Proceedings of the 2021 IEEE International
Conference on Intelligence and Safety for Robotics
Nagoya, Japan, March 4-6, 2021

Temporal characteristics of skin conductance and subjective intensity inresponse to unpleasant audio-visual and vibrotactile stimuli
Ibuki Tara, Shogo Okamoto, Yasuhiro Akiyama, and Yoji Yamada
used as warning stimuli [7]. Nonetheless, they can be linked
to positive emotions [8], [9], and they lead to a variety ofapplications. We used skin conductance for physiologicalmeasurement, which is suitable as an interface for humanmachine systems because of its quick response to stimuli andmeasurement accessibility. Further, the time-intensity method[10] was used to record subjectively reported emotionalstates.

Abstract— In this study, we measured and modeled thechanges in physiological and subjective signals when the participants were subjected to unpleasant stimuli, such as, a horror
movie and vibrotactile stimuli to the upper body. We measured
skin conductance as a physiological value and grip force, which
was adjusted to express the level of subjective unpleasantness.
We then modeled the skin conductance and grip force as
second-order lag systems and computed their characteristic
parameters. The damping ratios for skin conductance and grip
force were slightly greater than 1.0, and the natural frequencyof the grip force was greater than that of the skin conductance.
The response delay of the skin conductance was longer than thatof grip force by approximately 1 s. In terms of the behavior of
the skin conductance, the differences among individuals and thetypes of stimuli were almost null from the stimulus onset to thepeak, but after the peak, the differences were substantial. The
temporal changes in the subjective intensity were signiﬁcantly
different among participants. These characteristics are usefulfor designing emotionally responsive systems that use emotionalsignals to adapt their behavior.

II. M ETHOD
A. Unpleasant audio-visual and vibrotactile stimuli
We used a horror video [11] and vibrotactile stimuli. The
video was approximately three minutes long in which a man
found a polaroid camera and took photos of his room, then henoticed a devil in the photo, and ﬁnally, a real devil appearedand assaulted him.
We used vibrotactile stimuli to induce unpleasant emotions. As described in Section II C, vibrators were attached tothe eight points on the trunk of the participant and generated
a vibration of 70 Hz with a constant amplitude for 7 s.

I. INTRODUCTION
The demand for human-system cooperation technology,
such as collaborative and communication robots [1], [2] hasbeen increasing. Estimation of emotional states is a key
component of such technologies. Physiological indexes suchas heart rate, skin conductance [3], and facial images [4]are used to estimate the user’s emotional states. Estimatedemotional states are used by robotic and information systems
that behave or react in response to the emotional statesof the user. Alternatively, systems exist that use emotional
states that are subjectively reported by users. Hence, both
physiological and subjective indices are available for humansystem cooperation. Emotional states change temporally, andwe need to understand their dynamic characteristics to design
emotionally responsive systems. Many studies measured andmodeled temporal responses of physiological signals [5], [6].
However, few studies have compared the temporal characteristics of physiological and subjective responses to the samestimuli.
This research focused on the dynamics of emotional statesignals. We measured and modeled the temporal changes inthe skin conductance and subjectively reported emotional
states in response to unpleasant stimuli that evoke frightor warning, and speciﬁed their temporal characteristics. We
used audio-visual stimuli, such as a horror video and vibrotactile stimuli, to trigger emotional changes. Vibrotactile
stimuli tend to provoke unpleasant feelings and are often

B. Measurement of emotional responses
We measured the temporal evolution of skin conductanceas a physiological index. Skin conductance is calculated fromthe voltage difference between two electrodes attached to theskin surface. Sweating increases this value.
We used the time-intensity method [10] for recording subjective emotional state. In this method, the participant reports
the transient feeling level, typically by using a joystick orvisual-analog scale. In our study, a grip force was used toexpress the intensity because the gripping task did not incur
any visual loads. The participant was instructed to adjust thegripping force according to his/her transient unpleasantness.
C. Apparatus
A video player, vibrators, and the measurement of skinconductance and grip force were controlled or monitored
by a single microcomputer (mbed, ARM Ltd., England)
to share the same temporal base. A digital signage controller (BS/HD220, BrightSign, CA) received an ignitioncommand from the microcomputer, to play audio-visualcontent. Voicecoil motors (Vp408, Acouve Lab., Japan) wereattached to the chest, abdomen, waist, and shoulders througha vest. The vest was adjusted such that the voicecoil motors
were ﬁrmly in contact with the wearer’s body. We used
Bitalino (BPK160616, Plux-Wireless Biosignals, Portugal) tomeasure skin conductance. Pairs of electrodes were attachedto the palms of each participant after the palms were cleaned.

This study was in part supported by MEXT Kakenhi (20H04263).
All are with the Department of Mechanical Systems Engineering, Nagoya
University, Japan.

978-0-7381-4674-4/21/$31.00 ©2021 IEEE24•Fig. 1.

Latency: The time between the stimulus onset and thetime for the skin conductance or grip force to startrising,• Rising time: The time between the point at which theskin conductance or grip force starts rising and the timeat which its peak value is recorded,• Half-recovery time: The time between the peak andthe point at which the skin conductance or grip force
recovers to the half peak,• Recovery time constant: The time between the peak andthe point at which the skin conductance or grip force
recovers to 37% of the peak.
We selected two scenes that could strongly induce fright andfor which the beginning of the horror scenes could be clearlydeﬁned. There were some sections where the half-recovery
time or recovery time constant of the skin conductance could
not be calculated because the value converged to more than
50% or 37% of the peak amplitude. Hence, we could notacquire a sufﬁcient number of samples in the experiment ofvibrotactile stimuli, and we excluded the half-recovery andrecovery time constants from the analysis.
Furthermore, we computed two parameters describing thedynamics of systems. The response of skin conductance wasmodeled as the free vibration of an overdamped second-orderlag system [5]. There is no standard model for the timeintensity signals; hence, we also applied the second-orderlag system to the grip force to compare their temporal characteristics. For the model construction, the response signals
after the rising points were used. The model estimation wasperformed based on the least-squares method. The model
formula was,()(1)x(t) = A e−λ1 t − e−λ2 t ,Vibrotactile vest with eight high-ﬁdelity voicecoil motors.

Fig. 2. Four types of parameters were measured for the skin conductanceand grip force: latency, rising time, half recovery time, and recovery time
constant.

To record the subjective intensity of feeling, we used ahome-made grip dynamometer based on a strain-gauge-typed
loadcell (1040-0020-F000-RS, Tedea-Huntleigh, Israel). The
participant grasped the dynamometer in his/her dominant
hand.

where A determines the amplitude of the skin conductanceor grip force changes, and λ s are real positive values
to determine their response proﬁles. The second-order lag
elements were calculated using λ1 and λ2 as follows:

D. Experimental tasks
We conducted two experiments to measure human responses to audio-visual and vibrotactile stimuli. Four university students who were unaware of the study objectives
participated in two tasks.
In one experiment, the participants wore the vibrotactilevest and experienced vibrotactile stimuli in a resting statewith eyes closed. The change in the skin conductance wasmeasured. We did not inform the participants of the timing
to start the vibrotactile stimuli. This measurement was conducted one time for each individual.
In another experiment, the participant watched a threeminute horror video, and the subjective intensity of unpleasantness was measured by the time-intensity method using agrip dynamometer. During this task, the participant’s skinconductance was also recorded. The audio-visual stimulus
task was performed only once by each participant.

λ1 + λ2,
ζ= √2 λ1 λ2√
ωn = λ1 λ2 .

(2)
(3)

We excluded outlier proﬁles that included shapes that did
not ﬁt the curve of second-order lag systems and at least one
parameter that was out of the two times standard deviations.
IV. R ESULTS
Fig. 3 shows an example of the skin conductance change
after experiencing the vibrotactile stimuli. Fig. 4 showschanges in the skin conductance and grip force after experiencing the audio-visual stimuli, i.e., a horror scene. Fig. 5
shows the regression curves for the second-order lag systems.
Table I summarizes the mean and standard deviation of theparameters among the four participants.
For skin conductance, the latency and rising time wereapproximately 1.4 s and 1.9 s, respectively, irrespective ofthe type of stimuli. The differences among the individuals

III. A NALYSISAs shown in Fig. 2, four types of parameters [10], [12]were computed from the acquired time-series responses.
They include

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Fig. 3.

An example of skin conductance response to vibrotactile stimuli.

Fig. 5. Examples of measured and regression curves of the skin conductance and grip force.

Fig. 4. An example of skin conductance and grip force responses to audiovisual stimuli.

and scenes in the video were negligible. Conversely, thehalf-recovery and recovery time constants varied signiﬁcantly
among the participants and the type of stimuli. These tendencies of the response parameters are in accordance withthe general skin conductance characteristics [12], where thelatency is typically between 1 and 2, and the rising time isbetween 1 and 3.

TABLE IM EAN AND STANDARD DEVIATION OF THE RESPONSE PARAMETERS

Skin conductance

Grip force

Skin conductance

Audio-visual stimuli
Latency
Rising time
Half recovery time
Recovery time constant
λ1
λ2
ζ
ωn
Latency
Rising time
λ1
λ2
ζ
ωn
Vibrotactile stimuli
Latency
Rising time
λ1λ2
ζ
ωn

1.42 ± 0.27 s
1.87 ± 0.27 s
3.81 ± 3.61 s
4.15 ± 2.08 s0.45 ± 0.14
0.65 ± 0.11
1.03 ± 0.03
0.54 ± 0.11 rad/s
0.23 ± 0.13 s
0.56 ± 0.18 s
1.01 ± 0.15
2.47 ± 0.72
1.11 ± 0.07
1.56 ± 0.30 rad/s

The latency and rising time of subjective intensity wereshorter than those of the skin conductance by approximately
1 s, and on average, the subjective intensity began torise at 0.23 s after the stimulus onset, whereas the skinconductance began to change at 1.42 s on average. The
coefﬁcients of variation, that is, the ratios of the standard
deviation to the mean of grip force, tended to be larger thanthose for skin conductance. This is because the measured
value was consciously determined by the participants in thetime-intensity method, whereas the skin conductance valuechanged automatically. The natural frequencies, ωn , of thegrip forces were 1.56 ± 0.30 rad/s and larger than those forskin conductance (0.54 ± 0.11 rad/s), which indicates thatthe rising time to the peak was less for the grip force.
The damping ratios of the second-order lag models for skinconductance and grip force were slightly greater than 1.0.

1.43 ± 0.02 s
1.93 ± 0.26 s
0.33 ± 0.07
0.85 ± 0.10
1.12 ± 0.03
0.53 ± 0.09 rad/s

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V. C ONCLUSIONSIn this study, we measured and modeled the temporalchanges in skin conductance and voluntary grip force in
response to unpleasant audio-visual and vibrotactile stimuli
and analyzed their characteristics. The delay in skin conductance was longer than that of the subjective intensity
by approximately 1 s. The differences among the individuals and stimuli were almost null in the behavior of skinconductance from the stimulus onset to the peak, but thepost-peak differences were substantial. The rising time of
the grip force was substantially smaller than that of theskin conductance. Accordingly, the natural frequency of thegrip force was greater than that of the skin conductance.
These results collectively indicate that the grip force, as anindicator of subjective intensity of unpleasantness, changes
more rapidly than skin conductance. These differences in thesubjective and physiological responses to unpleasant stimuli
should be reﬂected in the design of emotionally responsive
human-machine systems. The number of participants in theexperiments was small, and we could not make statisticalconclusions. In the future, a larger number of participants
should be involved. Furthermore, although we used thesecond-order lag system for modeling the subjective grip
force, we need to consider the validity of this model for
subjectively reported emotional intensity.
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