One threshold to rule them all? Modification of the Index of Pupillary Activity to optimize the indication of cognitive load.

Cognitive load is an important source of information in performance situations. One promising non-invasive method is pupillometry. The Index of Pupillary Activity [IPA, Duchowski et al. 2018] performs a wavelet transformation on changes of pupillary dilations to detect high frequencies. This index is inspired by the Index of Cognitive Activity [ICA, Marshall 2000]. The IPA value is the sum of peaks exceeding a predefined threshold. The present study shows that it appears reasonable to adapt this threshold corresponding to the task. Fifty-five participants performed a spatial thinking test with six difficulty levels and two simple fixation tasks. Six different IPA values resulting from different thresholds were computed. The distributions of these IPA values of the eight conditions were analyzed regarding the validity to indicate different levels of cognitive load, corresponding to accuracy data. The analyses revealed that different thresholds are sensitive for different cognitive load levels. Contra-intuitive results were also obtained.