Transforming abstract plans into concrete actions

Most of us, at one time or another, will have used a pen to sign our name on a check or legal document—or written our name on a white board with a magic marker. Some of us may even remember writing our name in the air with a sparkler in our younger years, or in the wet sand of a beach using our toe. What is striking is that the strokes and swirls we produce in writing our name in these various ways are remarkably similar, even though the muscles that are recruited to make the movements are quite different. The ability to generate the same pattern of movements with entirely different sets of muscles suggests that, somewhere in the brain, there must be an abstract high-level representation of the required action that transcends the recruitment of a particular limb or group of muscles. The fact that we can generate the same actions with different limbs and muscles is often referred to as “motor equivalence.”

Although handwriting and other complex sequences of movements constitute the most common examples in the literature, simpler goal-directed movements such as pointing, reaching, and grasping also show motor equivalence. For example, we are able to point toward an object of interest with our finger, with our foot, or with a stick or some other implement (Fig. 1). Despite the large differences in the muscles that are recruited, however, the kinematics of the different pointing movements are often quite similar—and the goal and function of the movements are certainly the same. Again, it seems, some sort of abstract representation of even a simple action such as pointing is encoded in the brain and is independent of the particular limb or muscles that are used to perform the action. But where in the brain are these abstract …

↵¹Email: mgoodale{at}uwo.ca.