Background: Previous research has shown that experiencing motion stimuli negatively impacts cognitive performance. Objective: In the current study, we investigate whether this impact relates to Type-II spatial disorientation (SD), to motion stimulus magnitude, or to an interaction of these factors. Method: Stimuli for participants (n = 23) consisted of Earth-vertical yaw rotations on a rotating chair in a completely darkened room. In the surprise condition, the stimulus started with subthreshold acceleration, followed by suprathreshold deceleration to a non-zero velocity, inducing a sensation of rotation that is opposite to the actual rotation revealed when the lights were switched on. In the no-surprise condition, the same changes in velocity were used, but starting from (almost) zero velocity, which induced a sensation of rotation in the same direction as the actual rotation. Participants performed a self-paced arithmetic task and measurement of their cognitive performance started after the environment was revealed. Stimulus magnitude was operationalized through higher or lower peak suprathreshold deceleration. Results: The results revealed that counting speed decreased significantly when participants were surprised, constituting a large effect size. The proportion of counting errors likewise increased significantly when participants were surprised, but only in the high-magnitude condition. Application: The findings suggest that surprise caused by the recognition of SD has an involuntary disruptive effect on cognition, which may impact performance of piloting tasks. These results are relevant when modeling motion stimuli effects on performance, and when developing SD awareness training for pilots.@en