Effects of the Alignment Between a Haptic Device and Visual Display on the Perception of Object Softness

Virtual reality (VR) has been gaining popularity in surgical planning and simulation. Most VR surgical simulation systems provide haptic (pertinent to the sense of touch) and visual information simultaneously using certain alignments between a haptic device and visual display. A critical aspect of such VR surgical systems is to represent both haptic and visual information accurately to avoid perceptual illusions (e.g., to distinguish the softness of organs/tissues). This study compared three different alignments (same-location alignment, vertical alignment, and horizontal alignment) between a haptic device and visual display that are widely used in VR systems. We conducted three experiments to study the influence of each alignment on the perception of object softness. In each experiment, we tested 15 different human subjects with varying availability of haptic and visual information. During each trial, the task of the subject was to discriminate object softness between two deformable balls in different viewing angles. We analyzed the following dependent measurements: subject perception of object softness and objective measurements of maximum force and maximum pressing depth. The analysis results reveal that all three alignments (independent variables) have similar effect on subjective perception of object softness within the interval of viewing angles from -7.5^° to +7.5^°. The viewing angle does not affect objective measurements. The same-location alignment requires less physical effort compared with the other two alignments. These observations have implications in creating accurate simulation and interaction for VR surgical systems.