Jan Brascamp¹, Morgan Gaston¹; ¹Michigan State University

Perceptual decisions are central to decision-making research: by asking participants to select a simple response to a controlled stimulus, the decision process can be distilled to a tractable core. However, even classic two-alternative-forced-choice tasks—e.g. judging the direction of random-dot motion—include processing stages downstream from perception. A participant may perceive leftward motion, yet still must select and execute the corresponding motor response. These perceptual and post-perceptual stages may be serial or intertwined, but either way the decision is not purely perceptual. We leverage perceptual bistability and reflexive eye movements to study a perceptual decision process in isolation. Participants viewed an ambiguous structure-from-motion stimulus that can be perceived as rotating leftward or rightward. Perception at stimulus onset therefore reflects the visual system’s choice between two interpretations. To assess this decision without engaging response selection, we recorded gaze direction, which reflexively tracks perceived motion for such stimuli. Across a series of experiments, we first confirmed that gaze direction at stimulus onset reliably distinguishes the perceived rotation. We then asked whether gaze could reveal more fine-grained aspects of the decision process. Using manual reports, we replicated the prior finding that decisions are slower when they are more difficult: when the two percepts are a priori equally likely, participants take longer to report their percept than when one percept is strongly favored. This indicates that balanced percepts yield a more protracted decision process. Critically, gaze-following movements showed the same pattern: they were delayed on trials with relatively balanced percepts. This demonstrates, first, that the slowing arises within the perceptual decision itself rather than solely in response selection, and second, that gaze provides a suitable index of fine-grained decision dynamics, including decision duration. These results lay the groundwork for using gaze as a tool to study perceptual decision making independently from overt behavioral responses.