81 J. Cosmet. Sci., 74, 81–91 (March/April 2023) *Address all correspondence to Takayuki Kodama, kodama-t@tachibana-u.ac.jp Effects of Tactile Stimulation by Foams of Different Viscosities on Electroencephalogram Signals From Brain Regions Involved in Emotional Processing AKIO GODA, KAZUYA INOKO, MAYUMI KAGAMI, TAKAYUKI KUMEI, TOMOHIRO UEDA, KANO HIKINO AND TAKAYUKI KODAMA Department of Physical Therapy, Faculty of Health Sciences, Kyoto Tachibana University, Kyoto, Japan (A.G., T.U., K.H., T.K.) FANCL Research Institute, FANCL Corporation, Yokohama, Japan (K.I., M.K.) Accepted for publication July 01, 2023. Synopsis Objective evaluation of the influence of foam-induced tactile stimulation on participants’ emotions is important. This study aimed to quantify neurophysiological indices of the emotional effect of tactile stimulation by foam of different viscosities. Twelve healthy adult females were asked to wash their faces with three types of facial cleansing foams of low, medium, or high viscosity while electroencephalograms of their brain activities were recorded. Salivary cortisol levels were measured before and after face-washing. Foam-mediated tactile stimulation during face-washing increased activity in the orbitofrontal and anterior cingulate cortex. However, the increase in both regions was significantly smaller with the use of high viscosity foam than that with other foam types. With low viscosity foam, the neural activity was significantly higher when the foam was placed on participants’ faces compared to on their hands. Furthermore, the neural activity hemisphericity and the extent of changes in salivary cortisol levels during face-washing indicated that the high viscosity foam elicited more negative emotions than the low viscosity foams. The results suggest that tactile stimulation with foam causes emotional changes in participants according to foam viscosity, with low and medium viscosity foams causing pleasing sensations and high viscosity foams causing distinctly stressful emotions. INTRODUCTION During somatosensation, signals are generated on the dermal surface by touch stimuli. These signals are then transmitted to the central nervous system via peripheral nerves and are processed by neural networks.1 Through a wide range of tactile receptors located on the surface of our bodies, we constantly receive and process information from the external world. Tactile information can cause a myriad of emotional stimulations.2 For example, touching smooth or soft objects often elicits a sense of comfort.3 Studies suggest that the role of tactile information in creating pleasant emotions may be important in designing and developing appropriate stimuli to elicit emotional responses.4

82 JOURNAL OF COSMETIC SCIENCE Somatosensation plays a central role in human physiology,1 psychology,5 and biomedical engineering.6,7 Related research findings have wide application in the rehabilitation field, in innovations in electronic skin and artificial limbs, and in quality assessment of comfort in apparel and cosmetics. Yasasaka et al. used three tactile objects with plastic, cloth, and sandpaper surface materials and reported that differences in the features of the surface materials were associated with differences in affectivity, cognitive processes, and perceived vividness/familiarity.8 Kida and Shinohara examined the effects of tactile stimuli on participants’ emotions using wood, velvet, and a paintbrush.9 Gentle stroking with a soft velvet cloth produced the highest pleasantness ratings and the emotion was accompanied by activity in the frontal polar and orbitofrontal cortex (OFC). Greco et al. reported that the activity of alpha and beta frequency bands in the contralateral prefrontal and median cerebral cortex accompanied pleasant sensations of tactile stimulation.10 However, only a few reports have been published on the emotional response to foam- mediated tactile stimulation. We wash our faces and hands daily and repeatedly experience the foamy sensation of facial cleansers and hand soap on our faces and hands. Improving the emotional evaluation of washing with foam is important to provide a pleasant feeling, and will thereby promote hygiene education. Given that the qualities and properties of foam in facial cleansers and hand soaps can be adjusted by modifying the types and quantities of ingredients, the optimal quality of foam that provides comfort and ease of use has been examined. In recent years, research has focused on foam viscosity.11 It has been reported that a lower viscosity results in a finer foam. However, Ohmura et al. evaluated the perceived quality of foam using only subjective questionnaires.11 Objective evaluation of the influence of foam-induced tactile stimulation on participants’ emotions is an important next step. This study aimed to examine the emotional effect of tactile stimulation by foam with different viscosities through the recording of electroencephalogram (EEG) signals and salivary cortisol levels during the facial cleansing process. We hypothesized that a lower foam viscosity would generate more pleasant emotions, which should be reflected in neurophysiological parameters. METHODS PARTICIPANTS We included 12 healthy female adults (mean age: 20.2 ± 0.7 years) with no history of orthopedic or neurological diseases. The sample size was determined based on previous studies12,13 that conducted spatial analyses of neural activity during stimuli that elicit emotional changes, like those expected in the present study. This study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Review Committee of Kyoto Tachibana University (Approval No. 19-38). This study’s purpose, contents, and procedures were explained to the participants orally and in writing, and informed consent was obtained. SELECTIONS OF EXPERIMENTAL SAMPLES Three types of facial cleansers with different levels of viscosity were used as samples to assess the tactile effect of foam on participants’ emotions. The cleansers were unscented