Hands together?

Yesterday my piano teacher suggested, not for the first time, that I spend more time practising the left and right hand parts of musical pieces separately. When learning a new piece I'll often work out the notes and fingering with separate hands but try to play hands together as soon as I can. Melody, harmonies and dynamics make more sense when both hands play together, especially if the tune moves between the left and right hands.

I'm learning pieces for Grade 5 and last night practised separate hands for a couple of sections where I've been making mistakes. Playing with just one hand allowed me to pay more attention to each part and helped me to clear up a couple of small fingering errors. I was almost converted to the doctrine of separate hands until I came across this interesting blog entry Hands together? by Tara Gaertner, a piano-playing neurophysiologist. She's identified a small number of studies looking at how the brain manages control of right and left sided movement and listed two factors which impact on bilateral body control; one is the limited bandwidth of attention, the other is cross-cortical inhibition.

A complex movement can be broken down into a series of small chunks, each chunk can be slowly repeated, practising and refining the movement until it is embedded in memory. I almost wrote 'muscle memory' but there's no such thing. Motor memory is a consequence of re-inforcing the neuronal loops which encode the movement, thickening the myelin sheath of nerves which are activated during the movement and improving the accuracy and speed with which the action can be repeated. Attention has a small and finite limit and it's clearly easier to learn a new motor skill if all one's attention can be focused on movements of a single hand.

Cross-cortical inhibition occurs because humans have a natural tendency for mirror movements:  when one hand moves, the other hand tends to mirror its movements. This is seen in infants and young children, but as we develop motor control, we learn to stop these mirror movement. The motor cortex of each side of the brain gives off axons that travel through the corpus callosum (the thick fiber band that connects the two sides of our brains) to inhibit the motor cortex of the other side.  This cross-cortical inhibition is particularly pronounced when we’re only using one hand. When we play with our left hand, our right motor cortex is active, and it not only sends motor commands to our left hand, but it also sends commands to the left motor cortex, telling it not to move the right hand.  And vice versa when we play with our right hand. So, if we’re practicing only the right hand part of a piano piece, we’re probably learning to inhibit the left hand. Is it any wonder then, that when we go to play the song hands-together, it’s really difficult?  Perhaps hands-separate practice is a little bit counterproductive.