Mental Rotation - A Case of Embodied Action

A new view of mental rotation in humans is presented. Rather than being a perceptual phenomenon, mental rotation of objects is supposed to be an imagined action in the sense that its only difference to real action is the absence of motor output. A series of experiments is reported which shows that the difference in speed between mental and manual rotation are negligible and that performing rotational hand movements interferes with mental rotation and vice versa. It also could be shown that the preparation of rotational hand movements is already sufficient to influence mental rotation. The general role of motor processing in dynamic visual imagery is discussed, considering the underlying neurophysiology. Mental Rotation, Perception and Action Mental rotation occurs when a decision has to be made whether two objects differing in orientation are identical or mirror-images of each other. The angular disparity between stimuli is varied systematically and response times (RT) and errors are measured. The typical, most intriguing result found in many mental rotation studies is the almost perfect linear increase of RT with angular stimulus disparity. Together with introspective reports from participants and experimenters, these core findings led to the term "mental rotation", since it resembles the time course of a physical rotation with constant angular velocity. This analog nature of the mental rotation process, most impressively demonstrated by Cooper (1976) piqued psychologists' interest. The resemblance of mental rotation to external physical rotation, however, demands for a mental process mimicking external physical rotation, and it is still not clear how such an analog process is implemented in our brains. Recent electrophysiological studies could measure continuous changes in the activity pattern of cell assemblies in monkeys performing a visuomotor mental rotation task (Georgopoulos et al. 1989). They found that the neuronal population vector calculated from the cell assemblies' activity pattern continuously changed its direction prior to the onset of a movement pointing 90° to the left of a target light. The fact, that the neuronal population vector rotated prior and not in parallel to the movement makes it convenient to call it a mental rotation. These results demonstrate how analog operations can be performed by our brains and perhaps mental object rotation is performed by similar changes in the activity patterns of cell assemblies. Notably, these results were obtained by measuring cell activities in the motor cortex. Cognitive psychologists, so far, mainly have tried to explain the analog nature of mental rotation by looking for the relation between mental rotation and the perception of rotary motion. Shepard & Judd (1976) addressed this question by investigating apparent rotational motion and its relation to mental rotation. However, mental rotation speed was about 50 to 60°/s, whereas the slope of the optimum apparent motion function calculates to 1000°/s. Furthermore, Friedman & Harding (1990) showed that mental rotation speed depends on the axis of rotation, whereas the apparent motion illusion does not. They concluded that mental rotation and apparent motion don't have much in common. In addition mental rotation is strategic (Just & Carpenter 1985) compared to the largely automatic processing of apparent motion perception. Hence, the consultation of the perception of real motion as the main explanation for the mental rotation phenomenon is somewhat unsatisfactory. Mental rotation is obviously a much higher-level process than the perception of real or apparent motion. Nevertheless, there have been further, more direct attempts to prove the participation of rotary movement perception in mental rotation. Corballis & McLaren ( 1982) could show, that inducing a rotary aftereffect by means of a rotating textured disk influenced RT of the mental rotation of subsequently presented alphanumeric characters. Compared to the standard experiment, RT were increased when the aftereffect was in the opposite direction (discordant condition) of the presumed mental rotation. However, Jolicoeur & Cavanagh (1992) could exclude the participation of low-level motion analysis centers in the mental rotation process. Although finding slight differences in overall RT level when presenting alphanumeric stimuli in different surface media, there was no effect of surface medium on mental rotation rate. A pronounced effect was only produced when rotating the characters about a small angle by means of apparent mo From: AAAI Technical Report FS-96-02. Compilation copyright © 1996, AAAI (www.aaai.org). All rights reserved. tion. Concordant rotation of presented characters accelerated RT of mental rotation, whereas rotations opposite (discordant) to the presumed mental rotation direction led to delayed responses, compared to a neutral condition. Obviously, figure motion interacts to some degree with mental rotation. Summarizing their results, Jolicoeur & Cavanagh (1992) concluded that mental rotation occurs at a relatively high and perhaps abstract level of processing. This agrees with a principal difference between motion perception and mental rotation. Whereas motion perception is a rather automatic process, mental rotation is strategic and shares some characteristics with voluntary actions. Mental rotation can be started and stopped voluntarily (Cooper 1976) and even its speed is at free choice (Cooper & Shepard 1973). The processes engaged in motion perception might well participate in imagining rotating objects, but there should be a higher process steering these dynamic imaginations, because they are quasi completely under voluntary control. Considering its similarity to voluntary actions, it might be possible that motor or premotor processes are involved in mental rotation, a conclusion which Kosslyn (1994) meanwhile draws complementary to his earlier work. More precisely, this means that the processes engaged in rotary object manipulation might contribute to mental rotation. In this view, rotating something mentally is rather an imagined action than the perception-like imagination of an object in rotation. Let us assume that rotating an object mentally is something very similar to rotating it physically (e.g., with one's hand) in the sense that the same processes initiate and rule the rotation, whether it turns up mentally or it is actually performed. This "common-processing" assumption implies essentially two things. First, mental rotation should be commensurate with rotary object manipulation: factors that affect mental rotation should have the same effect on actual rotation. Second, both processes should be functionally connected, i.e., depend on each other. This could be investigated by having both tasks executed simultaneously, i.e., performing mental rotation while making rotational hand movements.