Good afternoon everyone, and welcome to my presentation. This presentation is about human perceptual
thresholds for motion stimuli. I think it is particularly relevant to those of you who
apply motion effects to audiovisual content for an immersive application, such as 4D cinema
and 4D rides. Let me start by saying a few words about us. My name is Jiwan Lee, and
the co-authors are Jaejun Park and Seungmoon Choi. I am a Ph.D. student under the supervision
of Professor Choi. We are in the interaction lab at POSTECH in Korea. This work was supported
by Samsung Science and Technology Foundation. The presentation is structured in three main
parts: background knowledge, absolute thresholds, and differential thresholds.
We first introduce some background knowledge related to our study. The starting point for
our research was the growing demand for motion effects in immersive applications. Motion
effect is employed for viewers’ more immersive and realistic experiences, and it serves as
vestibular feedback that somewhat coincides with the motion in a visual scene.
When 4D effect designers create motion effects, they combine six degrees of freedom of a motion
platform. (Video) Each degree of freedom corresponds to a rotation or translation around one of
the three axes in the three-dimensional space. For simplicity, here we call a motion that
moves only in a single degree of freedom a motion type. Designers, in general, use their
artistic flair to create motion effects manually. To create proper motion effects while using
different types of motion, they have no choice but to consider the perceptual performance
of humans. The human ability to perceive motion effects
can be quantified as two kinds of thresholds: absolute and difference. Absolute threshold
means the smallest amount of stimulation needed for a person to detect, and differential threshold
means the smallest amount of difference between two stimuli that can be detected.
When we summarize the measurements scattered across studies, please see these two tables,
there are some blanks. To make matters worse, these studies have experimented with different
experimental setups such as motion platforms or human body positions.
To provide a comprehensive basis for human vestibular perception, we measured both absolute
and differential thresholds for all six types of motion in the same setup. Based on our
experiences, we hypothesized that the sensitivity of rotation and translation for the z-axis,
I mean the earth-vertical axis, would be lower than that of motion for the other axes. That
is, yaw and heave, please see the rightmost chair motion, would have higher values of
both kinds of thresholds compared to the roll and surge, and pitch and sway.
We performed two psychophysical experiments to measure these kinds of thresholds. In both
experiments, we used the same motion platform shown in this slide and restricted participants
from receiving sensory information other than the vestibular information. This video shows
the six types of motion in the order of x-axis, y-axis, and z-axis with our motion platform.
To measure absolute thresholds of motion, as in Benson’s paper, we used a single cycle
of sinusoidal acceleration. We then found an appropriate intensity range for each motion
type by simple pilot test and used the ranges in the experiment. Using the motion stimuli,
we experimented according to the method of constant stimuli. It allows us to obtain the
entire psychometric function per motion type and thereby we are able to derive the motion
intensity of any perceptual probability. Please note that most studies reported only one point
of a psychometric function as a threshold, making it difficult to compare them with other
measurements or generalize. In this study, we defined the absolute threshold as the stimulus
intensity with a detection probability of 0.5. The probability value for a threshold
we chose is just a conventional choice. Five males and two females participated in the
experiment with an average age of 24. As a result, we obtained the six psychometric
curves. We colored the function, red for x-axis motion, green for y-axis motion, and black
for z-axis motion. When we ran the repeated-measures ANOVA with the absolute thresholds, the effect
of motion type on the three rotational thresholds was significant and also for the translational
thresholds. To measure differential thresholds of motion,
we used two cycles of sinusoidal acceleration as a motion stimulus. In particular, as in
other relevant studies, we also appended another half cycle of sinusoidal acceleration before
and after each stimulus. It was to prevent the motion platform from rattling due to the
waveforms starting and ending at non-zero velocity values. We set the intensity ranges
of the reference motion stimuli to take full advantage of the hardware performance of the
motion platform. We experimented according to the adaptive staircase method following
a two-up one-down rule. In this case, we are able to obtain one stimulus intensity, which
corresponds to a probability of 0.3 in the function. The method we employed here allowed
us to obtain the Weber fraction per motion type while efficiently measuring the differential
thresholds for multiple reference intensities. Seven males and three females participated
in the experiment with an average age of 26. As a result, we obtained a total of 18 measurements
and the six Weber fractions by regression. As with the absolute threshold results, we
colored the regression line, red for x-axis motion, green for y-axis motion, and black
for z-axis motion. According to the repeated-measures ANOVA, the motion type was a significant factor
on the rotational thresholds. As for translation, the differential thresholds were affected
by the motion type with marginal significance. Moreover, between the differential thresholds
and reference intensities, we found a strong linear relationship for roll, yaw and, surge
and a relatively weak linear relationship for the rest motion types.
Overall, including previously measured results, we were able to find one common pattern that
motion of the vertical axis shows the highest threshold. We hope that our results will be
useful for generating motion effects for 4D contents while considering the human sensitivity
to motion feedback.