With paper, three quarters of students mark up texts as they read. This includes highlighting key passages, underlining, drawing pictures and writing notes in margins. The digital text disrupts a technique called cognitive mapping, in which readers used physical cues such as the location on the page and the position in the book to go back and find a section of text or even to help retain and recall the information they had read.
Source: Hickey, Hannah (May 2, 2011) College students' use of Kindle DX points e-readers role in academia. http://www.washington.edu/news/2011/05/02/college-students-use-of-kindle-dx-points-to-e-readers-role-in-academia/