Transitions between discrete and rhythmic primitives in a unimanual task

Abstract

Given the vast complexity of human actions and interactions with objects, we proposed that control of sensorimotor behavior may utilize dynamic primitives. However, greater computational simplicity may come at the cost of reduced versatility. Evidence for primitives may be garnered by revealing such limitations. This study tested subjects performing a sequence of progressively faster discrete movements in order to “stress” the system. We hypothesized that the increasing pace would elicit a transition to rhythmic movements, assumed to be computationally and neurally more efficient. Abrupt transitions between the two types of movements would support the hypothesis that rhythmic and discrete movements are distinct primitives. Ten subjects performed planar point-to-point arm movements paced by a metronome: starting at 2 s, the metronome intervals decreased by 36 ms per cycle to 200 ms, stayed at 200 ms for several cycles, then increased by similar increments. Instructions emphasized to insert explicit stops between each movement with a duration that equaled the movement time. The experiment was performed with eyes open and closed, and with short and long metronome sounds, the latter explicitly specifying the dwell duration. Results showed that subjects matched instructed movement times but did not preserve the dwell times. Rather, they progressively reduced dwell time to zero, transitioning to continuous rhythmic movements before movement times reached their minimum. The acceleration profiles showed an abrupt change between discrete and rhythmic profiles. The loss of dwell time occurred earlier with long auditory specification, when subjects also showed evidence of predictive control. While evidence for hysteresis was weak, taken together, the results clearly indicated a transition between discrete and rhythmic movements, supporting the proposal that representation is based on primitives rather than on veridical internal models.

Motor cortical oscillations are abnormally suppressed during repetitive movement in patients with Parkinson’s disease

“…Thus, the decrement in movement amplitude and hastening that occurs near to and above 2 Hz in people with PD may reflect an inability to transition coordination patterns with an accompanying loss of movement stability. Two movements per second is also the rate at which movements typically change from discrete to continuous control (Huys et al, 2008; Sternad et al, 2013). Spencer and colleagues (2007) have hypothesized that discrete movement timing is predominantly controlled by the cerebellum whereas continuous movement timing may be dependent upon the basal-ganglia.…”

Motor primitives—new data and future questions

“…Different authors have considered MPs at several levels. MPs have been defined as kinematic elements (i.e., patterns of motion without regard to force or mass, e.g., strokes [1-3] or cycles [4,5]), or as kinetic elements (force-based, e.g., viscoelastic force-field primitives [6-9] and their associated muscle synergies[9-11]), or as neural drive circuits [12]. Because the nervous system appears to organize both the executed motion as well as prepare the contingent acts and corrections required for any common or unexpected perturbations in any complex body motion, it follows that the most fundamental MPs need to be useful as compositional elements in each of these contexts, i.e., the fundamental MPs must be adequate building blocks to rapidly construct the kinematic, kinetic, impedance and contingent response aspects of a complex movement.…”

Physiological modules for generating discrete and rhythmic movements: component analysis of EMG signals

“…Transitions from fast discrete movements to rhythmic movement (Sternad et al, 2013 ) as well as transitions from slowly continuous movement to sub-movements (Teeken et al, 1996 ; van der Wel et al, 2009 ) reveal that the underlying controls for discrete and rhythmic movements are based on the same modules. Our results provide further evidence that muscular synergies underlying both types of movement are the same.…”