Vol.7 No.4 2015
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Research paper : Mental fatigue measurement as application software on consumer devices (S. IWAKI et al.)−213−Synthesiology - English edition Vol.7 No.4 (2015) development of fatigue due to overnight mental workload.3.2 Autonomous measurement of flicker perception threshold without the examinerIn the conventional flicker test, the method of limits is used where the flashing frequency is gradually and continuously changed at fixed intervals and the subjects are asked to respond by pushing the button at the moment he/she subjectively perceives the flickering sensation, and it is difficult to eliminate the contamination from biases including the subject’s arbitrariness in responding to the perception of flickering sensation. This has not been a major problem as far as the method was used in the laboratory setting where the examiner and examinee sat face-to-face during data collection, hence the examiner could intervene as needed to avoid the possibility of the contamination. On the other hand, it is going to be an important issue to be solved when the method is applied to the measurement of flicker perception threshold in daily life where the examinee has to conduct the test autonomously without an examiner.To solve this issue, we introduced the forced-choice up/down (FCUD) method which adjust the stimulus parameter, brightness contrast in this case, adaptively corresponding to the subject’s response so that the parameter converges to the flicker perception threshold.[17] Specifically, as shown in the schematic diagram of Fig. 5, the subject is required to choose a flickering target stimulus among the multiple stimuli presented. If the subject’s reaction is correct, i.e., the subject chooses the flickering target stimulus which is randomly placed among other stationary alternatives, the brightness contrast of the target stimulus is decreased in the next trial to make the forced-choice task more difficult. Otherwise, the brightness contrast of the target is increased in the next trial until the subject can make a right choice. These steps are repeated until the brightness contrast is converged to the flicker perception threshold.The accuracy of the flicker perception threshold measurement can be improved by using FCUD algorithm since it allows denser sampling around the perception threshold compared to the original flicker test where the brightness contrast is changed at a constant rate regardless of the subject’s response (Fig. 6). The FCUD also contributes to reduce the amount of time required to complete the test by accelerating the convergence at the brightness contrast range where the flickering is less likely to be perceived (Fig. 6).4 Integration of technologies for fatigue measurement in daily lifeTo verify the effectiveness of the techniques described in subchapters 3.1 and 3.2 to be used for the fatigue measurement in daily life, we created a prototype system combined with an online database to manage the measured Normalized flicker perception threshold by CCFSNormalized flicker perception threshold by conventional flicker testy = 1.167 x - 0.174R2 = 0.7643p < 0.00010.60.70.80.91.01.10.70.80.91.01.1 Flicker perception thresholdFlicker perception threshold(b) Flicker perception threshold measurement proposed in this study (forced-choice up/down method)(a) Conventional flicker test (method of limit)・・・Range of the brightness contrast where flicker cannot possibly be perceivedfCorrectIncorrectCorrectCorrectCorrectIncorrectCorrectCorrectCorrectCorrectCorrectKeyresponsesFlicker perception threshold = (ΣiNxi)/NButtonpressButtonpressButtonpressButtonpressButtonpressRange of the brightness contrast where flicker cannot possibly be perceivedTimeTimeX1X2X3X4X5Incorrect responseCorrect responseCorrect responseTargetTargetTargetIncrease brightness contrast(+5)Decrease brightness contrast(-3)Decrease brightness contrast (-8)… Repeat until convergenceFig. 5 Schematic diagram for the method of determining the flicker perception threshold by the forced-choice up/down (FCUD) methodFig. 6 Differences in the methods for determining the flicker perception threshold in the conventional and the proposed methodsFig. 4 Correlation between the flicker perception threshold measured using the contrast-controlled flicker stimuli (CCFS) and the conventional frequency-controlled flicker stimuli[17]

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