Matthew Parrella¹, Isshori Gurung¹, Michael Grubb¹; ¹Trinity College

Abrupt onsets reflexively shift covert spatial attention. Our lab recently demonstrated that delivering trial-to-trial information about the probability of a peripheral onset modulated the magnitude of the attentional cueing effect (low-probability > high-probability). Although onsets were physically identical, pupil responses could have been modulated by the onset-probability signal, which provided information about the probability of the onset’s appearance. Specifically, anticipatory constrictions may have preceded high-probability onsets. Here, we tested this possibility in a new experiment using centrally-presented, luminance-matched onset-probability signals.  For half the participants, vertical lines at fixation signaled high probability (0.8) of onset appearance (a small, white, peripheral circle), while horizontal lines signaled low probability (0.2). These contingencies were reversed for the other half of participants. Importantly, both onset-probability signals had equal luminance. Participants fixated the onset-probability signal for 2,000 milliseconds before the onset was presented for 67 milliseconds, or omitted altogether, in line with the signaled probability. To maintain engagement, participants completed a simple localization task. We measured pupil area through the experiment using an EyeLink 1000. Preliminary evidence for an expectation-induced anticipatory constriction was obtained in Experiment 1 (n=15). However, this effect disappeared in Experiment 2 with a larger replication sample (n=39). Exploratory analyses of the two datasets uncovered a violation of a fundamental methodological assumption: despite being task-irrelevant, luminance-matched, and presented at fixation, a vertical onset-probability signal consistently generated larger pupillary constrictions, relative to the presentation of a horizontal signal. Statistical evidence for this orientation-dependent modulation was present in both experiments independently, and a combined analysis confirmed that this effect was not confounded by expectation. Our assumption—that one could eliminate a differential pupil response by using luminance-matched stimuli presented at the same location—was wrong. This seemingly benign manipulation demonstrated the real effects of stimulus design, no matter how inconspicuous, on pupillometric results.

Acknowledgements: Supported by NSF-2141860 CAREER Award to MAG