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要旨

Although we are constantly confronted with rotary movement of objects in daily life, little is known about human perception of three-dimensional rotations. We recently introduced a new paradigm to investigate the processing of visual information about rotations in space in which subjects are asked to mimic the rotary movement of objects displayed on a computer monitor by simultaneously changing the orientation of a hand-held object. From the recorded motion of this real object (a ball) we can extract the average responded rotation axis for each trial. Here, we report further experiments based on this approach. When watching a rotating object with a marked extension in a certain direction, the orientation of this object with respect to the axis of rotation has a distinct effect on the appearance of the motion. We show that, for rod-like objects, the variability in the recorded orientation of the rotation axis is lowest when the stimulus rotation axis is either aligned with or orthogonal to the main axis of the object: the standard deviation of signed angular error in arbitrary directions is typically 5°. When rotation axis and object axis are not aligned, variability is much higher, reaching a maximum at 45° misalignment. The variability found with objects that have no marked elongation fell between those extremes. Our findings confirm that subjects' responses can be highly reproducible. We conclude that alignment of the principal axes of an object with the axis of rotation facilitates subjects' mimicry of object motion.