Selective Deficit of Imagining Finger Configurations

Among the paradigms used to investigate motor imagery, the one for which the processing components are best understood is the paradigm that requires the participants to decide whether a hand presented in different orientations is a left or a right hand (Parsons, 1987). Parsons argued that in order to carry out this task, participants mentally rotate a representation of their own body part until it aligns with the stimulus. The time required to complete the mental rotation of the body part is an approximately linear function of the size of the angle required to superimpose the virtual to the presented image, as it is for the mental rotation of external, visual objects. However in contrast with the situation for objects, the imagined trajectory for the observer’s body part is strongly influenced by biomechanical constraints specific to its actual movement. Consistently with this position Kosslyn et al. (1998) argued on the basis of imaging data that motor imagery is involved in making implicit transformations of the viewer’s hands but not in that of external objects, and that the former operation recruits motor processes while the former does not. The question posed in this paper is whether neuropsychology can provide evidence in support of this view. While patients with a selective deficit in mentally rotating external objects have been documented (e.g. Bricolo et al., 2000), no cases with a selective deficit in rotating body parts have yet been described. The case presented in this paper is that of a patient with a selective deficit affecting his ability to mentally rotate his hands (and in particular finger configurations) whereas the mental rotation of external objects was preserved.