There are multiple ways in which the human visual system can encode objects. An object can be specified relative to the observer, to the environment, to its own intrinsic structure or to other objects in the environment. Each instance requires the adoption of specific spatial frames of reference. In general, reference frames provide a structure for specifying an object’s spatial composition and position.

Spatial reference frames can also be utilized in multiple ways to transform objects in the imagination. For example, if an observer wanted to construe an object at a different orientation without actually performing any actions, she could try at least two mental operations, each of which requires the rotation of a different reference frame with respect to a given stationary frame. She could either picture the object turning to its new orientation (object-relative or intrinsic reference frame) or she could imagine moving herself to the viewpoint corresponding to the new orientation (egocentric or relative reference frame). Both of these processes have been implicated in human beings’ ability to update objects across different viewpoints.

Despite the seeming importance of both of these processes, the majority of research has focused primarily on the first type: imagined object rotations. For example, the classic studies of Shepard and colleagues established that observers mentally rotate the axes of one object into congruence with those of another object in deciding whether their shapes are similar. Other studies have examined observers’ ability to predict the orientational outcome of single objects rotated about multiple axes. However, until recently, imagined rotations of the self have received less empirical consideration.

The goal of this paper is to provide a more comprehensive account of the role of spatial reference frames in mental rotation. We review studies from several related research domains – mental rotation, object recognition, perspective-taking, and motor imagery – to examine effects of multiple reference frames on imagined transformations of the self and of objects. This approach is specifically intended to shed light on the recent finding of inferior object (versus viewer) rotation performance, as evidenced by longer reaction times and higher error rates. We find that this discrepancy may be attributable to differences in the way the reference frames corresponding to each imagined rotation are transformed by the human cognitive system.

After a review of reference frames, the first main section of the paper focuses on factors affecting imagined object rotations. A recurring finding of the studies we reviewed is that imagining an object’s rotation is problematic when no information other than the object’s initial orientation is provided. This suggests a general deficit with imagining a cohesive rotation of the object’s intrinsic frame. For such tasks, observers are likely to depend on supplementary information from other frames, such as the environmental frame. As evidenced in the object recognition literature, imagined object rotations are further facilitated by view-specific encoding with respect to the relative frame.

The second main section of the paper focuses on factors affecting imagined viewer rotations. A review of several motor imagery studies indicates that imagined viewer rotations are less susceptible to misalignment with respect to the environmental frame, perhaps due to the inherently cohesive structure of the relative frame itself. We end with a review of research directly comparing imagined object- and viewer rotations, which provides further evidence of differences in the ways the respective reference frames of each type of rotation are transformed.

We begin our review with a brief discussion of spatial reference frames. As mentioned above, imagining an object rotating to your current viewpoint or imagining yourself rotating around an object to a new viewpoint require the adoption of different reference frames. In the first section, we describe the principle frames involved in such movements. In the second section, we examine how the object and viewer frames move with respect to the environmental frame.

Rotation of an object predominantly utilizes an object-relative or intrinsic reference frame, which is defined with respect to the object’s intrinsic top/bottom, front/back, and right/left axes. Rotation of the viewer around the object predominantly utilizes an egocentric or relative reference frame, which specifies the location of external objects with respect to the major up/down, front/back, and right/left axes of the observer’s body. The egocentric frame is often further broken down to relate objects to specific parts of the body. For example, retinocentric encoding specifies an object with respect to the nodal point of the eye. Headcentric encoding specifies an object with respect to the center of the head. Bodycentric encoding specifies an object with respect to axes of individual body parts, such as the hand.

The environmental reference frame specifies the cardinal directions of north, south, east, and west. However, it can also be thought of more specifically as pertaining to structures and planar surfaces that are usually fixed with respect to the environment, such as the walls, floor, and ceiling of a room. Thus, the turning of a room about an observer or object would constitute an environment rotation.

As pointed out by Hinton and Parsons (1988), a complete description of reference systems requires consideration of their relationship to other frames. Besides the different rotation frames involved, one major difference between imagined object and viewer rotations is the way their effective reference frames change with respect to the environmental frame. In imagined object rotations, the intrinsic frame moves with respect to environment, whereas the observer’s relative frame remains fixed. In imagined viewer rotations, the intrinsic frame remains fixed, and the relative frame moves with respect to the environment. The latter situation has been referred to as the “more radical change in our total experience” because it interferes with our natural inclination to be oriented with respect to the environmental frame. The ensuing conflict between physical and imagined viewpoints has lead some researchers to assert that imagined object rotations must necessarily be easier and more natural to perform than imagined rotations of the self.