The principal differences between auding and reading originate through the nature of the display from which information is extracted. It is the different qualities of the display which produce the major task variables distinguishing auding from reading. For example, the speech message display is characterized by a temporally linear, transient message which occurs at a presentation rate determined by the source rather than the auder. Even though, in face-to-face situations, the auder can exert an influence upon the nature and rate of message production by the speaker, he is not in direct control over it. Conversely, the printed display is characterized by an existent, virtually permanent message which can be scanned in a pattern and at a rate determined by the reader. He can directly control the scanning process by developing specific search strategies.

This durable display is partially responsible for the result that printed messages are sometimes more conducive to comprehension than speech messages. The persistence of the printed message permits the reader to either scan ahead for predictive information, or backward in a regressive manner, for repetition of the message. In a word, the reading display is "referable," that is, by its nature it allows the reader to reread the message-thereby facilitating comprehension. It is not the mere physical presence of the printed message, or any advantage associated with the visual modality, that accounts for the sometimes superior comprehension of difficult material through reading; rather it is the fact that reading easily permits preview, review, and study of the message as a function of its permanency.

Other display considerations that distinguish auding from reading pertain to the respective supra segmental and orthographical aspects of speech and' print. Supra segmental factors such as intonation and rhythm are typically contained within the speech display, but do not exist within the printed display. It is granted that certain punctuational symbols and grammatical techniques are utilized in writing to approximate these qualities, but they generally cannot properly convey the intended verbal message.

In regard to orthography, the written representation of a word occasionally imparts more information than that of a spoken word. For example, when a reader sees the individual words (homophones) site, sight, or cite presented in print, the spelling differences facilitate conceptualization. In auding, however, the meaning of homophones must be derived solely from contextual information. That is, the auder requires additional information in order to properly conceptualize homophones in the auditory message.

In the case of homophones, then, the graphic display contributes information not available in the acoustic display, while in the latter, supplemental contribute information not available in the graphic display. Interestingly, however, most tests of reading and auding ability do not emphasize these differences in thee auding and reading languages. Rather, they generally focus on the information commonly available in both language displays.