Does touch feedback make grasping in virtual reality more natural?
When people 'grasped' virtual objects with no touch feedback, the precision of their hand opening got worse for bigger objects the way perception does, but when a real object of the same size was there to touch, this size-dependent imprecision disappeared, as it does for normal grasping.
Source
Grasping Weber's Law in a Virtual Environment: The Effect of Haptic Feedback
Study at a glance
- Design
- Human experiment — Two between-experiment conditions (no haptic feedback vs. matched physical objects giving touch feedback) with object size, movement phase and practice block manipulated within subjects in an HTC Vive VR bimanual grasping task
- N
- N=27 · 14 undergraduates in Experiment 1 (no feedback); 14 different participants in Experiment 2 (haptic feedback), one of whom was excluded for not following instructions
- Population
- Healthy undergraduate students at Ben-Gurion University, Israel
- Outcome
- Just noticeable differences (within-subject SD of grip aperture) across object sizes and movement phases; grip aperture, maximum grip aperture, reaction time and movement time
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What they did
Participants wore a VR headset and reached with both hands to grasp the ends of virtual tube-shaped objects that were 15, 25, 35 or 45 cm long, with motion trackers on their forearms recording the distance between the hands. In Experiment 1, the hands met nothing at the end of the reach; in Experiment 2, a different group performed the same task but a physical polyester object of matching size sat where the virtual one appeared, so they felt it on contact. Each person did three blocks in which every object appeared 15 times (180 trials in total), and the researchers measured how variable the grip aperture was (the just noticeable difference, JND) at each tenth of the movement.
What they found
Grip apertures scaled clearly with object size in both experiments. Without touch feedback, JNDs in the final part of the movement grew steadily with object size (about 1.4, 1.5, 1.6 and 1.8 cm), which is the pattern Weber's law predicts for perception. With touch feedback, JNDs did not increase with size, and a direct comparison showed the two experiments' JND patterns differed significantly. In the feedback experiment, JNDs also shrank after the first block, suggesting practice improved precision, and in both experiments movements were slower than typical real-world grasps.
The limits
What it doesn't show
The samples were small (14 people per experiment, with one of the second group excluded) and the feedback comparison was between different groups rather than within the same people. The touch feedback came from real objects placed in the scene, so the study does not show that current haptic gloves or other VR feedback devices would have the same effect, a point the authors raise themselves. In the first block of each experiment the rise in JNDs with size could reflect a speed-precision tradeoff rather than genuine Weber's-law behaviour, and movements in both conditions were still atypical (slow, with early peak aperture). The measure at the moment of maximum grip aperture did not show the Weber pattern even without feedback, so conclusions depend on analysing later movement phases.
Key terms
- Weber's law
- The principle that the smallest detectable change in a stimulus grows in proportion to the stimulus's size, so perception is relative rather than absolute.
- Just noticeable difference (JND)
- The smallest difference that can be resolved; in grasping studies it is estimated as the within-person standard deviation of grip aperture for a given object size.
- Maximum grip aperture (MGA)
- The widest opening of the hands or fingers during a reach, usually reached before contact and scaled to object size.
- Perception-action dissociation
- The idea that vision for recognising objects and vision for guiding actions rely on partly separate processes, so actions can escape perceptual biases.
- Speed-precision tradeoff
- When faster movements are also more variable, which can make variability rise with object size for reasons unrelated to perceptual resolution.
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Quiz yourself
According to Weber's law, how should the just noticeable difference change as object size increases?
Common questions
Why does it matter whether grasping follows Weber's law?
Perceptual judgments of size follow Weber's law, but normal grasps of real objects usually do not. If a grasp shows Weber's-law variability, it suggests the action is relying on relative, perception-like size information rather than the precise, absolute coding typical of real-world action.
Did touch feedback change how wide people opened their hands?
Not in any major way; hand opening matched object size in both experiments. What changed was the variability pattern: without feedback it grew with size, while with feedback it stayed flat across sizes.
Does this mean commercial VR haptics would work as well?
Not necessarily. The feedback here came from real objects of exactly matching size, and the authors note that tactile gloves or other devices may still have precision and timing problems that could undermine the effect.
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