MIRROR movements are involuntary movements that mimic, and occur simultaneously with, voluntary movements on the opposite side of the body. The movements are known to occur because of a failure in communication between the two sides of the nervous system. They are thought to be normal during infancy and early childhood, but usually diminish with age and disappear altogether by the age of 10, following maturation of the corpus callosum, the massive bundle of nerve fibres connecting the left and right hemispheres of the brain.
A large genetic study published online in the journal Science now shows that mirror movements are caused by a single genetic mutation. The mutation is located within a gene that encodes a well-known protein involved in guiding growing nerve fibres to their proper destination during development, and gives rise to mirror movements because the connections between the two brain hemispheres fail to form properly.