One of the most intriguing findings of this new science of reading is that the literate brain actually has two distinct pathways for reading. One pathway is direct and efficient, and accounts for the vast majority of reading comprehension -- we see a group of letters, convert those letters into a word, and then directly grasp the word's meaning. However, there's also a second pathway, which we use whenever we encounter a rare and obscure word that isn't in our mental dictionary. As a result, we're forced to decipher the sound of the word before we can make a guess about its definition, which requires a second or two of conscious effort.
How the individual human brain learns to read