Exploratory Strategies in Haptic Softness Discrimination Are Tuned to Achieve High Levels of Task Performance

Haptic perception essentially depends on the executed exploratory movements. It has been speculated that spontaneously executed movements are optimized for the computation of associated haptic properties. We investigated to what extent people strategically execute movements that are tuned for softness discrimination of objects with deformable surfaces. In Experiment 1, we investigated how movement parameters depend on expected stimulus compliance. In a discrimination task, we measured exploratory forces for less compliant (hard) stimuli and for more compliant (soft) stimuli. In Experiment 2, we investigated whether exploratory force also depends on the expected compliance difference between the two stimuli. The results indicate that participants apply higher forces when expecting harder objects as compared to softer objects, and they apply higher forces for smaller compliance differences than for larger ones. Experiment 3 examined how applied force influences differential sensitivity for softness as assessed by just noticeable differences (JNDs). For soft stimuli, JNDs did not depend on force. For hard stimuli, JNDs were "worse" (higher) if participants applied less force than they use naturally. We conclude that applying high force is a robust strategy to obtain high differential sensitivity, and that participants used this strategy if it was required for successful discrimination performance.