Mounting evidence suggests that human motor control uses dynamic primitives, attractors of dynamic neural networks which require minimal central supervision. However, advantages for control may be offset by compromised versatility. Extending recent results showing that humans could not sustain discrete motions as duration decreased, this study tested whether smoothly rhythmic performance could be maintained as duration increased. Participants performed horizontal movements between two targets, paced by sounds that increased from 1s to 6s by 200ms per cycle and then decreased again. The instruction emphasized smooth rhythmic movements without interspersed dwell times. We hypothesized that: 1) when oscillatory motions slow down, smoothness decreases; 2) slower oscillatory motions are executed as a sequence of submovements; 3) the transition between oscillatory and submovements shows hysteresis. An alternative hypothesis was that 4) removing visual feedback restores smoothness, indicative of visually-evoked corrections causing the irregularity. Results showed that humans could not perform slow and smooth oscillatory movements. Harmonicity decreased with longer periods, dwell times between cycles appeared and became prominent at slower speeds. Velocity profiles showed an increase in the number of overlapping submovements with cycle duration. There was no evidence of hysteresis in the transition between these two types of movement. Eliminating vision had no effect, suggesting that intermittent visual feedback did not underlie this phenomenon. These results show that it is hard for humans to execute smooth rhythmic motions slowly. Instead, they 'default' to another dynamic primitive, and compose motion as a sequence of overlapping submovements.
- Discrete, Rhythmic
- Arm Movements
- Movement Primitives
- Copyright © 2016, Journal of Neurophysiology