While reading the document, we get a sense of the organization or structure of the document, and the emphasis intended by the author, by noting the format and layout. At a glance we can determine where we are in the document. The visually communicated structure also lets us quickly locate specific sections of the document. For many computer users this same information must be gleaned by listening to a flat synthesized voice which reads only text. In North America, screen readers are the most popular computer access systems for people who are blind.
Screen readers present aurally, information which is visually presented on the computer screen.
Translating a rich visual environment or document into the aural channel is very difficult. The primary constraint is that, while we can process many pieces of visual information virtually simultaneously, the aural channel is largely serial. When information is spoken using a monotone voice synthesizer, information can only be presented one piece at a time in a linear or serial fashion. Screen readers were originally developed when computers displayed only text. They were adequate and well suited to the DOS interface which also presented information in a linear fashion.