The voluntary control of motor imagery. Imagined movements in individuals with feigned motor impairment and conversion disorder.

The ability to volitionally control motor imagery was investigated by comparing the chronometry of real and imagined movements in a patient (AB) with conversion disorder who presented with paralysis of the left arm and hand and in a patient (MM) with an actual injury to the left arm. Control experiments investigated voluntary control of motor imagery in a group of healthy individuals who feigned a motor impairment with one limb and in one group who were instructed to move carefully and slowly. The visually guided pointing task was used to investigate the speed for accuracy trade-offs that occur as target size is varied for both real and imagined performance. In the healthy individuals, the speed for accuracy trade-off for both real and imagined performance on the motor task conformed to Fitts' law provided both the speed and accuracy of movements was emphasised. In MM, real and imagined performance was also within normal limits despite considerable pain and discomfort. In AB and in subjects feigning a motor impairment, motor task performance with the affected limb was slow and did not conform to Fitts' law. However, although imagined performance with the affected limb was generally slower than with the unaffected limb, it did conform to Fitts' law. These results suggest subjects cannot anticipate the effects of an actual limb injury. Furthermore, while they are able to control the general duration of imagined movements they have little voluntary control over their relative timing.