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.

From my very limited understanding, feedback is done by releasing chemicals. Not sure if hormone is the proper word but that's what comes to mind. Perhaps someone else here knows more about it.