Effects of the gravity direction in the environment and the visual polarity and body direction on the perception of object motion.

To process the motion of objects, humans need to consider information about up-down direction as obtained through various cues such as the gravity direction in the environment, visual polarity, and body direction. This study investigates the effects of up-down direction, as obtained from these cues on motion perception, with a focus on acceleration perception. We presented the participants with moving objects that had various acceleration speeds and measured the physical acceleration to be perceived as constant velocity. We examined the effect of the up-down direction from the visual polarity by changing the relationship between the up-down direction indicated by the gravity direction cue and the up-down direction indicated by visual polarity by manipulating the posture of the observer. The results showed that the up-down direction received by the gravity affected motion perception. Moreover, the up-down direction indicated by the visual polarity affected motion perception when the observer's body direction and the physical gravity direction were different. On the other hand, up-down direction indicated by the visual polarity did not affect motion perception when the body direction coincides with physical gravity direction. Overall, the results suggest that the up-down directions indicated by the gravity, visual polarity, and body direction are integrated non-linearly in the perceived acceleration of visual motion.