Attention and sensation in functional motor disorder
Functional motor disorder (FMD), also called psychogenic motor disorder or conversion disorder, describes impairments of motor function where there is no evidence of organic disease. The diagnosis is usually confirmed by positive clinical signs, such as Hoover's sign, in which normal power returns when attention is diverted away from the affected limb. This suggests that selective attention is an important determinant of these functional symptoms. The present study is the first specifically to explore the shifting of spatial attention in relation to the side of FMD. We tested 14 patients with unilateral functional upper limb weakness on three tasks requiring detection of visual targets close to the affected or unaffected hand, or touches to the hand itself. Targets were preceded by central cues promoting voluntary shifts of attention, or peripheral cues promoting automatic shifts. We observed a reduced response to visual and/or tactile targets on the affected side in around half of the patients, by comparison with age-matched controls, indicating that some degree of detection cost often accompanies FMD. Additionally, although the patient group showed normal cueing effects on the visual tasks, they had a unilateral absence of cueing effect on the affected side in the tactile task. Consideration of the data in the context of recent theory suggests that the abnormality may be not in the shifting of attention itself, but rather in the consequences of attending to the affected side. Specifically, the expected cueing effects may be absent on the affected side, because attention to a functionally weak limb increases the perception of the symptom, including any reduced sensory response. This preliminary research suggests promising new lines of investigation into the role of attention, and particularly somatic attention, in FMD.