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Chapter 7 • Alternative and Augmentative Communication 189

The classification of AAC systems is not a precise taxonomy and the foregoing catego-
ries should be seen as independent, with classification in one area not necessarily indi-
cating or precluding classification in another. For example, a low-tech system such as a
communication book can be used to support both receptive communication, through
modelling of usage or aided language stimulation, and expressive language. Similarly, a
modified mainstream system could provide a variety of selection sets, thus supporting
both a symbol and fully literate user.

Components of an AAC System

While the development of robust low-tech and unaided systems is a crucial part of AAC
intervention, this section will focus primarily on the components of high-tech devices
and systems. All high-tech AAC systems are characterised by three key, interrelated
components: an input method, a selection set or language processor and an output
method (Cook et al., 2014). These components are interrelated, in that decisions made
about one may have an impact on decisions made about the others.

Input Methods

Access to AAC devices can be either direct or indirect. Direct access involves the user
pointing directly to the item they wish to select, using either a finger or stylus to access
a touchscreen, or by using a pointing device. Indirect access involves the use of one or
more switches and scanning through the available options within the selection set to
make a choice. As a general rule, direct access methods provide the AAC system with
continuous input such as the signals from a mouse controlling a cursor, and indirect
systems provide discrete input, such as the press of a switch or keyboard key to produce
a specific action. As such, while the selection of a direct or indirect access method will
be specific to the individual, it is generally held that a direct access method is faster
and more flexible, although it requires a greater level of motor control and coordina-
tion (Griffiths and Addison, 2017). The choice of a direct or indirect access method will
be highly specific to the individual and will also impact on a number of other choices
about the AAC system, including the layout and presentation of items within the selec-
tion set.
Many hardware options are available to clinicians selecting an access system for a per-
son using AAC (Chapter 5).
In recent years, the advent of eye-gaze control technology has provided a nonphysi-
cal direct access method to some users whose physical disabilities might have previously
precluded the use of direct access. While eye-gaze technology does not require a user to
physically interface with an AAC device and therefore may be appealing for those with
limited or impaired movement, the cognitive elements of such an access method are only
beginning to be understood, particularly for younger users with congenital disabilities
(Borgestig et al., 2016; Light and McNaughton, 2013; Myrden et al., 2014).