A new brain imaging study shows the difference, in terms of brain activity, between a person feigning having a paralysed arm and a patient with conversion paralysis - that is, paralysis with no clinically identifiable neurological cause.
Conversion paralysis is one manifestation of conversion disorder, previously known as hysteria, which was made famous by the nineteenth century French neurologist Jean-Martin Charcot (pictured) and later, by his students Pierre Janet and Sigmund Freud. The label "conversion" disorder comes from the idea that an emotional complaint is somehow converted into a physical symptom.
In the current study, Yann Cojan and colleagues scanned the brain of a 36-year-old woman with conversion paralysis, as she completed a version of the Go / No Go task. Trials began with a signal telling her which hand to respond with, followed, after a delay, by a green or red signal (Go / No Go), which indicated whether the response should be made or withheld (it was green on 75 per cent of trials).
The woman, divorced with two children, had recently recovered from a physical illness and had suffered a stressful relationship break up. Her complaint was of paralysis in her left hand, despite no identifiable neurological cause. The woman's brain activity during the task was compared with that of several healthy controls, a minority of whom were asked to feign having paralysis in their left hand.
As expected, the researchers found suppressed activity in the right primary motor cortex of the female patient when she attempted to move her "paralysed" hand (you'll remember that the right hemisphere controls the left side of the body). A similar suppression was observed in the controls who were feigning paralysis.
However, unlike in the controls, the researchers also observed in the patient's brain increased connectivity between the right motor cortex and midline structures, including the ventromedial prefrontal cortex and the precuneus, which is found in the parietal lobe. These brain regions have previously been associated with self-monitoring, mental imagery and autobiographical memory, thus raising the intriguing possibility that this anomalous activity could represent the brain basis for emotional interference with motor control.
Two other key findings emerged. The brain region normally associated with consciously inhibiting a prepared response - the inferior frontal gyrus - was not activated when the woman failed to move her "paralysed" hand (but it was activated when the controls feigned paralysis on "Go" trials). Moreover, there was evidence of preparatory motor activity in the woman's brain during Go trials with her "paralysed" hand, thus supporting her claim to be willing a movement to occur.
"Taken together, our results may help better understand the brain pathways by which self-awareness becomes distorted in these patients [with conversion disorder] and how the mind may take control over the body during conversion," the researchers said.
Cojan, Y., Waber, L., Carruzzo, A., & Vuilleumier, P. (2009). Motor inhibition in hysterical conversion paralysis. NeuroImage, 47 (3), 1026-1037 DOI: 10.1016/j.neuroimage.2009.05.023
Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.