ABML2011OS1-2

2011年11月3日-5日 東京（芝浦工業大学）

視覚知覚・認知計測技術と定量的疲労評価への応用
Quantitative measurement of human visual perception and its application to mental fatigue assessment○ 岩木直（産総研），原田暢善（フリッカーヘルスマネジメント(株)）Sunao IWAKI, National Institute of Advanced Industrial Science and Technology (AIST)
Nobuyoshi HARADA, Flicker Health Management Corp. (FHM)
Abstract: We have been developing techniques (1) to visualize spatiotemporal brain activities to investigate neuralmechanisms underlying the human visual cognitive functions, especially the 3-D object perception from motion, and (2) toevaluate mental fatigue quantitatively by measuring perceptual performance of the visually presented flicker, namely, theflicker-perception threshold (FPT) on consumer devices. In this paper, we describe a method to solve the long-standing
problem that the original flicker test inherently has, namely, arbitrariness and subjectivity in the subjects' response durin gdetermining FPT. We developed (a) a new method to control a subjective flickering sensation by changing the contra st of
the visual stimuli instead of changing temporal frequency, and (b) a new paradigm, in which forced-choice task
performance was used to determine FPT to eliminate arbitrariness of the subjects’ response. The proposed techniques wereimplemented as application softwares, which enables us to measure mental fatigue quantitatively using consumer mobile
devices such as smart phones and PCs.
Key Words: Visual cognitive function, Flicker perception threshold, Mental fatigue, Forced-choice task performance,
MEG

2. Contrast-controlled flicker stimuli (CCFS)
Instead of changing a temporal frequency as in the original
flicker test, we developed a method to control the degree ofsubjective flickering sensation by changing the contrast between
on and off periods of the flickering visual stimuli (contractcontrolled flicker stimuli: CCFS), which can be implemented
even on the display devices with fixed frame rates.
To validate the compatibility of the proposed method (CCFStest) with the original flicker test, we conducted fatigue

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Normalized FPT
1.04
1.00

Sleep

measurement tests in which 12 subjects were required tocontinue their office work, typically word processing on PCs,
overnight until 8:30 am next morning. Temporal changes inmental fatigue on the subjects were evaluated every 2 hours
from 2:30 pm on the first day by (a) the CCFS flicker test
implemented on standard PC with simulated 30 fps fixed frame
rate, (b) the CCFS flicker test implemented as a cell-phone
application with 15 fps fixed frame rate, (c) the original flicker
test (Shibata RDF-1), (d) subjective rating with visual analog
scale (VAS) (3), and (e) Japanese version of standard
questionnaire for subjective fatigue edited by Japan Society ofOccupational Health (4).
Figure 1 shows the results of the fatigue measurements on aSwimming

1. Introduction
Critical flicker frequency (CFF), defined as a frequency atwhich intermittent light stimuli appear to be steady for thehuman observer, is known to become lower as mental fatigue
accumulates (1). Although CFF has decades of history as areliable index of mental fatigue (flicker-test) (2), it requires a
dedicated device to measure, which impeded the diffusion of themethod to be used in general public.
The primary aim of this study is to develop a system toevaluate mental fatigue quantitatively and objectively by using
measurements of the flicker-perception threshold (FPT) on
consumer-devices such as cell-phones and PCs on which we do
not have precise control of the temporal frequency of visual
stimuli due to fixed display refresh rates (typically 30 or 60
frames per second (fps)).
The main topics of this research
were,(i) to develop and validate visual stimuli which is capable ofgradually change the degree of subjective flickering
sensation on raster-scan display devices with fixed frame
rates,(ii) to design a psychophysical procedure to solve thelong-standing problem that the original flicker test
inherently has, namely, arbitrariness and subjectivity inthe subjects' response during determining FPT, and(iii) to construct a system that can store and retrieve measured
data online.

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(a) FPT – PC (30 fps)(b) FPT - cell-phone (15 fps)(c) FPT - original flicker

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16:30

20:30 0:30
4:30
Time (HH:MM)

8:30

12:30

Fig. 1 Results of the FPT measurements during overnight
office work setting using (a) the CCFS implemented as
PC software, (b) the CCFS implemented on standard
cell-phone application, and (c) the original flicker test.

Normalized FPT

Normalized FPT obtained byoriginal flicker test

1.00.90.8

0.7 0.8 0.9 1.0 1.1
Normalized FPT obtained by
CCFS flicker test on PC

typical subject, in which normalized FPT values measured in
conditions (a), (b) and (c) were depicted. Results obtained byproposed CCFS tests ((a) and (b)) showed temporalcharacteristics similar to that obtained by the original flicker
test (c).
The results of the correlation analysis (Fig. 2) showed that theFPT values obtained from CCFS tests on PC have significant
correlation with those obtained from the original flicker tests(r=0.874, p=0.0077).
3. Forced-choice task for FPT detection (FC-FPT)In the original flicker test, measurements of FPT rely on thevoluntary response from the subjects on the subjective sensationof flicker. The arbitrariness on the subjects’ response may
introduce arbitrary biases on the fatigue measurementsTime
14:30 (Day 1)

8:30 (Day 2)

14:30 (Day 2)

depending on subjects’ intention to manipulate the results.
This could pose major problem especially when the proposed
technique was applied to the fatigue measurements in daily life
where no one would be supervising the measurement sessions.
To solve this problem, we designed a forced-choice task for
FPT detection (FC-FPT) in which subjects were required tochoose a CCFS target among other alternative non-flickering
stimuli (Fig. 3). The contrast of the CCFS targets weredecreased if the subject responded correctly, and the contrasts
were increased if the subject’s response was incorrect. TheFC-SPT task was repeated until the target contrast reached theconvergence. Results of the FPT measurements using FC-FPT
showed similar traces with the results obtained by original
flicker test during the overnight fatigue measurement sessions(Fig. 4)
4. Implementation on consumer devices and online
database system
The original idea of developing FC-FPT test by using CCFS
stimuli was to make the quantitative mental fatigue
measurement available in daily life. We have developed
application programs to assess temporal changes in mental
fatigue by measuring flicker perception threshold with CCFS
and FC-FPT detection algorithms. The application programsLong-term fatigue
data storaget

Decrease contrast

Incorrect
response・・・
Increase contrast

Internet
FPT test on
consumer devicest

Decrease contrast

Display FPT history(fatigue evaluation)

Incorrect
response

・・・
Target0.92

Correct
response

・・・

Target0.96

Fig. 4 Comparison between FPTs measured by the original
flicker test (gray line) and the FC-FPT test (black line) in
the typical subject during the overnight fatigue
measurement session.

Fig. 2 Results of the correlation analysis between FPT values
obtained by the CCFS flicker tests on PC and those by theoriginal flicker tests, both normalized by the values at thefirst measurements at 2:30 pm on the first day.

Target

FPT by the originalflicker test
FC-FPT

1.0

0.88

0.7
0.6

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Sleep

1.1

1.08

y = 1.167x - 0.174
R² = 0.7643
p=0.0077

tFig. 5 Fatigue measurement database system based on the FPT
test on consumer mobile devices and online datastorage.

・・・ Repeat until convergenceFig. 3 FPT detection using forced-choice task (FC-FPT)
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run on consumer mobile devices including smart phones that are
gaining popularity in recent years.
To store and retrieve measured FPT data seamlessly without
costing extra effort, we implemented an online fatigue
measurement database system (Fig. 5). The database isaccessible from the FC-FPT measurement application software
installed on the mobile device and the stored data are retrieved
on-demand in the form of line chart on which temporal changesof measured FPT values are displayed to show the personal
history of mental fatigue.
5. Conclusion
We developed a new method to control subjective flickering
sensation by changing the contrast of the flickering visual
stimuli instead of changing temporal frequency, and a newparadigm, in which forced-choice task performance was used todetermine the flicker-perception threshold to eliminate
arbitrariness of the subjects’ response. The proposed techniques
were implemented as application softwares to measure mental
fatigue quantitatively using consumer mobile devices such as
smart phones and PCs.
References(1) E. Simonson and N. Enzer, Measurement of fusion
frequency of flicker as a test for fatigue of the central
nervous system, Indust. Hyg. & Toxicol., vol. 23, pp. 83-89,
1941.
(2) W. Karwowski, International Encyclopedia of Ergonomics
and Human Factors, 2nd Edition, CRC Press, 2006.
(3) S. Grant et al., A comparison of the reproducibility and thesensitivity to change of visual analogue scales, Borg scales,and Likert scales in normal subjects during submaximal
exercise, Chest, vol. 116, pp. 1208-1217, 1999.
(4) http://square.umin.ac.jp/of/

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