Vol.7 No.4 2015
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Research paper : Mental fatigue measurement as application software on consumer devices (S. IWAKI et al.)−211−Synthesiology - English edition Vol.7 No.4 (2015) if there is a presentation of sensory stimuli to evoke visual, auditory, or somatic perception, as well as a way to collect responses from the user, it may be possible to achieve a system using a device to which we have daily access.2 Scenario for realizing a fatigue measurement system that can be used in daily lifeWe decided to develop a technology that enables the quantitative evaluation of fatigue based on the changes in flicker perception that is presented visually using general-use electronic devices such as smart phones, personal computers, or car navigation systems.There is a frequency threshold at which “flickering” can be perceived when the frequency of the intermittent point-light stimulus is gradually decreased (this is called the critical fusion frequency or critical flicker frequency, CFF) (Fig. 1). Since it was first reported in 1941 that CFF declined with accumulation of fatigue,[7] it became widely known as a quantitative measure for degree of fatigue. Since CFF has the properties that (i) change monotonically in time with continuous workload (that is, accumulation of fatigue), and (ii) are stable with small fluctuations between measurements, it has been used as an important research tool in the field of industrial health, industrial physiology, and traffic psychology (flicker test).[12][13] The flicker test is thought to measure the flicker perception threshold that changes according to the excitability of the central nervous system including the brain cortex or the change in the level of arousal due to accumulation of fatigue. Since there are personal differences in CFFs, it is reasonable to measure CFF of the subject at a normal state, and to determine the fatigue condition at a given time based on “how much the measured CFF changed compared to the normal condition.” For example, caution must be issued when CFF is decreased 5 % compared to the normal state, or one must take a break if it decreases 10 % or more. According to the previous research, in the case where CFF decreases 10 % or more, it is known that the cognitive and behavioral performance degrades significantly as represented by the worsening of scores on a simple math test.[14]Major technological issue when conducting the flicker test using personal computers and smartphones is “how to measure the flicker perception threshold using the display of general-purpose electronic devices.” In the conventional flicker test, LED is used to gradually change the flashing frequency of visual stimulus by 0.1 Hz units to determine the threshold at which the subject perceives the flickering sensation. On the other hand, for the display screen of smartphones or PCs, the vertical synchronizing frequency (refresh rate) is fixed at a certain value (15 or 30 Hz with typical cell phone displays; 60 Hz with PCs), and the flashing frequency cannot be controlled at 0.1 Hz accuracy. Therefore, it is necessary to develop a method to evaluate the flicker perception threshold that is compatible with CFF but not dependent on the changes in the flashing frequency of visual stimulus.Also, in the conventional flicker test, the method of limit, where the flashing frequency is gradually and continuously changed at certain intervals and the subject is asked to push a button at a point when he/she subjectively perceives the flickering, is used to determine the frequency threshold of the flicker perception. With this method, it is difficult to remove the contamination from habituation and expectation as one takes the flicker test repeatedly, as well as the bias from subject’s arbitrariness or intention to manipulate the results. These issues had not been critical problems in conventional use such as in a laboratory setting where the examiner and the subjects sat face-to-face while collecting the data, and thereby it had been possible to maintain the subject’s motivation for accurate data measurement. However, in the case of measuring the flicker perception threshold autonomously without an examiner in daily life, avoiding the bias from arbitrary operation by the examinee during measurement becomes an issue that must be taken into serious consideration.We developed techniques to bring the reliable fatigue measurement method based on the flicker perception threshold, which is originally used for academic research in the laboratory setting, to our daily life. We also constructed a prototype system by integrating the elemental technologies, and conducted fatigue evaluation experiments in the real environment to verify the effectiveness of our approach.3 Elemental technologies to realize a simple fatigue measurement system that can be used dailyAs mentioned above, to realize the quantitative evaluation of mental fatigue in the daily environment by flicker perception threshold measurement that has been used for research in a lab setting, it is mandatory to solve the following technological issues:(a) To develop the elemental technology that allows the measurement of flicker perception threshold, which was measurable only on specialized devices, to be made on Critical Flicker Frequency (CFF)30 Hz100 Hz50 HzFlickering sensationFixed lightFig. 1 Critical flicker frequency (CFF)

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