We investigated whether surface texture (i.e., stochastic roughness) influences softness perception during direct touch interactions with elastic, textured stimuli. Using a Bayesian adaptive modeling approach and a 2AFC task, we evaluated participants' ability to discriminate the softness of stimuli that varied in both their stochastic surface roughness (Hurst exponent) and material elasticity. To explore potential interactions between these features, we conducted two discrimination experiments, testing stimuli from two distinct ranges of elasticity. All participants performed the task using pressing. Results show that softness discrimination was determined primarily by material elasticity, with no discernible influence of surface features. The findings suggest that humans effectively isolate elasticity-based information from smaller-scale surface topography or texture during direct pressing with the finger.

One may notice a relatively wide range of tactile sensations even when touching the same hard, flat surface in similar ways. Little is known about the reasons for this variability, so we decided to investigate how the perceptual intensity of light stickiness relates to the physical interaction between the skin and the surface. We conducted a psychophysical experiment in which nine participants actively pressed their finger on a flat glass plate with a normal force close to 1.5 N and detached it after a few seconds. A custom-designed apparatus recorded the contact force vector and the finger contact area during each interaction as well as pre- and post-trial finger moisture. After detaching their finger, participants judged the stickiness of the glass using a nine-point scale. We explored how sixteen physical variables derived from the recorded data correlate with each other and with the stickiness judgments of each participant. These analyses indicate that stickiness perception mainly depends on the pre-detachment pressing duration, the time taken for the finger to detach, and the impulse in the normal direction after the normal force changes sign; finger-surface adhesion seems to build with pressing time, causing a larger normal impulse during detachment and thus a more intense stickiness sensation. We additionally found a strong between-subjects correlation between maximum real contact area and peak pull-off force, as well as between finger moisture and impulse.