The opponent-process theory is a theory of color vision in humans. It states that the brain processes color by registering the difference in activity between the three different types of cone cells in the eye. Each type of cell registers the wavelength of a particular spectrum of colors. The brain uses information about changes in the cone cells in order to construct a color image. When you close your eyes, the brain continues to register the difference in activity between the three types of cone cells. Because it takes a few seconds for the activity in your cone cells to stop, your brain continues to “see” the images even though your eyes are closed.