increased salivation evoked by vibrotactile stimula‐ tion, we recorded changes in heartbeat frequency and pupillary reflex during stimulation. We reported that pulse frequency changes during vibrotactile stimulation. A decrease in pulse frequency and a contraction in pupil diameter suggest parasympathetic activity (Prin‐

© 2012 Hiraba et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Hiraba et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 network (Principles of Neural Science, 2000b).

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 temperature-

Optimal Vibrotactile Stimulation Activates the Parasympathetic Nervous System

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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

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‐

controlled, quiet room, as described in previous papers (Hiraba et al. 2008, 2011).

**2.3. HRV analysis during vibrotactile stimulation**

**2.4. Pupillography during vibrotactile stimulation**

this experiment.

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 stimulation led to mental stability due to parasympathetic activity.
