Final answer:
The opponent-process theory explains why it is more difficult to distinguish a light increase in a well-lit room compared to a darkened room.
Step-by-step explanation:
The opponent-process theory offers the best explanation for why people find it more difficult to distinguish a light increase in an already well-lit room compared to an identical light increase in a darkened room.
This theory states that color is coded in opponent pairs, such as black-white, yellow-blue, and green-red. Some cells in the visual system are excited by one of the opponent colors and inhibited by the other. When there is already a high level of light in a well-lit room, the cells that are excited by the opponent color are already firing at a high rate, making it more difficult to distinguish a light increase. In a darkened room, however, the cells firing at a low rate can easily detect a light increase.
For example, in a well-lit room where there is already a high level of green light, the cells that are excited by green and inhibited by red are already firing at a high rate. When there is a slight increase in green light, the firing rate of these cells doesn't change much, making it difficult to perceive the change. In a darkened room, the cells that are excited by green and inhibited by red are firing at a low rate. When there is a slight increase in green light, the firing rate of these cells increases significantly, making it easier to perceive the change.