Abstract
Pupil size reflects a proxy for neural activity associated with global luminance, arousal and cognitive processing. Microsaccades are also modulated by arousal and cognitive processes. Are the effects of arousal and cognitive signals on pupil size and microsaccades coordinated? We hypothesize that if pupil size and microsaccades are coordinately modulated by these processes, pupil size immediately before microsaccade onset, as an index for ongoing processing, should correlate with microsaccade responses during tasks that alter these signals. Here, we examine the relationship between pupil size and microsaccade responses in tasks that include variations in global luminance, arousal, and cognitive control. In Experiment 1, an emotional acoustic stimulus was presented under two different global luminance levels while the participants maintained fixation upon a central fixation point. Microsaccades that occurred during stimulus presentation were analyzed. Higher microsaccade peak velocities correlated with larger pre-microsaccade pupil responses. In contrast, pupil responses evoked by global luminance signals did not correlate with microsaccade responses. In Experiment 2, to examine cognitive signals related to voluntary saccade preparation, we used an interleaved pro- and anti-saccade task, in which subjects were instructed, prior to target appearance, to either automatically look at the peripheral target (pro-saccade instruction) or to suppress the automatic response and voluntarily look in the opposite direction from the target (anti-saccade instruction). Microsaccades occurred during saccade preparation instruction were analyzed, showing higher microsaccade peak velocities correlated with larger pre-microsaccade pupil responses in the anti-saccade condition. The present study provides evidence for a tight coupling between pupil size and microsaccade responses. Given the central role of the superior colliculus in microsaccade generation, these results suggest the involvement of the superior colliculus for the pupil arousal and cognitive modulations, but not for the pupil luminance modulation.