Hello, my name is Judith Hartfill, I am a Ph.D. student at the Human-Computer Interaction
group at Universität Hamburg and I will present our paper “Analysis of Detection Thresholds
for Hand Redirection during Mid-Air Interactions in Virtual Reality”.
We analyzed detection thresholds for redirected virtual hands with a redirection method where
the virtual hand was decelerated, so that the real had to be moved further. The blue
hand in the video shows the user’s real hand position and was not visible during the
experiment. Handedness and motion direction were of particular
interest to us, so we included both hands and 6 different motions per hand. We applied
our findings in a confirmatory study. Using the hands is one of the most intuitive
ways to interact with a virtual environment, and camera-based hand tracking is supported
by more and more head-mounted displays so that no additional hardware like tracking
gloves are needed. Humans perceive the position of body parts,
such as hands, through multisensory integration by merging information from the visual system,
as well as proprioceptive and kinesthetic information.
Due to the dominance of the visual system in this process, perception of the hands’
real position can be manipulated in VR. If these manipulations are small enough, they
can stay unnoticed by the user. Decelerated virtual hands might be beneficial
in therapeutic applications, for example for pain therapy or as preventive exercises for
the elderly. Users could perform larger movements than they would actually do so that the exercises
could be performed more intensively. It could furthermore reduce patients’ fear of pain,
as patients could be encouraged to perform motions that they would usually avoid.
Here is a short overview of related works. There are already works that reported detection
thresholds for angular or gain-based hand redirection methods. However, the setup and
focus of our work are quite different. Previous works looked at pointing motions
or letting the participants move their hands freely, while in our setup we used 6 different
predefined motions: both directions on all three spatial axes.
While other works only\h visualized the participants'\h
hands, we used a whole-body avatar to enhance
the feeling of embodiment. The avatars’ hand size and arm length were adjusted for
each participant. We used the hand tracking of the Oculus Quest
2, which supports accurate finger tracking. Furthermore, both hands were included in our
setup, while in previous works only the right hand regardless of the user’s handedness
was used. We estimated detection thresholds using a
staircase method, which will I will explain in more detail later.
The redirection method we used applies a linear gain to the virtual hand on all axes. We used
gain factors between 0.4 and 1 for the decelerated movements, where 1 is identical to the actual
hand movement. The redirection was applied to the hand root bone, so that only the hand
position, not the fingers, were manipulated. For the avatar's arm motion, we used an inverse
kinematics approach. We analyzed detection thresholds for both
directions on each spatial axis: inward and outward, forward and backward, and upward
and downward. In the image, you can see the start and end
positions of each motion for both hands. After each trial/movement, participants were
asked to answer the following question:\hWas the movement of the virtual hand identical
or not identical to your real hand? We were especially interested in two things.
First, is there a difference in detection thresholds between the movement directions
on each axis? And second, do\h detection thresholds differ
between the dominant and non-dominant hand? We expected a higher sensitivity in the dominant
hand. Participants proceeded through 480 trials
in total, in 24 conditions (6 motion directions, 2 Hands: left and right, 2 redirection modes:
decelerated and accelerated). Although we were especially interested in decelerated
redirections, accelerated hand movements were included to avoid habituation effects.
Each condition had two starting points or staircases, as you can see in the graph, starting
from the highest and lowest gain factor in that condition (1 and 0.4 for decelerated
movements). If the movement of the virtual hand was perceived to be identical, in the
next trial more redirection was applied. For the decelerated redirection mode, this means
a lower gain factor. If the movements of the hands were perceived to be not identical,
less redirection was applied in the next trial (gain factor increased in decelerated mode).
We calculated the detection thresholds by taking the mean of the last 5 trials from
both staircases, as the staircases had developed around the participants' true detection threshold
until then. We had 20 participants, 12 of them were male
with a mean age of 25.3 years. The detection thresholds for each movement direction are
in a range between 0.727 and 0.552. We found a significant main effect for movement
direction, with a higher sensitivity for inward motions compared to outward motion and the
same for upward and downward as well as forward and backward.\h\h
We could not find significantly
different detection thresholds for the dominant or non-dominant hand. Standard deviations
for each condition are between 0.119 and 0.167. Based on this, more conservative detection
thresholds can be derived.\h
We applied the detection thresholds\h into a simple playful application\h\h
and conducted a small confirmatory study with six\h participants, three younger and three older adults.\h
The task of the game was to touch soap bubbles\h to free the butterflies within them. Participants\h\h
were asked to play two versions of the game: first\h with “normal” hands and after that with redirected\h\h
hands, where we applied redirection on each\h movement direction according to the thresholds\h\h
we found. One participant explicitly noticed hand\h movement was manipulated in the second version,\h\h
one other one said that bubbles were harder to\h reach but reported that real and virtual hands\h\h
were aligned all the time and hypnotized, that the\h bubbles were further away in the second version.\h\h
The other four participants did not notice\h any difference between both versions.
To summarize our work: We found significantly different detection thresholds for decelerated
hand motions for both motion directions on each spatial axis. So for an optimal experience,
redirection should be applied according to the lowest detection threshold (forward 0.727)
or the virtual hand position needs to be reset to avoid drifting over time. Interestingly,
an effect of handedness could not be found. The confirmatory study showed that detection
thresholds mainly hold when applied in a game context. If these values also work for pain
patients and how a therapeutic application based on unnoticeable hand redirection could
be developed remains to be investigated.