In these experiments, two plates were grasped between the thumb and the index finger and squeezed together along a linear track The force resisting the squeeze, produced by an electromechanical system under computer control, was programmed to be either constant (in the case of the force discrimination experiments) or linearly increasing (in the case of the compliance discrimination experiments) over the squeezing displacement, After completing a set of basic psychophysical experiments on compliance resolution (Experiment I), we performed further experiments to investigate whether work and/or terminal-force cues played a role in compliance discrimination, In Experiment 2, compliance and force discrimination experiments were conducted with a roving-displacement paradigm to dissociate work cues (and terminal-force cues for the compliance experiments) from compliance and force cues, respectively. The effect of trial-by-trial feedback on response strategy was also investigated. In Experiment 3, compliance discrimination experiments were conducted with work cues totally eliminated and terminal-force cues greatly reduced. Our results suggest that people tend to use mechanical work and force cues for compliance discrimination, When work and terminal-force cues were dissociated from compliance cues, compliance resolution was poor (22%) relative to force and length resolution. When work cues were totally eliminated, performance could be predicted from terminal-force cues. A parsimonious description of all data from the compliance experiments is that subjects discriminated compliance on the basis ofterminal force.To a first approximation, the mechanical behavior of all deformable solid objects can be expressed as! = F' . + Kx + Bx + mx, which represents the relationship between the total force (f) applied on the object and the corresponding displacement (x), velocity (x), and acceleration (i); the frictional force (F'.), linear stiffness (K), viscosity (B), and mass (M) are the physical parameters that distinguish one object from another (we use lowercase letters for variables and uppercase for parameters). It is our goal to study manual resolution ofall these physical variables and parameters and to provide basic psychophysical information that can be used to (1) advance our understanding of manual perception of object properties, (2) guide the development of design specifications for haptic interfaces that not only sense position and force commands from the human operator but also display such information to the operator in teleoperation and virtual environment systems (see, e.g., the recently published book on systems of this type edited by Durlach & Mavor, 1994), and (3) improve the design of autonomous robots that must make use of manual sensing and manipulation. This is the third in a series ofpapers concerned with how individual physical properties of objects are perceived. In the first paper of this series (Durlach et al., 1989), we reported the results ofa variety ofexperiments in which the subject was required t...

To explore the possibility that multisensory information may be useful in expanding the range of haptic experiences in virtual environments, psychophysical experiments examining the influence of sound on the haptic perception of stiffness were carried out. In these experiments, subjects utilized the PHANToM, a six-degree-of-freedom haptic interface device with force-reflection along three axes, to feel the stiffness of various virtual surfaces. As subjects tapped on the different virtual surfaces, they were simultaneously presented with various impact sounds. The subjects were asked to rank the surfaces based on their perceived stiffness. The results indicate that when the physical stiffnesses of the surfaces were the same, subjects consistently ranked the surfaces according to sound, i.e., surfaces paired with sound cues that are typically associated with tapping harder surfaces were generally perceived as stiffer. However, when sound cues were randomly paired with surfaces of different mechanical stiffnesses, the results were more equivocal: naïve subjects who had not used the PHANToM previously tended to be more affected by sound cues than another group of subjects who had previously completed a set of stiffness discrimination experiments without sound cues. The possible implications of this result for the design of multimodal virtual environments and its comparison to prior work by some of the authors on the effects of vision on haptic perception are discussed.

To study the impact of visually presented spatial cues on the human perception of mechanical stiffness in virtual environments, a three degree of freedom, force-reflecting haptic interface, the Planar Grasper, was utilized. In a series of psychophysical experiments on the discrimination of stiffness of two virtual springs, subjects pressed the springs and felt the corresponding displacements and forces through their hands, in addition to seeing the deformation of the springs displayed graphically on a computer monitor. Unknown to the subjects, the relationship between the visually presented deformation of each spring and actual deformation was systematically varied between experimental trials. This relationship ranged from fully registered (visual deformation was equal to the actual deformation of each spring) to completely interchanged (visual deformation of the softer spring was equal to the deformation of the harder spring for that force and vice versa). The results demonstrated a clear visual dominance over the kinesthetic sense of hand position. The subjects essentially ignored all kinesthetic hand position information regarding spring deformation, and based their judgment on the relationship between the visual position information and the indentation force sensed factually. This caused an increasing misperception of stiffness with increasing mismatch between the visual and haptic position information, culminating in totally erroneous judgments when the two were interchanged. These results indicate such haptic illusions can be exploited to overcome some of the limitations of haptic interfaces and to enhance the range of haptic experience.