Opposing neurobiological structures and functions exist throughout the mammalian brain. This can be seen at every neurological level, from the individual neuron through the communication between entire brain regions. The brain operates through excitatory and inhibitory stimulation controlled by various neurotransmitters. This is particularly evident when examining appetitive/aversive stimuli associated with approach/avoidance behaviors. It is this paradigm of opposite forces which likely lays the foundation for opponent pro cess.