PERSISTENT MIRROR MOVEMENTS - FORCE AND TIMING OF MIRRORING ARE TASK-DEPENDENT

A simple isometric motor task was used to quantify intended and uninte nded finger movements in two subjects (father and son) with persistent mirror movements. One hand voluntarily changed grip force between thu mb and index finger at different amplitudes and frequencies, while the other hand was to maintain a constant force. During all experimental conditions the ''steady'' hand showed insuppressible, highly cross-cor related contractions, compatible with bilateral distribution of a sing le motor command to the spinal cord. However, these associated movemen ts were not strictly mirror images, nor did they show a fixed relation ship to the voluntary movements across experimental conditions. The ra tio of mirror to voluntary movement ranged from 1.4 to 19.1% and from 3.4 to 78.4% in the two subjects and was directly related to voluntary strength and speed. At maximum speed, mirror activity tended to prece de voluntary activity, while it was delayed in slow force changes. Com parable time lags were not found in control subjects instructed to sim ulate mirror movements. We conclude that neuronal mechanisms in additi on to bilateral corticomotoneuronal connections are at work in persist ent mirror movements.