The importance of being upright: use of environmental and viewer-centered reference frames in shape discriminations of novel three-dimensional objects.

We investigated the frame of reference that people use to make shape discriminations when their heads are either upright or tilted. Observers made same-different judgments of pairs of novel three-dimensional objects that were aligned along their length within the frontal-parallel plane and rotated in depth around an axis parallel to their own axes of elongation. The aligned objects were displayed vertically, tilted 45 degrees, or horizontally with respect to the environmental upright, but the distance of each pair from the upright was irrelevant to resolving the angular disparity between the stimuli for the same-different judgment. Nevertheless, when observers' heads were upright, the time to encode the stimuli was a linear function of the distance of the stimuli from the environmental upright, whereas when observers' heads were tilted 45 degrees, encoding times for tilted and vertical stimuli did not differ and were faster than the times to encode horizontal stimuli. We interpreted these data to mean that observers either rotate or reference the top of an object to the environmental upright, and they can use either a gravitational or retinal reference frame to do so when either they or the objects are not upright.