Congenital mirror movements: a clue to understanding bimanual motor control.

Mirror movements (MM) are involuntary movements of one side of the body that accompany and mirror intentional movements on the opposite side. Physiological MM can occur during normal childhood development, probably owing to corpus callosum immaturity. Pathological congenital MM may be clinically isolated or part of a complex congenital syndrome, including Kallmann syndrome, Klippel-Feil syndrome, and congenital hemiplegia. Congenital isolated MM are usually familial. Recently, heterozygous mutations of the DCC gene, with autosomal dominant inheritance, were shown to cause some cases of MM. The pathogenesis of congenital MM may involve (i) abnormal interhemispheric inhibition between the two motor cortices; (ii) functional alteration of motor planning and motor execution; and/or (iii) abnormal persistence of the ipsilateral corticospinal tract. Fundamental and clinical research is providing novel insights into the complex underlying molecular pathways, and recent experimental work has identified several mechanisms that may mediate the motor network dysfunction. In this review, we analyze clinical, genetic, neurophysiologic, and neuroimaging data on congenital MM, and discuss how this knowledge may improve our understanding of bimanual motor control.