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