The parieto-frontal circuits are the basic compositions of the main elements of the cortical motor system. These circuits depend on the motor area to receive afferent information from the parietal areas. The input in one area is predominant, containing full amounts of information. The other input area is known as moderate or weak. When the input is moderate or weak, it contains additional secondary information. Each parietal area is connected to several motor areas. However, it only makes privileged contact with one motor area. Exceptions to this include the prefrontal gyrus, where the parietal area sends an equal amount of fibers to many motor areas. This interaction is vital because the activity in the facial muscles is due to voluntary control of the direct and indirect pathways that are corticobulbar pathways. Facial muscles often respond to emotional influences by these pathways also. Most of our emotions are expressed more intensely on the left side than the right side of the face. The reason for the asymmetry however, remains unclear, a commonly concluded theory is that the right side of the hemisphere has an advantage in emotional processing than the left hemisphere. To examine facial muscle movement often, transcranial magnetic stimulation (TMS) is used.