In the Tactile Discrimination of Compliance, Perceptual Cues in Addition to Contact Area Are Required

Abstract

In our ability to discriminate compliant, or ‘soft,’ objects, we rely upon information acquired from interactions at the finger pad. We have yet to resolve the most pertinent perceptual cues. However, doing so is vital for building effective, dynamic displays. By introducing psychophysical illusions through spheres of various size and elasticity, we investigate the utility of contact area cues, thought to be key in encoding compliance. For both active and passive touch, we determine finger pad-to-stimulus contact areas, using an ink-based procedure, as well as discrimination thresholds. The findings indicate that in passive touch, participants cannot discriminate certain small compliant versus large stiff spheres, which generate similar contact areas. In active touch, however, participants easily discriminate these spheres, though contact areas remain similar. Supplementary cues based on stimulus rate and/or proprioception seem vital. One cue that does differ for illusion cases is finger displacement given a volitionally applied force.

An elasticity-curvature illusion decouples cutaneous and proprioceptive cues in active exploration of soft objects

“…In the simulation, stress/strain distributions at contact locations and the skin surface deflection are quantified as the cutaneous cues. In the experiments, contact area is derived from a contiguous area on the skin surface with a superthreshold contact pressure (16,17). Furthermore, the deflection of the skin surface is the contour of a deflection profile in the contact plane (21,28).…”

“…Moreover, stimulus compliance at times has been parameterized by its stiffness rather than its modulus (19,22,28), which can be confounding for naturalistic objects of identical stiffness but differing in geometry (28). Furthermore, stimuli with flat surfaces do not fully mimic the contact profile of the skin surface's contacting an elastic object (8,9,16). Herein, we address these issues by building spherical stimuli with combinations of radii and elasticity which can recapitulate properties of ecologically compliant materials, such as fruit.…”

“…The compliant stimuli were constructed from a room temperature curing silicone elastomer (BJB Enterprises, Tustin, CA; TC-5005 A/B/C). To achieve the desired modulus, based on prior calibrations (16), corresponding ratios of cross-linker were added and mixed. These formulations were then cast into 3-D printed molds of three radii (4, 6, and 8 mm) and cured to become stimulus tips.…”

“…The gross contact area between the stimulus surface and finger pad was measured by the ink-based method (16,20). An overview of this method is shown in Fig.…”

“…This percept is informed by some combination of cutaneous inputs from mechanosensitive afferents signaling skin deformation and proprioceptive inputs signaling body movements. Efforts to define the precise cues within skin deformation and body movements have focused on contact area at the finger pad (14)(15)(16)(17)(18), spatiotemporal deformation of the skin's surface (19)(20)(21), and kinesthetic inputs of displacement, force, and joint angle (22)(23)(24)(25). Such an array of sensory contact inputs, mediated by independent cortical mechanisms, are recruited and integrated in the primary somatosensory cortex, and form the perceptual basis from which compliances are recognized and discriminated (26).…”