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

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-velocity cues in passive touch psychophysical experiments. Additionally, a novel ink-based method to mark the finger pad was used to measure contact area per stimulus, simultaneously with displacement and force. Stimulus compliances were chosen to span a range both harder and softer than the finger pad. There were three major findings. First, compliances harder than the finger pad are more readily discriminable by force cues – i.e., when displacement is controlled between stimuli. Second, the harder stimuli are discriminable at lower forces when force-rate cues are available – i.e., when displacement-velocity is controlled between stimuli – than vice versa. In contrast, compliances softer than the finger pad are equally discriminable at low forces, regardless of control mode. Third, selecting particular force-rates can confuse observers. For example, the harder compliances are less discriminable if the more compliant of the two is indented at a greater force-rate than if the more compliant of the two is indented at a lesser force-rate. These findings are important for the next generation of tactile rendering displays and robotic exploration strategies.