Maintaining concentration on demanding cognitive tasks, such as vigilance (VG) and working memory (WM) tasks, is crucial for successful task completion. Previous research suggests that internal concentration maintenance fluctuates, potentially declining to sub-optimal states, which can influence trial-by-trial performance in these tasks. However, the timescale of such alertness maintenance, as indicated by slow changes in pupil diameter, has not been thoroughly investigated. This study explored whether "pupil trends"-which selectively signal sub-optimal tonic alertness maintenance at various timescales-negatively correlate with trial-by-trial performance in VG and WM tasks. Using the Psychomotor Vigilance Task (VG) and the Visual-Spatial 2-back Task (WM), we found that human pupil trends lasting over 10 seconds were significantly higher in trials with longer reaction times, indicating poorer performance, compared to shorter reaction time trials, which indicated better performance. The Attention Network Test further validated that these slow trends reflect sub-optimal states related to (tonic) alertness maintenance rather than sub-optimal performance specific to VG and WM tasks, which is more associated with (phasic) responses to instantaneous interference. These findings highlight the potential role of detecting and compensating for non-optimal states in VG and WM performance, significantly beyond the 10-second timescale. Additionally, the findings suggest the possibility of estimating human concentration during various visual tasks, even when rapid pupil changes occur due to luminance fluctuations.Significance Statement Using biomarkers to estimate human concentration levels can adaptively enhance performance in daily activities. Theoretically, the pupil diameter, which measurably fluctuates over several seconds, could mirror real-time concentration in demanding tasks like vigilance (VG) and working memory (WM). Although capable of accurately estimating concentration in the presence of rapid luminance changes, empirical evidence linking these pupil measures at the slow timescales to trial-by-trial VG and WM task performance is lacking. This study demonstrates that the 10-second pupil trend accurately reflects these tasks' performance, underscoring its potential for daily concentration assessment.
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