By analysing brain activity, researchers found that the brain regulates its resource use and tries to identify the most essential information.A recently completed study indicates that the human brain avoids taking unnecessary effort. When a person is reading, she strives to gain as much information as possible by dedicating as little of her cognitive capacity as possible to the processing.

Source: The brain uses minimum effort to look for key information in text | University of Helsinki

The study is at the start, but preliminary results show that the brain processes text information using as little effort as possible. There is more brain activity on complex words, the researchers believe because the brain is trying to figure out the importance in the words in the overall text.