Complex sentence: Premovement neuronal activity has been widely experimented upon in three major motor fields of the frontal cortex with the goal of comparing the neuronal activity which comes from visual signals, versus neuronal activity which comes form non-triggered or self-paced movements.
Split into simpler sentences: Premovement neuronal activity has been widely experimented upon in three major motor fields of the frontal cortex. The goal of this experimentation is to compare the neuronal activity which comes from visual signals, versus neuronal activity which comes from non-triggered or self-paced movements.