Luminance matching in cognitive pupillometry is not enough: The curious case of orientation

Abrupt onsets reflexively shift covert spatial attention. Recent work demonstrated that 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 information about the probability of the onset’s appearance. Specifically, anticipatory constrictions may have preceded high-probability onsets. Here, we tested this hypothesis using centrally-presented, luminance-matched onset-probability signals. For half the participants, vertical signaled high probability (0.8) of onset appearance (a small, white, peripheral circle), while horizontal signaled low probability (0.2). Contingencies were reversed for the other half. Participants fixated the onset-probability signal for 2,000 milliseconds before the onset was briefly presented or omitted, in line with the signaled probability. To maintain engagement, participants completed a simple localization task. Preliminary evidence for an anticipatory reduction in pupil area was obtained in Experiment 1. However, this effect disappeared in Experiment 2 with a larger replication sample. Exploratory analyses uncovered a violation of a fundamental methodological assumption: despite being task-irrelevant and perfectly luminance-matched, vertical onset-probability signals consistently generated smaller pupil areas, relative to horizontal signals in both Experiments 1 and 2. Interestingly, this “orientation effect” was stronger in the second half of the experimental session, and in a third experiment, we significantly reduced its magnitude by changing the locations of the task-relevant stimuli. In short, across three experiments (self-reported gender: 52 female, 26 male, 1 non-binary) we show that even with perfect luminance-matching, unforeseen changes in cognitive state can modulate pupillometric measurements. Significance Statement Cognitive pupillometry often relies on the assumption that matching stimuli for luminance will control for unwanted influences on pupil area. Here, we demonstrate that a simple, task-irrelevant white line—identical in luminance, size, and spatial location—elicited systematically smaller pupil areas when oriented vertically rather than horizontally. This orientation effect was present in two independent datasets; it emerged with task experience; and it was attenuated by changing the locations of task-relevant stimuli. Our findings reveal an unexpected, and potentially hidden, confound in cognitive pupillometry. In light of the pupil’s remarkable sensitivity, researchers should carefully consider whether stimulus features beyond luminance—such as orientation and its interaction with broader experimental design choices—inadvertently affect pupil-based measures of cognitive processes.