Reading basically involves transforming a text, which is a graphic representation, into thought, or meaning. It used to be thought that this was simply a matter of combining letters into words, words into sentences and sentences into meanings. However, over the last thirty years, psychologists and linguists, using a variety of experimental techniques, have discovered that things are much more complex. Several models of the reading process have been put forward to account for the experimental findings. A key element in explaining reading is the amount to which what the brain already knows affects perception of what is being read (top-down processing). This idea was initially thought to be in contrast to earlier ideas that reading was a linear progression from page to understanding (bottom-up processing), but newer research seems to indicate that both elements play important parts in reading.