Abstract
Introduction
High physical and cognitive strain, high pressure, and sleep deficit are part of daily
life for military professionals and civilians working in physiologically demanding
environments. As a result, cognitive and physical capacities decline and the risk of
illness, injury, or accidents increases. Such unfortunate outcomes could be prevented by
tracking real-time physiological information, revealing individuals’ objective fatigue
levels. Oculometrics, and especially eyeblinks, have been shown to be promising
biomarkers that reflect fatigue development. Head-mounted optical eye-trackers are a
common method to monitor these oculometrics. However, studies measuring eyeblink
detection in real-life settings have been lacking in the literature. Therefore, this
study aims to validate two current mobile optical eye-trackers in an unrestrained
military training environment.
Materials and Method
Three male participants (age 20.0 ± 1.0) of the Swiss Armed Forces participated in this
study by wearing three optical eye-trackers, two VPS16s (Viewpointsystem GmbH, Vienna,
Austria) and one Pupil Core (Pupil Labs GmbH, Berlin, Germany), during four military
training events: Healthcare education, orienteering, shooting, and military marching.
Software outputs were analyzed against a visual inspection (VI) of the video recordings
of participants’ eyes via the respective software. Absolute and relative blink numbers
were provided. Each blink detected by the software was classified as a “true blink” (TB)
when it occurred in the software output and the VI at the same time, as a “false blink”
(FB) when it occurred in the software but not in the VI, and as a “missed blink” (MB)
when the software failed to detect a blink that occurred in the VI. The FBs were further
examined for causes of the incorrect recordings, and they were divided into four
categories: “sunlight,” “movements,” “lost pupil,” and “double-counted”. Blink frequency
(i.e., blinks per minute) was also analyzed.
Results
Overall, 49.3% and 72.5% of registered eyeblinks were classified as TBs for the VPS16
and Pupil Core, respectively. The VPS16 recorded 50.7% of FBs and accounted for 8.5% of
MBs, while the Pupil Core recorded 27.5% of FBs and accounted for 55.5% of MBs. The
majority of FBs—45.5% and 73.9% for the VPS16 and Pupil Core, respectively—were
erroneously recorded due to participants’ eye movements while looking up, down, or to
one side. For blink frequency analysis, systematic biases (±limits of agreement) stood
at 23.3 (±43.5) and −4.87 (±14.1) blinks per minute for the VPS16 and Pupil Core,
respectively. Significant differences in systematic bias between devices and the
respective VIs were found for nearly all activities (P < .05).
Conclusion
An objective physiological monitoring of fatigue is necessary for soldiers as well as
civil professionals who are exposed to higher risks when their cognitive or physical
capacities weaken. However, optical eye-trackers’ accuracy has not been specified under
field conditions—especially not in monitoring fatigue. The significant overestimation
and underestimation of the VPS16 and Pupil Core, respectively, demonstrate the general
difficulty of blink detection in the field.
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