Abstract
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.
Highlights
Humans use information from several modalities in everyday life, especially when interacting with objects in their environment
This study provided preliminary hints that vision may be weighted more than it should if visuo-haptic softness perception were optimal
5 Conclusions In summary, we demonstrated that participants can infer the softness of natural deformable stimuli from the visual effects of stimulus exploration alone, and that this secondary visual information is integrated with the primary haptic information in visuo-haptic judgments
Summary
Humans use information from several modalities in everyday life, especially when interacting with objects in their environment. Many material properties of an object (such as thermal conductivity, weight or compliance) are most linked to haptics. One important material property is softness, which is the psychological correlate of the compliance of a surface. Compliance is defined by the relation between the force applied to an object and the object’s deformation, including the position of its surface. The haptic sense directly measures both force and position information, which are necessary to judge softness. The visual sense can directly measure position changes, but intrinsic visual cues to force are hardly available. When a compliant object comes into contact with an indenter, such as our finger or another object, our vision can provide us with information concerning the time-course and pattern of the object’s surface deformation around the contact region.
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