Excerpted from Chapter 1 of Introduction to Language Pathology, Fourth Edition, by David Crystal, Ph.D., D.Sc., & Rosemary Varley, Ph.D.

Copyright © 1998 Whurr Publishers Ltd. All rights reserved. First published for exclusive distribution in North America by Paul H. Brookes Publishing Co. in April 1999. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

When would you say that someone was communicatively 'disabled'? Sometimes the disability is fairly obvious; but by no means is it always so. Let us begin with the most obvious case. Everyone would agree that there will be problems if a person lacks ability in one or more of the main modes of language use (speaking, listening, reading, writing) and in the various components of non-verbal communication; and such disabilities are common. There are many who totally lack the ability to communicate in speech, or who have severe hearing impairments, or who cannot read and write. But these disabilities, it should be noted, are not equal in importance. We have already noted that the speech-hearing route is the primary modality in language. Disabilities in speech-hearing have more fundamental effects than problems with reading and writing. Within the primary modality, it is speech that generally attracts the most attention because it is so much more obvious a facility to develop and use than is hearing and understanding. Considering the relative ease with which you are able to disguise lack of understanding of another's talk, for instance, in a lecture, with the embarrassing experience of having to speak upon a subject or answer a question on which you know very little.

Could there be anything more serious than the complete absence of ability to speak and to understand speech? That there are indeed such possibilities becomes clear when we put the study of language into the broader context of communication as a whole. At least if you are hearing impaired and without speech, and so denied easy use of the auditory-vocal channel of communication, you can communicate via visual channels through reading and writing or by gesture and signing. Given this perspective, the possibility of more serious breakdowns in communication than in speech-hearing alone is perhaps more obvious. A combination of vocal-auditory and visual disability, for example, will post special problems. Such problems would identify the population of "deaf-blind" children and adults. It is a disability that was first widely publicized when the story of Helen Keller was told. In such cases, tactile bases of communication have to be developed.

But language pathology is concerned with disorders beyond failures of sensory systems (hearing and vision) and movement systems (speaking and writing). Sensory systems are routes along which information travels to the brain; they allow the brain to monitor both the internal bodily environment and the external world for salient information. Movement systems permit action, or the modification of our environment in ways consistent with our needs; our visual receptors may inform us that a good friend is approaching, so we act by turning and producing a greeting. The brain lies at the centre of this information-processing system, and damage to the brain results in communication disorders that cannot be resolved simply by changing the route of information input (for example, from hearing to vision or to tactile information) or the kind of output (for example, from speech to writing or to signing). Individuals with damaged brains present language pathologists with some of their most challenging problems.