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 compa ring the chronometry of real and imagined movements in a patient (AB) with conversion disorder who presented with paralysis of the left arm and band a nd in a patient (MM) with an actual injury to the left arm. Control experim ents investigated voluntary control of motor imagery in a group of healthy individuals who feigned a motor impairment with one limb and in one group w ho were instructed to move carefully and slowly. The visually guided pointi ng task was used to investigate the speed for accuracy trade-offs that occu r as target size is varied for both real and imagined performance. In the h ealthy individuals, the speed for accuracy trade-off for both real and imag ined performance on the motor task conformed to Fitts' law provided both th e speed and accuracy of movements was emphasised. In MM, real and imagined performance was also within normal limits despite considerable pain and dis comfort. In AB and in subjects feigning a motor impairment, motor task perf ormance 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 in jury. Furthermore, while they are able to control the general duration of i magined movements they have little voluntary control over their relative ti ming. (C) 2000 Elsevier Science Ltd. All rights reserved.