Abstract
Psychological factors such as task difficulty and attentional breadth (i.e. the eccentricity at which covert spatial attention is allocated) have been shown to affect pupil size. Most studies investigating these factors looked at their effect on phasic pupil responses (e.g. moment-to-moment fluctuations in pupil diameter), and it is still unclear whether they affect tonic pupil size (e.g. baseline levels of pupil diameter). Our study aimed to address visual and attentional-load confounds found in previous research by varying attentional breadth and task difficulty independently. We conducted two experiments investigating the effects of: (1) attended eccentricity; and (2) task difficulty, on both tonic and phasic pupil size. Participants performed a visual discrimination task with targets appearing bilaterally at different eccentricities. In Experiment 1, we varied both factors independently in a blocked manner, which allowed us to assess only differences in tonic pupil size. Task difficulty did not affect tonic pupil size, which is striking because its effects on phasic pupil size are well documented. Attended eccentricity also did not affect tonic pupil size. In Experiment 2, we wanted to assess the effects of attended eccentricity on phasic pupil size, in line with previous studies. We found that phasic pupil responses to stimuli were larger when participants directed covert attention towards more eccentric regions, even when controlling for visual input and task difficulty. As in Experiment 1, tonic pupil size had no relation with task difficulty. In general, our experiments show that changes in task difficulty are likely not reflected in changes in tonic pupil size. Furthermore, our results suggest that pupil size increases with increasing eccentricity of covert spatial attention (attentional breadth), even when keeping task difficulty constant.