Evaluating the Physics and Repeatability of Human Brushers in Delivering Affective Touch

Research in affective touch often utilizes a pleasant stroking paradigm, with touch typically delivered at velocities between 0.3-30 cm/s and forces about 0.4 N. These values are derived from perceptual and physiological responses of touch receivers rather than natural behaviors of touchers. Herein, we observe untrained touchers in delivering pleasant touch as gentle strokes to a receiver's forearm through high-resolution force and position measurement using an instrumented brush. Twenty participants delivered about eleven strokes in each of five trials, yielding data on stroke velocity, force, duration, and length. The cohort deployed forces (0.37 N ± 0.24, 2 SD) near 0.4 N, but stroking velocities (13.7 cm/s ± 8.8, 2 SD) slightly higher than typically considered "optimal" (1-10 cm/s). We observed no correlation between stroke force and velocity, which led us to consider these factors jointly in characterizing an individual's brushing strategy. Moreover, while the cohort of participants exhibited a compact range of forces and velocities, individuals tended to occupy only subsets of this range with high repeatability across trials. Altogether, the findings suggest a need to further evaluate the velocity range between 10-30 cm/s and to jointly consider force, velocity, and consistency in characterizing an individual's brushing strategy.