Neural alpha oscillations and pupil size differentially index cognitive demand under competing audio-visual task conditions.

Cognitive demand is thought to modulate two often used, but rarely combined, measures: pupil size and neural alpha (8-12 Hz) oscillatory power. However, it is unclear whether these two measures capture cognitive demand in a similar way under complex audio-visual task conditions. Here we recorded pupil size and neural alpha power (using electroencephalography), while human participants of both sexes concurrently performed a visual multiple object-tracking task and an auditory gap-detection task. Difficulties of the two tasks were manipulated independent of each other. Participants' performance decreased in accuracy and speed with increasing cognitive demand. Pupil size increased with increasing difficulty for both the auditory and the visual task. In contrast, alpha power showed diverging neural dynamics: Parietal alpha power decreased with increasing difficulty in the visual task, but not with increasing difficulty in the auditory task. Furthermore, independent of task difficulty, within-participant trial-by-trial fluctuations in pupil size were negatively correlated with alpha power. Difficulty-induced changes in pupil size and alpha power, however, did not correlate, which is consistent with their different cognitive-demand sensitivities. Overall, the current study demonstrates that the dynamics of the neurophysiological indices of cognitive demand and associated effort are multi-faceted and potentially modality-dependent under complex audio-visual task conditions. SIGNIFICANCE STATEMENT Pupil size and oscillatory alpha power are associated with cognitive demand and effort, but their relative sensitivity under complex audio-visual task conditions is unclear as is the extent to which they share underlying mechanisms. Using an audio-visual dual-task paradigm, we show that pupil size increases with increasing cognitive demands for both audition and vision. In contrast, changes in oscillatory alpha power depend on the respective task demands: Parietal alpha power decreases with visual demand but not with auditory task demand. Hence, pupil size and alpha power show different sensitivity to cognitive demands, perhaps suggesting partly different underlying neural mechanisms.