The Oculomotor Signature of Expected Surprise

Expected surprise could be defined as the anticipation of the uncertainty associated with the future occurrence of a target of interest. We hypothesized that spatial expected surprise could have a different impact on anticipatory and visual gaze orientation. This hypothesis was tested in humans using a saccadic reaction time task in which a cue indicated the future position of a stimulus. In the ‘no expected surprise’ condition, the visual target could appear only at the previously cued location. In other conditions, more likely future positions were cued with increasing expected surprise. Anticipation was more frequent and pupil size was larger in the no expected surprise condition compared with all other conditions. The latency of visually-guided saccades increased linearly with the logarithm of surprise but their maximum velocity did not.In conclusion, before stimulus appearance oculomotor responses were altered probably due to increased arousal in the no expected surprise condition. After stimulus appearance, the saccadic decision signal could be scaled logarithmically as a function of surprise (Hick’s law). However, maximum velocity also reflected increased arousal in the no surprise condition. Therefore, expected surprise alters the balance between anticipatory and visually-guided responses and differently affects movement kinematics and latency.