Haptic interfaces for virtual environments: perceived instability and force constancy in haptic sensing of virtual surfaces.

Haptic interfaces are becoming more commonplace in virtual environment and teleoperation systems. There is a growing need to not only continue to improve hardware platforms and rendering algorithms, but evaluate human performance with haptic interfaces. This review summarizes two recent studies inspired by perception problems in using haptic interfaces to interact with virtual environments. The first study evaluated perceived quality of virtual haptic textures and discovered several types of perceived instability and their sources. We found that the buzzing type of perceived instability was most likely due to the mechanical resonance of the haptic interface hardware, and the aliveness type of perceived instability due to our inability to sense the slight movements of our hands in free space. The second study focused on the motor strategy employed during interaction with a virtual surface via a force-feedback haptic interface. We found that users tended to maintain a constant penetration force into a virtual surface when interacting with the surface. This can result in a reversal in perceived relative surface heights if the taller surface is rendered with a lower stiffness, thereby resulting in an erroneous perception of the virtual environment being rendered. For both studies, possible solutions to improving human perception of virtual and remote objects via hardware and/or software are discussed.     

Haptic perception: A tutorial

This tutorial focuses on the sense of touch within the context of a fully active human observer. It is intended for graduate students and researchers outside the discipline who seek an introduction to the rapidly evolving field of human haptics. The tutorial begins with a review of peripheral sensory receptors in skin, muscles, tendons, and joints. We then describe an extensive body of research on “what” and “where” channels, the former dealing with haptic perception of objects, surfaces, and their properties, and the latter with perception of spatial layout on the skin and in external space relative to the perceiver. We conclude with a brief discussion of other significant issues in the field, including vision-touch interactions, affective touch, neural plasticity, and applications.     

Haptic perception of wetness

In daily life, people interact with textiles of different degrees of wetness, but little is known about the mechanics of wetness perception. This paper describes an experiment with six conditions regarding haptic discrimination of the wetness of fabrics. Three materials were used: cotton wool, sponge-structured viscose and thin viscose. Two ways of touching were investigated: static touching, in which only thermal cues were available, and dynamic touching, in which additional mechanical cues were available. For dynamic touching, average Weber fractions for discrimination were around 0.3, whereas for static touching, they ranged from 0.34 to 0.63. The results show that people can make use of the additional mechanical cues to significantly improve their discrimination performance. There was no significant difference between Weber fractions for the three materials, showing that wetness can be judged as a separate perceptual quantity, independent of the material.     

Haptic and visual perception of roughness

In this study, we are interested in the following two questions: (1) how does perceived roughness correlate with physical roughness, and (2) how do visually and haptically perceived roughness compare? We used 96 samples of everyday materials, such as wood, paper, glass, sandpaper, ceramics, foams, textiles, etc. The samples were characterized by various different physical roughness measures, all determined from accurately measured roughness profiles. These measures consisted of spectral densities measured at different spatial scales and industrial roughness standards (R(a), R(q) and R(z)). In separate haptic and visual conditions, 12 na?ve subjects were instructed to order the 96 samples according to perceived roughness. The rank orders of both conditions were correlated with the various physical roughness measures. With most physical roughness measures, haptic and visual correspondence with the physical ordering was about equal. With others, haptic correspondence was slightly better. It turned out that different subjects ordered the samples using different criteria; for some subjects the correlation was better with roughness measures that were based on higher spatial frequencies, while others seemed to be paying more attention to the lower spatial frequencies. Also, physical roughness was not found to be the same as perceived roughness.     

Haptic perception of texture gradients

To pick up 3-D aspects of pictures is arguably the most difficult problem concerning tactile pictorial perception by the blind. The aim of the experiments reported was to examine the potential utility of texture gradients in this context. Since there is no theoretical basis for predicting absolute values of 3-D properties from 2-D patterns read by the finger pads, the abilities of participants to perceive gradients lying between known maxima and minima were assessed. Experiment 1 involved blindfolded sighted participants making verbal magnitude estimations of texture-gradient magnitudes corresponding to plane surfaces at different slants. In experiment 2 the participants' task was to orient a surface at a slant corresponding to the texture gradients depicted tactually, and experiment 3 required early-blind participants to attempt the same task. The results revealed that participants can scale the magnitudes of texture gradients with high precision and that they can also accurately produce surface slants from depictions, providing the extreme conditions are clearly defined and there are opportunities for learning. Texture gradients appear as informative to the blind as they do to the sighted. To what extent these data can be generalised to other gradients and textures or to other projections of 3-D scenes remains to be investigated.     

Haptic perception of spatial relations

There are some indications that haptic space like visual space is not Euclidean (e.g. Blumenfeld, 1937 Acta Psychologica 2 125-174). In a series of experiments, we investigated the haptic perception of spatial relations in a systematic way. We restricted ourselves to a horizontal plane at waist height. Blindfolded subjects were asked to perform three tasks with their right hand: (i) a reference bar was presented under four different orientations and subjects were asked to rotate a test bar such that it felt to be parallel to the reference bar; (ii) subjects had to rotate two test bars in such a way that they felt collinear; (iii) subjects had to point a test bar in the direction of a marker. Bars and marker could appear at nine different locations. In all experiments large systematic deviations (up to 40 degrees) were made. The deviations strongly correlated with horizontal (right-left) but not with vertical (forward-backward) distance. Subjects showed qualitatively identical trends but the size of the deviations was strongly subject-dependent. In addition, a significant haptic oblique effect was found. These results provide strong evidence that haptic space in non-Euclidean.     

Haptic perception of linear extent

The perception of linear extent in haptic touch appears to be anisotropic, in that haptically perceived extents can depend on the spatial orientation and location of the object and, thus, on the direction of exploratory motion. Experiments 1 and 2 quantified how the haptic perception of linear extent depended on the type of motion (radial or tangential to the body) when subjects explored different stimulus objects (raised lines or solid blocks) varying in length and in relative spatial location. Relatively narrow, shallow, raised lines were judged to be longer, by magnitude estimation, than solid blocks. Consistent with earlier reports, stimuli explored with radial arm motions were judged to be longer than identical stimuli explored with tangential motions; this difference did not depend consistently on the lateral position of the stimulus object, the direction of movement (toward or away from the body), or the distance of the hand from the body but did depend slightly on the angular position of the shoulder. Experiment 3 showed that the radial-tangential effect could be explained by temporal differences in exploratory movements, implying that the apparent anisotropy is not intrinsic to the structure of haptic space.     

Haptic identification of curved surfaces

In two experiments, the active haptic identification of three-dimensional mathematically welldefined objects is investigated. The objects, quadric surfaces, are defined in terms of the shape index, a quantity describing the shape, and curvedness, a quantity describing overall curvature. Both shape index and curvedness are found to have a significant influence on haptic shape identification . Concave surfaces lead to a larger spread in responses than convex ones. Hyperbolic surfaces show a slight tendency to be identified with more difficulty than elliptic ones. Surfaces with a high curvedness are identified more easily than those with a low curvedness. Results from experiments with constant and with random curvedness are indistinguishable . It is concluded that shape index and curvedness are psychophysically not confounded.     

Haptic perception of linear extent.

The perception of linear extent in haptic touch appears to be anisotropic, in that haptically perceived extents can depend on the spatial orientation and location of the object and, thus, on the direction of exploratory motion. Experiments 1 and 2 quantified how the haptic perception of linear extent depended on the type of motion (radial or tangential to the body) when subjects explored different stimulus objects (raised lines or solid blocks) varying in length and in relative spatial location. Relatively narrow, shallow, raised lines were judged to be longer, by magnitude estimation, than solid blocks. Consistent with earlier reports, stimuli explored with radial arm motions were judged to be longer than identical stimuli explored with tangential motions; this difference did not depend consistently on the lateral position of the stimulus object, the direction of movement (toward or away from the body), or the distance of the hand from the body but did depend slightly on the angular position of the shoulder. Experiment 3 showed that the radial-tangential effect could be explained by temporal differences in exploratory movements, implying that the apparent anisotropy is not intrinsic to the structure of haptic space.     

Haptic form perception: Relative salience of local and global features

John B. Pierce Laboratory and Yale University, New Haven, Connecticut When we examine objects haptically, do we weight their local and global features as we do visually, or do we place relatively greater emphasis on local shape? In Experiment 1, subjects made either haptic or visual comparisons of pairs of geometric objects (from a set of 16) differing in local and global shape. Relative to other objects, those with comparable global shape but different local features were judged less similar by touch than by vision. Separate groups of subjects explored the same objects while wearing either thick gloves (to discourage contour-following) or splinted gloves (to prevent enclosure). Ratings of similarity were comparable in these two conditions, suggesting that neither exploratory procedure was necessary, by itself, for the extraction of either local or global shape. In Experiment 2, haptic exploration time was restricted to 1, 4, 8, or 16 sec. Limiting exploration time affected relative similarity in objects differing in their local but not their global shape. Together, the findings indicate that the haptic system initially weights local features more heavily than global ones, that this differential weighting decreases over time, and that neither contour-following nor enclosure is exclusively associated with the differential emphasis on local versus global features.     

Haptic after-effect of successively touched curved surfaces

A flat surface is more often judged to be convex after the touching of a concave surface than after the touching of a convex surface. This haptic after-effect increases with the time of contact with the curved surface till it saturates, and it decreases with the time-lapse between the touching of the first surface and the next one. In this paper, the haptic after-effect of two successively touched spherically curved surfaces is investigated. It is found that both surfaces contribute to the after-effect, but the after-effect is not additive. The time course of the after-effect of two successive surfaces can be described by a first-order integrator with a single time constant of about 7 s and an amplitude equal to the difference between the saturation levels of the after-effects of the two surfaces when measured in isolation. The new saturation level is therefore equal to that of the after-effect of the most recently touched surface.     

Haptic dominance in form perception: vision versus proprioception.

An experiment placed vision and touch in conflict by the use of a mirror placed perpendicular to a letter display. The mirror induced a discrepancy in direction and form. Subjects touched the embossed tangible letters p, q, b, d, W, and M, while looking at them in a mirror, and were asked to identify the letters. The upright mirror produced a vertical inversion of the letters, and visual inversion of the direction of finger movement. Thus, subjects touched the letter p, but saw themselves touching the letter b in the mirror. There were large individual differences in reliance on the senses. The majority of the subjects depended on touch, and only one showed visual dominance. Others showed a compromise between the senses. The results were consistent with an attentional explanation of intersensory dominance.     

Haptic form perception: relative salience of local and global features.

When we examine objects haptically, do we weight their local and global features as we do visually, or do we place relatively greater emphasis on local shape? In Experiment 1, subjects made either haptic or visual comparisons of pairs of geometric objects (from a set of 16) differing in local and global shape. Relative to other objects, those with comparable global shape but different local features were judged less similar by touch than by vision. Separate groups of subjects explored the same objects while wearing either thick gloves (to discourage contour-following) or splinted gloves (to prevent enclosure). Ratings of similarity were comparable in these two conditions, suggesting that neither exploratory procedure was necessary, by itself, for the extraction of either local or global shape. In Experiment 2, haptic exploration time was restricted to 1, 4, 8, or 16 sec. Limiting exploration time affected relative similarity in objects differing in their local but not their global shape. Together, the findings indicate that the hepatic system initially weights local features more heavily than global ones, that this differential weighting decreases over time, and that neither contour-following nor enclosure is exclusively associated with the differential emphasis on local versus global features.     

Haptic perception of parallelity in the midsagittal plane.

Previous studies [Perception 28 (1999) 1001; Perception 28 (1999) 781] on the haptic perception of parallelity on a horizontal plane showed that what subjects haptically perceive as being parallel deviates considerably from what is physically parallel. The deviations could be described with a subject-dependent orientation gradient in the left-right direction. The gradients found in the bimanual conditions were significantly larger (about 70%) than those in the unimanual conditions. The questions to be answered in the present study are the following: (1) Does the haptic perception of parallelity in the midsagittal plane also show systematic deviations from veridicality? (2) Are the unimanual and bimanual performances again quantitatively but not qualitatively different? The set-up consisted of a plate positioned in the midsagittal plane of the subject. The subject touched the right side of the plate with his/her right hand and the left side with the left hand. The results show again large systematic deviations. The major part of the deviations can be described by means of a subject-dependent orientation gradient in the vertical direction. The quantitative (but not qualitative) difference between the unimanual and the bimanual conditions is much larger in the midsagittal plane than in the horizontal plane.     

Haptic perception of the distance walked when blindfolded.

The ability to detect the distance walked when blindfolded using only haptic information generated by the walking activity was investigated. Participants walked a straight path until told to stop, turned, and attempted to return to their starting point. The path was completely featureless. Counting was prevented. Blindfolded, sighted participants traveled with a long cane or a trained sighted guide. Gait was varied or distorted. In all experiments the return distance was a linear function of the set distance, with some participants giving and some conditions resulting in remarkably sensitive performances. The magnitude of errors was a linear function of step length.