So it looks in the paper they're using a theoretical framework called the Free Energy Principle developed by Prof Karl Friston at UCL which postulates that brains have evolved to minimise the amount of information surprise between their own internal generative model of the world and the observable world.
The paper uses this as the basis of learning and it looks like the neurons are given a random (electrical) stimulus when they miss the ball and a predictable stimulus when they hit it.
The paper uses this as the basis of learning and it looks like the neurons are given a random (electrical) stimulus when they miss the ball and a predictable stimulus when they hit it.