When people stare at a bright color for too long, for example, red, and look away at a white field they will perceive a green color. Activation of one member of the pair inhibits activity in the other. This theory also helps to explain some types of color vision deficiency. For example, people with dichromatic deficiencies must match a test field using only two primaries. Depending on the deficiency they will confuse either red and green or blue and yellow. The opponent-process theory explains color vision as a result of the way in which photoreceptors are interconnected neurally. The opponent-process theory applies to different levels of the nervous system. Once the neural system passes beyond the retina to the brain, the nature of the cell changes and the cell responds in an opponent fashion. For example, the green and red photoreceptor might each send a signal to the blue-red opponent cell farther along with the system. Responses to one color of an opponent channel are antagonistic to those to the other color. That is, opposite opponent colors are never perceived together – there is no "greenish red" or "yellowish blue".