When small holes are felt with the tongue they are perceived to be larger as compared to when felt with the index finger. It is an open question why the size perceived with the tongue differs from size perceived with the finger. We suggest that differences in perceived size are due to differences in the effector's deformation at the edge of the explored hole, which correlate with the effector's pliability. The more pliable the effector is, the more it will deform and the hole will be perceived to be larger. In two experiments we tested this hypothesis. In Experiment I we manipulated the force applied by the tongue, and thus the tongue's deformation. As predicted, the tongue perceived holes to be larger when higher forces were applied. Experiment II investigated how the toe perceives size as compared to the tongue and finger. The toe is less pliable than the finger and the finger is less pliable than the tongue. Also as predicted, holes at the toe were perceived to be smaller than holes at the finger and considerably smaller than holes at the tongue. The findings corroborate our pliability-deformation hypothesis of haptic size perception. Drewing, K., Bruckbauer, S., & Szöke, D. (2015, June). Felt hole size depends on force and on the pliability of the effector. In 2015 IEEE World Haptics Conference (WHC) (pp. 100-105). IEEE.
The perception of softness is the result of the integration of information provided by multiple cutaneous and kinesthetic signals. The relative contributions of these signals to the combined percept of softness was not yet addressed directly. We transmitted subtle external vertical forces to the exploring human finger during the exploration of deformable silicone rubber stimuli to dissociate the force estimates provided by the kinesthetic signals and the efference copy from cutaneous force estimates. This manipulation introduced a conflict between the cutaneous and the kinesthetic/efference copy information on softness. We measured Points of Subjective Equality (PSE) of manipulated references to stimuli which were explored without external forces. PSEs shifted as a linear function of external force in predicted directions - to higher compliances with pushing and to lower compliances with pulling force. We found relative contribution of kinesthetic/efference copy information to perceived softness being 23% for rather hard and 29% for rather soft stimuli. Our results suggest that an integration of the kinesthetic/efference copy information and cutaneous information with constant weights underlies softness perception. The kinesthetic/efference copy information seems to be slightly more important for the perception of rather soft stimuli. Metzger, A., & Drewing, K. (2015, June). Haptically perceived softness of deformable stimuli can be manipulated by applying external forces during the exploration. In 2015 IEEE World Haptics Conference (WHC) (pp. 75-81). IEEE.
An object's softness is stereotypically judged by pressure movements indenting the surface [1]. In exploration without movement constraints, participants repeat such indentation movements. We investigated how people modulate executed peak forces for different indentations depending on stimulus softness. Participants performed a 2AFC discrimination task for stimulus pairs from one of 4 softness categories. We assumed that movement control at different exploration moments is based on variations in the predictive and sensory signals available. We manipulated availability of predictive signals on softness category, by presenting either stimuli of the same category in a blocked fashion (high predictability) or by randomly mixing stimuli from different categories (low predictability). Effects of sensory signals were examined by contrasting first and last indentation, as sensory signals are hardly available when initiating exploration but gathered during exploration. The results show that participants systematically apply lower forces when sensory or predictive signals indicate softer objects as compared to harder objects. We conclude that softness exploration can be considered as a sensorimotor control loop, in which predictive and sensory signals determine movement control. Further, the results indicate a high importance of predictive processes throughout the entire exploration, as effects of predictive signals maintain in the last indentation. Lezkan, A., & Drewing, K. (2015, June). Predictive and sensory signals systematically lower peak forces in the exploration of softer objects. In 2015 IEEE World Haptics Conference (WHC) (pp. 69-74). IEEE.
