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.

Softness perception intrinsically relies on haptic information. However, through everyday experiences we learn correspondences between felt softness and the visual effects of exploratory movements that are executed to feel softness. Here, we studied how visual and haptic information is integrated to assess the softness of deformable objects. Participants discriminated between the softness of two softer or two harder objects using only-visual, only-haptic or both visual and haptic information. We assessed the reliabilities of the softness judgments using the method of constant stimuli. In visuo-haptic trials, discrepancies between the two senses' information allowed us to measure the contribution of the individual senses to the judgments. Visual information (finger movement and object deformation) was simulated using computer graphics; input in visual trials was taken from previous visuo-haptic trials. Participants were able to infer softness from vision alone, and vision considerably contributed to bisensory judgments (∼35%). The visual contribution was higher than predicted from models of optimal integration (senses are weighted according to their reliabilities). Bisensory judgments were less reliable than predicted from optimal integration. We conclude that the visuo-haptic integration of softness information is biased toward vision, rather than being optimal, and might even be guided by a fixed weighting scheme.

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In this experiment we investigated the influence of stimulus properties on exploratory movement parameters in active touch. More precisely, we investigated whether and to what extent variations in stimulus compliance influence the executed finger force and velocity of the exploratory movements. Therefore, we varied the compliance of silicon rubber stimuli with deformable surfaces. Participants freely explored pairwise presented stimuli with a bare finger, and were asked to select the softer one. We found that both exploratory force and velocity depended on the compliance of the stimulus. Our results suggest in particular that observers strategically adapt their maximum finger force to the expected softness of the stimulus.