Complexity science involves the study of situations where the behavior of a system is not easily understood by studying its parts in isolation, but depends crucially on understanding how the parts interact (the weather, turbulence, and the economy are among the most familiar examples of complex systems). Complex systems analysis has been increasingly successfully applied by psychologists and neuroscientists to the study of perception, action, and cognition. The NSF-funded research project by psychologists James Lackner and Paul DiZio at Brandeis University is an example of the application of complexity science to human movement and motor control.
Lackner and DiZio take the approach of beginning with a deceptively simple question, study it in a deceptively simple experimental paradigm, and analyze the task and behavior with a deceptively simple model. The question is how far must a person be moved off-balance before they will take a step to avoid a fall. With the Hold & Release (H&R) technique, the experimenter applies a small force to the sternum of the research participants, which they resist in order to maintain balance. When the force is released participants must make postural adjustments to maintain balance and if the force is large enough a step is required. It appears that a (relatively) simple linear model -- the one-leg inverted pendulum model -- can account for participants' postural adjustments in this situation. The model breaks down, however, when the force is large enough to require the participant to take a step in order to maintain balance. This requires the transition to a more complex non-linear model which assumes the two legs operate in a coordinated but asymmetrical fashion. Lackner and DiZio will test this model by performing the H&R task in a rotating room. This simple next step adds additional forces (especially rotational Coriolis forces that serve to pull the movement away from the intended trajectory but that change continuously as a function of the limb's velocity) and additional degrees of freedom for participants' responses that provide a rigorous environment for testing the new model. The model is refined enough to make detailed predictions about participants' stance, about which combinations of forces will push them to the stepping threshold, and even with which foot the step will be made.
If successful, Lackner and DiZio will have succeeded in taking a major step towards unifying the study of posture and locomotion, which until now have been studied as essentially separate processes. It is clear that any improvement in our understanding of motor control can have implications for the assessment and treatment of movement disorders, but this approach is particularly promising because H&R measurements are easy to make and the detailed model is so tightly coupled to the measurement paradigm. The interdisciplinary nature of complex systems research is also demonstrated by this project, in which psychologists Lackner and DiZio will employ a Ph.D. physicist as a post-doctoral researcher. Undergraduate physics majors regularly assist with Lackner and DiZio's research.
