Reliability of oculometrics during a mentally demanding task in young and old adults

The sensitivity of oculometrics to the changes in mental load has already been investigated in several studies. However, the reliability of oculometrics remains unexplored, particularly under concurrent mental demands. To address this, we recruited 20 young and 18 elderly healthy adults to perform a functional computer task with three levels of mental load (i.e., low, medium, and high) on two days interspaced by at least seven days. We tracked the participants' eye movements and pupil size during the task and computed the characterizing features of saccades, fixations, blinks, pupillary responses as well as nonlinear dynamics of pupillary responses and gaze trajectories. In addition, we recorded the trace of the computer mouse for performance measurement and acquired subjective ratings of the perceived task load. Among the investigated oculometrics, saccadic peak velocity (SPV) and its rate of changes across saccade amplitude (SVA) were influenced by the level of the task load in both the young and elderly adults, and the effects remained consistent across days. Reliability assessments revealed good to excellent test-retest reliability and acceptable absolute reliability in SPV, SVA, and the duration of fixations and saccades. The perceived load and task performance were monotonically altered with the task load levels. These findings may provide practical implications on mental load quantification in occupational fields.