Degrees of freedom

"Motor complexity provides an adaptive advantage for interacting with the environment since higher complexity permits more flexibility in motor output. This is based on Bernstein’s (1967) concept of skill as a reflection of mastering redundant degrees of freedom in motor learning. He posited that the body is composed of multiple biomechanical degrees of freedom that can be utilized in several ways to achieve the same goal. Motor control is contingent on the ability to use these degrees of freedom to flexibly interact with the environment. In normative development, when the motor system is immature or in a state of early motor learning, the system constrains the degrees of freedom to gain some control and produce stable movements for a given task. This results in simple movements such as the repetitive kicking that Thelen and Fisher (1983) observed. As the motor system matures or learning progresses, degrees of freedom are released to permit greater specificity and efficiency of movement. This is supported by findings that higher motor complexity is associated with more accurate motor task performance (Deutsch and Newell, 2001; Mosconi et al., 2015).

Our model elaborates on Bernstein’s (1967) postulate by emphasizing an important role of sensorimotor integration in the ability to release biomechanical degrees of freedom to produce controlled, complex movements. Sensorimotor integration involves communication between the sensory and motor systems in the brain allowing for: (a) the use of sensory input to generate an accurate and efficient motor plan (e.g., through an inverse model); and (b) the use of self-generated and external sensory feedback to monitor and correct error in the movement (e.g., updating the forward model in the ongoing movement or for future movements; see (Wolpert et al., 1998) for a description of these processes in the cerebellum). Under our framework, sensory input allows the motor system to plan and execute accurate movements by making optimal use of the available degrees of freedom and to correct error in the movement by exploiting the available degrees of freedom to make adjustments that are appropriate to the error.

In the case of stereotyped behavior in healthy infants, our model suggests that the infant brain does not efficiently integrate sensory information with the motor system because either the sensory and motor regions of the brain are immature, or the infant has limited experience with or access to sensory information (e.g., due to immobility). Poor sensorimotor integration prevents the infant from using his/her degrees of freedom flexibly and efficiently, limiting his movements to simple, stereotyped behaviors. With maturity, the infant is able to integrate sensory information with motor behavior permitting him to release degrees of freedom to produce complex movements.

Similarly, our model posits that deficits in sensorimotor integration contribute to the emergence and maintenance of stereotyped behavior in clinical disorders. Whether it is caused by atypical development of sensorimotor circuitry, degenerative processes, or another form of altered neural function, these alterations could contribute to the persistence of stereotyped behavior by disrupting the sensory inputs that are required to inform and diversify motor repertoires."

Shafer, R. L., Newell, K. M., Lewis, M. H., & Bodfish, J. W. (2017). A Cohesive Framework for Motor Stereotypy in Typical and Atypical Development: The Role of Sensorimotor Integration. Frontiers in Integrative Neuroscience, 11, 19. http://journal.frontiersin.org/article/10.3389/fnint.2017.00019/full

References

Bernstein’s N. A. (1967). The Co-Ordination and Regulation of Movements. Oxford: Pergamon Press Ltd.

Deutsch K. M., Newell K. M. (2001). Age differences in noise and variability of isometric force production. J. Exp. Child Psychol. 80, 392–408. 10.1006/jecp.2001.2642 

Thelen E., Fisher D. M. (1983). The organization of spontaneous leg movements in newborn infants. J. Mot. Behav. 15, 353–377. 

Mosconi M. W., Mohanty S., Greene R. K., Cook E. H., Vaillancourt D. E., Sweeney J. A. (2015). Feedforward and feedback motor control abnormalities implicate cerebellar dysfunctions in autism spectrum disorder. J. Neurosci. 35, 2015–2025. 10.1523/JNEUROSCI.2731-14.2015 

Wolpert D. M., Miall R. C., Kawato M. (1998). Internal models in the cerebellum. Trends Cogn. Sci.2, 338–347. 10.1016/S1364-6613(98)01221-2