|Series| Theories of CRM
What is a Coordinated Rhythmic Movement?
I started writing a blog post this week, in which it (like many of my posts) featured Coordinated Rhythmic Movement (CRM; Kelso 1981) rather heavily. Then I realised, I haven’t actually spelt out exactly what a CRM is! With that in mind, I stopped writing that post and started writing this one.
A CRM (AKA rhythmic coordinated movements and rhythmic movement coordination) is any movement that uses at least one limb or oscillator (something that moves back and forth in a regular rhythm) that is rhythmically coordinated with another limb or oscillator. Within this definition, a CRM is not limited to a single organism. That is, the limb of one organism can be rhythmically coordinated with another limb of the same organism, of another organism, or that of a computer controlled oscillator.
Bimanual and Unimanual CRM
A bimanual CRM simply means that a single organism is controlling two oscillators. This is in contrast to unimanual CRM, in which one organism controls one oscillator and the other oscillator is controlled by an external source (be it another organism or a computer controlled oscillator).
CRM is Everywhere
CRM is a ubiquitous behaviour found across various types of life on earth. In the air, birds rhythmically coordinate their wings to fly. On the ground, animals rhythmically coordinate their limbs to traverse different surfaces. In water, animals rhythmically coordinate their limbs to move throughout their aquatic environment. Many of these rhythmic movements are coordinated between several limbs of the same organism. One example of a bimanual coordinated rhythmic movement is that of bird flight. In order to maintain flight, both wings have to be rhythmically coordinated in synchrony (see figure 1).
If one wing were to become slightly out-of-sync with the other, this would result in a drastic destabilisation of the bird’s flight. If the bird was unable to regain stability (by restoring the synchronous coordinated rhythmic movement of the wings) the consequence of such a destabilisation would likely be fatal. Are such destabilising events common? To answer that question I’ll pose another question: How many birds have you seen fall out of the sky in a spiral like fashion (seemingly because their wings are out-of-sync)? Not many, right?
That’s because these types of drastic destabilisation events are incredibly rare, at least for bird flight. Interestingly, there seems to be an intrinsic stability with certain types of coordinated rhythmic movements. One of which is the synchronous CRM that is quintessential of stable bird flight. This synchronous type of CRM is known as an in-phase movement.
The other form of CRM that seems to be intrinsically stable are anti-phase movements. I like to think of these movements as perfectly out-of-sync. Each oscillator is in perfect opposition to the other oscillator. Keeping with the avian theme, think of the bipedal movements of the ostrich (See figure 2) as an archetypal example for anti-phase bimanual CRM.
Talking in Cycles
To help explain, I’m going to walk you through the process of one full cycle that each oscillator completes, which in this case is one leg of the Ostrich. Then, I will explain how when these oscillators are rhythmically coordinated in anti-phase, this results in the bipedal movement of the ostrich.
All cycles need a starting point; a sensible place to start is with the planting of the foot. Once the foot has planted the act of pushing the limb into the surface results in the propulsion of the organism. This leaves the leg posterior to the initial planting position. In order to maintain propulsion, or simply slow down without a sudden stop (which would have drastic consequences for the organism) the leg must be reset to the initial planting position. This resetting process is done in a particular way not to impede the other limb, slow down the organism or destabilise the movement. Once the leg has been fully reset, the cycle is complete.
The reason that this type of movement is called anti-phase is because each oscillator (leg) is in perfect opposition with the other oscillator. That is, when one leg is at the beginning of the cycle, the other is at the half-way point of the cycle. This phase-distance between each oscillator is relatively unchanging.
What About Speed Variation?
Even in the event of speeding up and slowing down, the phase-distance between each oscillator still remains relatively unchanged. These changes in speed are achieved by varying the frequency in which these cyclic movements occur, not by changing the phase-distance between the oscillators.
Why Do These Types of Movements Seem to be Inherently Stable?
There are various theories that have different explanations as to why these movements seem to be inherently stable. The language developed by Haken, Kelso, & Bunz (1985) has left a heavy duty stamp on the discipline. Within this work, they developed the HKB model (Haken et al., 1985) which described the intrinsic stability of in-phase and anti-phase movements as attractors. This model is a major part of the history of the Dynamical Systems Approach/Theory. Though the theory has changed and developed over time, the early work still has a huge influence on both the language and the research questions of today. In order to fully understand the breadth of the discipline, this theoretical approach cannot be ignored. As one of the dominant theories of the discipline, the work of the Dynamical Systems Approach will require a particular level of attention.
Another approach that I will be spending a lot of time getting to grips with is Geoffrey Bingham’s Perception/Action model of CRM (Bingham, 2001, 2004a, 2004b). Where Kelso’s model (Haken et al., 1985) is descriptive in its explanation of the basic phenomena; Bingham’s model is explicit. It meticulously lays out both the organisation and coupling of the perceptual and action components.
Outside of these two models, I have already started to cover alternative approaches, such as Rhythm Setting. My approach will be to leave no rock unturned when it comes to the variety of explanations, all of which deserve scientific exploration and analysis.
What’s with all the bird talk?
Coming back to our examples, you might be wondering: What’s with all this bird talk? Maybe you’re thinking I’m a Bill Oddie fan-boy who spends all his time researching the avian genus. Alas, that’s not the case (sorry Bill).
Rather than birds, most of the CRM research is actually done on human subjects. These CRM tasks themselves can vary quite a lot. Some involve the coordination of two fingers (one as each oscillator), some involve the coordination of a leg and an arm and others two limbs that belong to two different people.
Why Study Them?
Despite the methodological variety, these tasks are always relatively simple. They require the individual to both perceive and control the target movement. This movement may be totally new to that individual, thus require learning. Within this paradigm, we can access learning data. We can see how learning this new movement has changed (if at all) the ability to produce other movements (such as in-phase and anti-phase). The simplicity of the CRM paradigm allows for greater experimental control. This, combined with the tools it provides to study learning, make it an ideal model system for studying learning, perception and action. Using a start small and build approach, is a perfectly logical way of getting to grips with some fundamental questions about how we learn to perceive and act in our world.
CRM in the lab: More Technical Jargon
Bingham, G. P. (2001). A perceptually driven dynamical model of rhythmic limb movement and bimanual coordination. Proceedings of the 23rd Annual Conference of the Cognitive Science Society, 75–79.
Bingham, G. P. (2004a). A Perceptually Driven Dynamical Model of Bimanual Rhythmic Movement (and Phase Perception). Ecological Psychology, 16(1), 45–53. http://doi.org/10.1207/s15326969eco1601_6
Bingham, G. P. (2004b). Another Timing Variable Composed of State Variables : Phase Perception and Phase Driven Oscillators. Advances in Psychology, 135, 421–442.
Haken, H., Kelso, J. a. S., & Bunz, H. (1985). A theoretical model of phase transitions in human hand movements. Biological Cybernetics. http://doi.org/10.1007/BF01068748
Kelso, J. A. S. (1981). On the oscillatory basis of movement. Bulletin of the Psychonomic Society, 18, 63.