For a period of time, Energizer Batteries used mock commercials for seemingly boring, mundane products. A few seconds into the mock commercial, viewers heard the distinctive drumbeat of the Energizer Bunny before it marched across the screen. Several viewers said that when they heard the drums, they looked at and attended to the commercial because they liked the bunny. However, over time, viewers stopped paying attention to the commercial. What theory suggests that if a stimulus doesn't change, over time we adapt or habituate to it and begin to notice it less

The theory which suggests that if a stimulus doesn't change, over time we adapt or habituate to it and begin to notice it less is the adaptation level theory.

Explanation:

According to the adaptation-level theory, after being exposed to the same stimulus repetitively or for a longer period of time, we become less sensitive and, eventually, insensitive to that stimulus. For example, when we enter a bakery, we immediately identify the smell of bread, bagels etc. But after just a few minutes inside the bakery, our receptors get used to the stimulus and we no longer smell baked goods.

That is the case with the commercial. At first, the drumbeat functioned as a stimulus, and people were interested in seeing the Bunny. After being repetitively exposed to that same sound and image, people gradually grew used to it, not feeling interested in looking anymore.