The sound of one hemisphere clapping
What are the neurological correlates of “one hand clapping”, the most famous Zen koan in the world?
First, the koan challenges the processing patterns occurring in the association areas of the brain, where input from various sensory subsystems is integrated. In this case, we are asked to dissolve the normal association made in this area between the aural clapping input and the visual input involving two hands.
Second, the koan seeks to perturb the normal mapping between the motor commands to move the arm and the visual and proprioceptive inputs that the arm is actually moving. Put simply: can we will one of our arms to move, and believe it did, without it actually moving?
V. S. Ramachandran discusses a related situation in his brilliant book, Phantoms in the Brain. He describes the phenomenon where stroke patients believe they can move, or have moved, limbs which are in fact paralyzed.