When I walk, it feels like a unified action. I mean this in contrast to something like climbing a ladder whence I am extremely aware of the left-right-left-right nature of the commands I must send to my limbs in order to achieve my ascent.
I was thus quite surprised to learn from a paper appearing in journal last year that there appear to be completely separate control mechanisms for operating each of one’s legs while walking. I had (somewhat naively) assumed that my coherent ambulatory experience implied a single underlying motor-program or brain-circuit.
The authors of this paper showed that human beings have no trouble at all walking on a pair of treadmills (one for the left leg and one for the right) moving in opposite directions. Further, they had people abruptly switch between various combinations of directions (forward and forward, backward and forward, forward and backward, backward and backward) and speeds, with short periods (5-10 minutes) of readjustment. Because we essentially never encounter these types of situations in out every day experience, and yet adapt to them very rapidly, the authors concluded (sensibly, I think) that we must have distinct regulators of leg movement for walking.
On some level this is unsurprising, clearly it is possible to move one leg independently from the other. However my assumption above is not completely without basis; it was Charles Sherrington who won a nobel prize for the discovery that cats can execute a walking motion using only the neurons in their spinal cord. In a somewhat troublesome to consider series of experiments, he demonstrated that spinal-cord severed cats (no communication between spinal cord and brain) whose weight is mostly supported while their feet rest on a moving treadmill can go for a rudimentary stroll. These cats were in effect walking reflexively.
What is intriguing about this whole situation is the degree to which our consciousness has access to what is going on in our neurons. Obviously we don’t have to determine individual muscle tensions or relationships between contraction and flexion when we move, instead we have ideas like “kick the ball” or “walk up the stairs” and our subconscious translates those into motor output. But could it be possible to gain access to that information? Highly trained athletes and those who must be extremely in tune with their bodies probably have a much greater degree of control, but they probably never feel a motor neuron’s spike rate change as they command it to apply more force. Thus, at some level, we simply do not have conscious control of our bodies.
This is a bit unsettling, but it is also exciting because it means that we really must reframe the way we think about the relationship between minds and brains. At least, I must not take for granted that my consciousness is a total reflection of what is happening in my brain.
1. Choi JT, Bastian AJ. (2007) Adaptation reveals independent control networks for human walking. Nat Neurosci. 10(8):1055-62.