In Chapter 7 Carr describes advantages and disadvantages of getting content online. He focused mostly on the disadvantages as you might expect as the book is called The Shallows.
First, reading a hypertexted document reduces overall comprehension. This occurs not only from following the links. Having to decide whether to follow the link requires cognitive load. This takes part of the brain’s attention away from deeper comprehension and making connections.
Second, competing media do not enhance the deepness with which something is learned. Having video embedded in a text document, or a text crawl on a video divided of experiment participants so much that they could not remember as much afterwards as control groups that worked with one media. Pictures and sound together may be one exception to not using mixed media.
Third, people skim long text as web content, or skip it altogether.
Carr speculates that the brain gets better at speedily categorizing whether information is relevant and important with more web use. Users also get slightly better working memories. The cost of these gains comes from weakening of brain cells for calm linear thought and lengthy contemplation. Users get better at hunting for information and worse at creating it or reflecting on it.
I could use this for designing lessons in online courses. If deep thought is required to understand material then I’ll hold links and competing media to another page, rather than integrating them in the traditional way. I will limit the amount of text on a single page to discourage skimming if deep reading is required.