## 4 Discussion  
In this study, we developed a multimodal stimulator for the study of human tactile perception and cognitive function. The tactile sensations of warmth, vibration, and pressure can be presented individually or simultaneously, and the stimulus parameters of tac-tile stimulus type, intensity, and time can be quantitatively controlled using a microcontroller. In addition, the three stimuli can be presented in the same location, which is the area below the fingers where most sensations are felt in everyday life. From the pilot experiments with human subjects, changes in neuronal activity were confirmed in all types of stimuli. From the evaluation of the actuator, the system showed that the intensity of the warmth sensation ranged from 26 \({}^{\circ}\)C to 36 \({}^{\circ}\)C, the vibration sensation ranged from 0.2 g to 2.1 g, the frequency ranged from 25 Hz to 250 Hz, and the pressure sensation ranged from 0.2 psi to 3.8 psi in five levels. Although fluctuations of up to \(\pm\)0.5 \({}^{\circ}\)C and \(\pm\)0.4 psi occurred for warmth and pressure, respectively, the subjects did not perceive the change. In the case of warmth, the rising time to the maximum temperature was about 2 s and the falling time was about 3.8 s. The thermo-tactile driver can control the rise time of the temperature because it can digitally control the magnitude of the current flowing through the electric wire. This will also allow for additional experiments on the perceptual and cognitive aspects of the difference in temperature-change time. However, the return to the initial temperature cannot be controlled, as it uses natural convection with the finger resting on the actuator. In the case of vibration, it has the advantage of being able to present the same intensity from the low frequency of 25 Hz to the range of 225 Hz to 250 Hz [18], which is the most sensitive range for humans, including the 130 Hz to 180 Hz range [19], which is the vibration frequency used by most cellular phones. This corresponds to the frequency range [18,20] detected by the four representative mechanoreceptors, which will enable the study of various aspects of tactile sensation. In the case of pressure sensation, the rising time to the maximum pressure was about 0.4 s and the falling time was about 0.1 s, so the repetition rate can be presented up to 0.5 s, or 2 Hz, unless it is a sustained pressure-sensation experiment. In general, research on tactile stimuli requires repeated experiments to generate meaningful results. For this reason, the framework of the actuator must be made of a rigid, low-deformation material. In this study, the actuator 3D-printed using ABS (acrylonitrile butadiene styrene) filament, which is a high-strength and heat-resistant material, so that it can be consistently used without deforming even when the force applied by the subject, compressed air, and temperature change repeatedly. Three single stimuli (W, V, and P) and combination stimuli (W + V, V + P, and W + V + P) were presented to the bottom of the right index finger, and the ERD/S spectrum was analyzed in the somatosensory area (C3) of the brain. Event-related desynchronization (ERD) refers to the decrease brain signal power in the \(\alpha\) band and \(\beta\) band in response to external stimulation. Event-related synchronization (ERS) shows an increase in signal power in the same band in response to the stimulus [17,21]. Although ERD/S analysis is mainly used in motor research, it has also been used in tactile research in the sensorimotor rhythm (SMR) region of 10 to 30 Hz because vibration and pressure also trigger fine muscle movements [22]. In this study, a similar trend was observed, i.e., ERD occurred after stimulus presentation followed by ERS [23,24]. It is believed that the ERD occurred due to attention processing by tactile perception after the stimulus presentation, and the subsequent ERS occurred due to neuronal activity due to cognitive processing of the stimulus [24].