The voluntary control of motor imagery. Imagined movements in individuals with feigned motor impairment and conversion disorder
Introduction
Motor imagery is a dynamic state in which an individual mentally simulates the performance of a specific motor action [3], [10]. Three converging lines of evidence suggest that real and imagined movements are controlled within the same neurocognitive networks.
- 1.
Behavioural studies in normal individuals show that the speed of imagined motor performance is highly correlated with the speed of actual motor performance [3], [4], [5], [10], [11], [13]. Furthermore, the speed and accuracy of imagined movements are subject to the same environmental and physiological constraints as real motor performance with both described by Fitts’ law [9].
- 2.
Neuroimaging studies consistently find patterns of activation that are common to both the mental simulation of motor performance and the generation of overt motor movements [6], [16], [19].
- 3.
Studies of patients’ lesions in the motor areas of the frontal lobe, basal ganglia or cerebellum find that abnormalities in motor imagery are qualitatively and quantitatively similar to impairments in actual motor task performance [8], [12], [17].
To determine the extent to which individuals exercise voluntary control over motor imagery or can remember the time necessary to perform motor actions we investigated imagined motor task performance in two different conditions. First, we assessed imagined movements in a patient with a unilateral motor impairment due to conversion disorder. While motor impairment can be severe in conversion disorder, careful neurological and psychiatric investigation excludes disruption to central nervous system (CNS) motor areas. However, the motor impairment in conversion disorder does not arise from a deliberate attempt to malinger. Instead, it is due to a complex psychiatric syndrome that may have an organic basis of its own that does not involve CNS motor areas [2], [14], [15]. In this experiment, normal subjects were also required to deliberately feign an injury to one arm and then imagine performing a motor task with the injured arm. The condition is akin to malingering, although the important difference is that subjects in this experiment could not gain financially or emotionally from feigning the arm injury. Nonetheless, subjects can give compelling demonstrations of feigned injuries. As the motor impairment associated with conversion disorder and feigned injury does not arise from disruption to CNS motor areas we hypothesised that motor imagery would be unimpaired. In a second experiment, we determined the effect of an actual limb injury on real and imagined motor task performance with that limb. We also sought to disrupt the conventional relationship between movement duration and target width by giving the motor task to healthy subjects and emphasising that performance be accurate rather than both fast and accurate.
Section snippets
Subjects
All subjects participated in the study after giving written informed consent and handedness was determined in all subjects with the Edinburgh Handedness Inventory.
Apparatus and design
The Visually-Guided Pointing Task (VGPT) was used to measure movement durations for real and imagined movements in all experiments. The task and procedure was modified from that used by Sirigu and colleagues [1], [17], [18]. On each trial, subjects were presented with a piece of clear plastic upon which was drawn an 80 mm vertical line. A black target box was also drawn on the plastic with its closest edge 30 mm away from the vertical line (see Fig. 1, Ref. [14]). A set of 10 different stimuli
Data analysis
Data from the visually guided pointing task was collated according to task (real or imaginary), condition (normal or feigned injury; speed and accuracy or accuracy only) and target width. For each subject, the mean movement duration of the two trials for each target width was calculated for each condition. For the healthy controls, these were used to calculate a group mean for each target width. For the control means and for the individual performance of AB and MM a least squares regression
Experiment 1
Fig. 1 shows the relationship between mean movement duration and target width for the real and imaginary motor task under the normal and feigned injury condition in the control group. The linear and logarithmic regressions fitted to this data are shown in Table 1. For the normal condition, the logarithmic regression gave the best fit of the data for both the real and imagined movements made with the dominant and non-dominant hands. For the feigned injury condition, the logarithmic regression
Discussion
The results of the current study show that while individuals do have some voluntary control over the duration of imagined goal-directed movements, they cannot exert voluntary control over speed and accuracy relationships in imagined motor task performance. When performing the motor sequencing task and imagining performance on the same task under normal conditions, healthy subjects showed the expected speed-accuracy trade-off, described by Fitts’ law, for both real and imagined movements
Acknowledgements
The authors would like to thank Scott Johnson for suggesting Experiment 2 and Scott Johnson and an anonymous reviewer for their helpful comments on an earlier version of this paper.
References (19)
The neurological basis of motor imagery
Behavioural Brain Research
(1996)- et al.
The timing of mentally represented actions
Behavioural Brain Research
(1989) - et al.
Motor imagery of a lateralised sequential task is asymmetrically slowed in hemi-parkinson’s disease
Neuropsychologia
(1995) - et al.
Asymmetries between dominant and non-dominant hands in real and imagined motor task performance
Neuropsychologia
(1999) - et al.
The effect of an external load on the force and timing components of mentally represented actions
Behavioural Brain Research
(2000) - et al.
Slater revisited — 6 year follow-up study of patients with medically unexplained motor symptoms
British Medical Journal
(1998) - et al.
Mentally simulated movements in virtual reality: does Fitts’ law hold in motor imagery?
Behavioural Brain Research
(1996) - et al.
Mapping motor representations with positron emission tomography
Nature
(1994) - Decety J, Grezes J, Costes N, Peranio D, Jeannerod M, Procyk E, Grassi F, Fazio F. Brain activity during observation of...
Cited by (57)
What about conversion disorder in pain clinic?
2019, DouleursNeurobiologic theories of functional neurologic disorders
2016, Handbook of Clinical NeurologyCitation Excerpt :They employed new technology (functional imaging) and made an interpretation of the data (orbitofrontal and anterior cingulate activation during attempted movement of the weak leg inhibiting prefrontal cortex activation) that was relatively free from speculation about psychologic underpinnings of the disorder. Studies by Spence et al. (2000) and Maruff and Velakoulis (2000) found what appeared to be normal preparatory activity for upcoming movement in single patients with functional paralysis, but then differences from healthy controls and those feigning paralysis with respect to prefrontal cortex activation. This was interpreted as the presence of abnormal prefrontally driven inhibition of a normally functioning motor system, echoing previous theories such as those of Ludwig (1972) and Whitlock (1967).
Self-Awareness Disorders in Conversion Hysteria
2015, The Neurology of Consciousness: Cognitive Neuroscience and NeuropathologyLying in neuropsychology
2014, Neurophysiologie Clinique