A functional analysis of human mirror movements

In this report, an experimental method for investigating the effect of variations in force, amplitude, and frequency of oscillations of the isometric pincer grasp on mirror movements (MM) is described. Normal adults (N = 24) were used as experimental subjects. The magnitude of MM was greater at relatively high static force levels maintained by the passive hand (26% of maximum volitional force or MVF) than at low force levels (4% MVF) in the passive hand. By contrast, the magnitude of MM was smaller at relatively large (20.5% MVF) than at small (9.5% MVF) mean force levels of the active hand when the static force in the passive hand as well as the amplitude and range of oscillation frequencies of the active hand were held constant. The magnitude of MM relative to active hand oscillation was smaller when the active hand oscillated around a relatively large amplitude (22% of MVF) than when it did so around a small amplitude (8% of MVF) at constant mean force levels and oscillation frequencies. The absolute magnitude of MM was not affected by this change in oscillation amplitude. In all experiments, the magnitude of MM was consistently less at relatively high than at low frequencies of oscillation in the active hand. Intrinsic variables of motor control have a major effect on the inducation and modulation of MM, and their contribution may be as great as that of subject variables. The present results differed substantially from those of others who have used similar experimental paradigms. The major difference was the direction of change in power transfer from the active to the passive hand with changes of force in the active hand. Possible reasons for these differences are discussed.