Before revision:  For the firing of  VN neuron, the learning gain degree {L}_g, corresponding to the modulation gain ratio, increases with increasing  learning cycle, and it saturates at about the 300th cycle.
After revision:  For the firing of the VN neuron, the learning gain degree {L}_g, corresponding to the modulation gain ratio, increases with increasing learning cycle, and it saturates at about the 300th cycle.