Deciphering Contact Interactions and Exploration Strategies Underlying Tactile Perception of Material Softness

Our sense of touch is essential and permeates in interactions involving natural explorations and affective communications. For instance, we routinely judge the ripeness of fruit at the grocery store, caress the arm of a spouse to offer comfort, and stroke textiles to gauge their softness. Meanwhile, interactive displays that provide tactile feedback are becoming normal and ubiquitous in our daily lives, and are extending rich and immersive interactions into augmented and virtual reality. To replicate touch sensation and make virtual objects feel tangible, such feedback will need to relay a sense of compliance, or “softness”, one of the key dimensions underlying haptic perception. As our understanding of softness perception remains incomplete, this study seeks to understand exploratory strategies and perceptual cues that may optimally encode material softness. Specifically, we employ methods of computational finite element modeling, biomechanical experimentation, psychophysical evaluation, and data-driven analysis. First, we characterize the functional roles of physical contact cues, by studying a tactile illusion phenomenon where small-compliant and large-stiff spheres are naturally indistinguishable. In modulating contact interactions between the finger pad and stimuli, we found that pressing an object into the finger does not fully reveal its softness, but pressing actively does. Thus, our percept of softness is a product of both sensation and volition and depends upon both tactile afferents in the skin and musculoskeletal proprioception. Second, in exploring both engineered and ecological soft objects, we investigate how cues are optimally evoked and integrated under one’s active control. By varying exploration time, we observe that exploratory strategies are finely tuned to elicit efficient contact force and finger movements for superior performance. Third, considering inherent differences and constraints among individuals’ skin mechanics, we investigate individual differences in eliciting tactile cues, exploratory strategies, and thus, perceptual sensitivity. We characterized the skin material properties of individuals’ finger pads and evaluated their contact interactions in performing discriminative touch. The results indicate that an individual’s tactile acuity is constrained by their skin softness, but could be improved under volitional control of their exploratory movements. Overall, this work may aid in engineering the next-generation wearable haptic displays, which must be more tangible, compatible, and perceptually naturalistic.