Vol.7 No.4 2015
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Research paper : Mental fatigue measurement as application software on consumer devices (S. IWAKI et al.)−212−Synthesiology - English edition Vol.7 No.4 (2015) general-purpose display devices.At the same time, it is necessary to deal with new issues that may arise in the course of transferring the fatigue measurement from laboratory setting to our daily environment. Particularly, the following technology needs to be developed to realize the fatigue measurement in our daily life:(b) To develop a mechanism that allows users to conduct the test autonomously, that is, to develop the elemental technology that allows the measurement of flicker perception threshold, which was originally dependent on the subjective reporting of the examinee under the supervision of the examiner, in an objective manner even by the user alone.3.1 Measurement of flicker perception threshold using the general-purpose display: Contrast-controlled flicker stimuli (CCFS)First, we developed a technique to allow the CFF measurement on general-purpose display devices, based on the previous research that there is a systematic relationship between the flicker perception threshold in the flashing frequency and the brightness contrast (a difference in brightness between ON and OFF of the flashing light stimuli).[15][16] Figure 2 shows the relationship between the flashing frequency and the brightness contrast at which the observer perceives a flickering sensation (flicker perception threshold: FPT). (edited based on References [15] and [16]). The FPT is affected by changes in both the flashing frequency and the brightness contrast. When the measurement value in normal conditions is the black line, it changes to the grey line in conditions of fatigue (Fig. 2). This means that under the same brightness contrast, flicker is readily perceived as the flashing frequency decreases, and under the same flashing frequency, flickering is readily perceived as the brightness contrast increases. In the conventional flicker test, the frequency at which the subject perceived the flicker (CFF) was measured under the constant brightness contrast. In contrast, the change of FPT due to fatigue can be also characterized by using the brightness contrast threshold under the constant flashing frequency. Using this characteristic, it is possible to measure the change of FPT due to fatigue in an image display device with fixed refresh rate. We devised the contrast-controlled flicker stimulus (CCFS) method[17] that allows fatigue measurement with accuracy equivalent to the conventional flicker test, by changing the brightness contrast between high (ON) and low brightness (OFF) of the flashing visual stimuli, rather than changing the flashing frequency as in the conventional flicker test. Figure 3 shows the measurement results of the state of mental fatigue accumulation as the subject engages in the office work such as creating materials on a computer from 14:30 to 8:30 the next day, using: (i) the CCFS implemented on a cell phone with display of refresh rate of 30 Hz; (ii) the CCFS implemented on a personal computer with display of refresh rate of 60 Hz; and (iii) the specialized device (RDF-1, Sibata Scientific Technology Ltd.) for conventional flicker test (average of 12 subjects; error bar represents the standard deviation). In all methods, accumulation of mental fatigue due to the overnight work and the recovery of fatigue by short naps were appropriately measured as in the conventional flicker test. It was also found that the results obtained by CCFS flicker test were significantly correlated with the test results of the conventional flicker test (Fig. 4).These results show that the FPT measurements by CCFS implemented on the widely available consumer devices can be used as an alternative to the original flicker test which requires the specialized device in evaluating the progressive Fig. 3 Results of the fatigue measurement under overnight workload[17]Measurements conducted by contrast-controlled flicker stimuli (CCFS) application implemented on a cell phone (thin grey line) and a PC (thin black line), and by a specialized device for conventional flicker test (thick line).Fig. 2 Relationship between the flashing frequency and ON/OFF brightness contrast in the flicker perceptionFlicker perception threshold in normal stateFlicker perception threshold in fatigue stateChange of perception threshold by fatigue on contrast domainChange of perception threshold by fatigue on frequency domainFlashing frequencyON/OFF brightness contrast of flashing stimulusFlicker perception threshold normalized by the first measurementTime (HH:MM)(a) FPT-CCFS on PC (30 fps)(b) FPT-CCFS on cell (15 fps)(c) FPT-original flicker0.750.800.850.900.951.001.0512:3016:3020:300:304:308:3012:30Nap

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