Affective Touch With Mid-Air Ultrasound: A Psychophysical Comparison With Real Materials

Affective touch has attracted significant interest due to its positive effects on child development and adult well-being. Studies showed that light and slow stroke of soft brush on hairy skin elicits affective sensation by activating C-Tactile fibers. Mid-air haptics, which uses focused ultrasound to create tactile sensations in free space, offers a promising method for eliciting affective changes. However, it remains poorly understood to what extent mid-air haptics can produce the perception similar to a brush stroke on hairy skin. Here, we conducted a psychophysical study comparing the tactile perception of mid-air ultrasound stimulation with that of real materials (sandpaper, aluminum resin, urethane rubber, cloth, and soft brush). Participants rated each stimulation on the dorsum of hand along three affective touch dimensions (pleasantness, surprise, ticklishness) and three discriminative touch dimensions (smoothness, softness, warmness). Univariate analyses revealed that ultrasound stimuli differed from aluminum resin, sandpaper, and urethane rubber in one or more dimensions of discriminative touch. Ultrasound stimulation with 10 Hz amplitude modulation significantly felt more pleasant than sandpaper, while no consistent differences between ultrasound and real textures ratings was observed in surprise and ticklishness. Multivariate analyses showed that ultrasound stimuli with amplitude modulation at 10 Hz and 100 Hz were significantly closer to real textures than unmodulated ultrasound. Questionnaires showed that over half of the participants identified ultrasound stimulation as a brush or cotton. These findings suggest that mid-air haptics can evoke perceptual attributes overlapping with those of real textures, including soft brushes, on hairy skin. Moreover, amplitude modulation may enhance the perceptual resemblance to realistic textures.