The opposite process theory is an explanation of how the experiences of certain sensory and neurological phenomena are related to each other. Simply put, the body efficiently processes opposite experiences, such as fear and pleasure, in the same place, making it difficult for people to experience both at the same time. When stimulation at such a site evokes an experience, a person may experience an "afterimage" of the opposite experience after the stimulation has ended. Two areas where opponent process theory has been applied include understanding color vision and exploring the close connections between seemingly opposing feelings. This theory can also explain phenomena such as taste.
In understanding color vision, the opponent process theory is very important. Simply put, rods and cones perceive things in terms of black/white, red/green, and blue/yellow. When someone looks at a delicious red apple, for example, it stimulates the red/green cones. If someone looks at the apple for at least 30 seconds and then looks at a blank sheet of paper, a green ghostly image will be seen.
The oppositional process theory explains why people do not describe colors as "greenish-red" or "bluish-yellow," but see colors as "yellowish-green." The individual cones cannot simultaneously process green and red, but the green/red and blue/yellow cones can activate at the same time. It also explains why some people are red/green color blind, because cones sensitized to these wavelengths don't work properly or the brain can't receive signals from them.
This theory is also used to explain what happens when linked emotions are activated. One famous study included military paratroopers. When the men first jumped out of the plane, they were terrified, with all the physical symptoms of fear. Upon landing, they experienced a wave of relief. Upon repeated exposures to the experience, fear occurred in increasingly shorter durations, and relief turned to pleasure, turning skydiving from a terrifying activity to an exciting one.
Some people may have noticed that when very intense feelings are evoked, they are followed by a longer, less intense emotion that may be in opposition. Sometimes this secondary emotion sets in before the first one has completely dissipated, creating mixed feelings. Researchers have theorized that the opponent process theory may play a role in addiction, with people seeking drugs to get high to avoid low. While the opponent process theory cannot fully explain the complex processes involved in addiction, it certainly may be a contributing factor.