Affective touch is a well-established regulator of emotional well-being, and there is growing interest in designing robotic devices that can replicate its benefits. Yet, most affective touch systems developed so far rely on simple linear stroking patterns, even though natural human touch is rarely so uniform. Specifically, the role of stroking trajectory in shaping affective experience has remained unexplored. This study investigated how the predictability of stroking touch influences pleasantness, subjective affect, and physiological arousal when delivered by a robot to the forearm. 22 healthy adults experienced three stroking trajectories (straight, sine, and sawtooth) at a constant CT-optimal velocity for two minutes each. The trajectories were each administered twice in randomized order. Perceived pleasantness, subjective valence and arousal, and heart rate variability (HRV) were measured for each trial. Statistical analyses showed that the straight trajectory was more pleasant than the sawtooth, while both the straight and sine trajectories resulted in lower subjective arousal than the sawtooth. No significant differences were found for valence or physiological arousal. These findings suggest that stroking predictability modulates subjective arousal, though the subtle trajectory differences were insufficient to produce measurable HRV changes under low-arousal conditions. In addition, a strong positive correlation between perceived pleasantness and valence was observed, raising the theoretical question of whether pleasantness in affective touch studies may be functionally equivalent to valence. The present study argues that stroking trajectory is a meaningful design parameter for robotic affective touch systems, with practical implications for the development of wearable haptic devices and companion robots seeking to modulate human affect.
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