Department of Exercise Sciences


Distinct modulation of event-related potentials during motor preparation in patients with motor conversion disorder

Rebekah L. Blakemore, Brian I. Hyland, Graeme D. Hammond-Tooke, J. Greg Anson
Published in PLOS ONE April 2013, Volume 8, Issue 4

Introduction

Conversion disorder is a poorly understood syndrome, thought to be triggered by psychological stressors such as trauma or conflict, in which patients present with neurological symptoms that cannot be explained by any underlying neuropathology. Motor disorders include tremor, paralysis, and paresis, in which patients demonstrate unexplained muscle weakness during intentional movement. Previous studies have used functional imaging to investigate neural correlates of conversion paresis; however to date, no data are available about direct time-dependent changes in cortical processing associated with preparation for movement.

 

Aim

We investigated whether conversion paresis patient show distinct electroencephalographic (EEG) markers associated with their unconscious movement deficits.

 

Method

  • Six unilateral upper limb conversion paresis patients, 12 feigning participants asked to mimic weakness and 12 control participants performed a precued reaction time task, requiring movements of either hand, depending on precue information (Figure 1A).
  • Performance measures (force, reaction and movement time), and event-related EEG potentials (ERP) were compared, between groups and across hands and between hemispheres, using linear mixed models analyses.
Blakemore-2013-Fig-1

 

Results

  • Feigners generated the same between-hand difference in reaction and movement time as expressed by patients, even though no specific targets were set nor feedback given on performance (Figure 1B & 1C).
  • We found novel ERP signatures specific to patients. When the symptomatic hand was precued, the P3 ERP component accompanying the precue was dramatically larger in patients than in feigning participants (Figure 3C & 3E).
  • Additionally, in patients the earlier N1 ERP component was diminished when the precue signalled either the symptomatic or asymptomatic hand (Figure 3A & 3B).
Blakemore-2013-Fig-3

 

Interpretation

These results are consistent with previous suggestions that lack of awareness of the origin of their symptoms in conversion disorder patients may result from suppression of brain activity normally related to self-agency. In patients the diminished N1 to all precues is consistent with a generalised reduction in cognitive processing of movement-related precues. The P3 enhancement in patients is unlikely to simply reflect changes required for generation of impaired movements, because it was not seen in feigners showing the same behavioural deficits. Rather, this P3 enhancement in patients may represent a neural biomarker of unconscious processes, including additional emotional loading, related to active suppression of brain circuits involved in the attribution of self-agency.

 

Acknowledgements

The authors wish to acknowledge Professor Elizabeth Franz (Department of Psychology, University of Otago) and Dr Jon Shemmell (School of Physical Education, University of Otago) for their early discussion and comments, and Nigel Barrett, Gavin Kennedy and Glenn Braid (School of Physical Education, University of Otago) for technical, electronic and programming support.