Force-Rate Cues Reduce Object Deformation Necessary to Discriminate Compliances Harder than the Skin

Abstract

Grasping and manipulating an object requires us to perceive its material compliance. Compliance is thought to be encoded by relationships of force, displacement, and contact area at the finger pad. Prior work suggests that objects must be sufficiently deformed to become discriminable, but the utility of time-dependent cues has not been fully explored. The studies herein find that the availability of force-rate cues improve compliance discriminability so as to require less deformation of stimulus and finger pad. In particular, we tested the impact of controlling force-rate and displacement-rate cues in passive touch psychophysical experiments. An ink-based method to mark the finger pad was used to measure contact area per stimulus, simultaneously with displacement and force. Compliances spanned a range harder and softer than the finger pad. The results indicated harder compliances were discriminable at lower peak forces when the stimulus control mode was displacement-rate (0.5 N) compared to force-rate (1.3 N). That is, when displacement-rate was controlled to be equal between the two compliances, the resultant force-rate psychophysical cues could be more readily discriminated. In extending prior studies, while some magnitude of finger pad deformation may be sufficient for discriminability, temporal cues tied to force afford more efficient judgments.

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

“…In particular, through a series of biomechanical measurements, the contact area between the finger pad and stimulus was quantified to determine if the illusion case spheres would generate similar cutaneous contact. Contact area was measured directly, using an ink-based procedure (20). Measured contact area is commensurate with the cutaneous cues predicted in the finite element simulation.…”

“…Second, at the behavioral level, we prioritize exploratory force to optimize our perception of object compliances in relevant contexts (23,29,32,33). Indeed, the availability of forcerelated cues improve discriminability by reducing the necessary deformation of the skin (20). Similarly, for the exploratory procedure of pinch grasp, we control the grip force within a safety margin, informed by skin mechanoreceptors, to prevent slipping or overly grasped (34).…”

“…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).…”

Multi-Layered Perceptual Model for Haptic Perception of Compliance

“…It is necessary to extract effective features from the interaction data according to specific experimental condition and perception tasks. Studies [13] have shown that time-dependent cues, usually information about force and deformation changing with time, could convey compliance in a manner more efficiently and rapidly. Lawrence [14] introduced the quality metric "rate-hardness" and linked it to the human perception of hardness of the rigid surface via a psychophysical study.…”

“…This method mapped each different of physical quantity into a proportion in perceptual responds. But such a simple mechanism of static mapping would lose lots of useful information brought by time-series cues of the interaction process (such as force and movement), which has been confirmed to have significant influence on haptic perception of compliance [13][14][15]. In order to combine movement and force information together instead of static perception models, Rank and Hirche [17] proposed a kind of dynamic state observer model to predict the perceptual thresholds varying with the specific interaction movement with the environment.…”