Page 469 - Biomedical Engineering and Design Handbook Volume 2, Applications

TECHNOLOGY AND DISABILITIES 447

available when an on-screen key is highlighted by mouse cursor movement: clicking and dwelling.
In the latter the user keeps the mouse pointer on an on-screen key for an adjustable, preset time and
the key is entered. The on-screen feature also allows entry by scanning, using a hot key or switch-
input device. Several keyboard configurations are included, and an auditory click may be activated
to indicate entry of a character.
The graphical user interface (GUI) is used as the selection set for mouse entry. There are limita-
tions to the use of the GUI for people with disabilities. The first of these is that the GUI requires sig-
nificant eye-hand coordination. Pointing devices rely on a significant amount of coordination
between the body site (hand, foot, or head for most individuals with disabilities), executing the
movement of the screen pointer and the eyes following the pointer on the screen and locating the tar-
gets. Second, it relies on a visual image of the screen for operation. If either of these is not present
(e.g., muscle weakness or limited vision), then the GUI is difficult to use, and it must be adapted or
an alternative found.
The sensory feedback provided by the particular type of pointing device can vary widely. For
individuals who have some vision, a major form of feedback is vision, that is, following the cursor
on the screen. Devices controlled by the hand (e.g., mouse, trackball, joystick) also provide rich tactile
and proprioceptive feedback. Head- and eye-controlled pointing devices provide much less sensory
feedback, and other means, such as an LED that lights when the signal is being received are included
to aid the user. The type of sensory feedback affects the user’s performance, and the more feedback
available, the more likely the use will be successful.

15.3.2 Adapted Computer Outputs

User output from the computer is typically provided by either a video display terminal (VDT), a
printer, or speech or sounds. Use of any of these requires an intact sensory system. Alternatives that
can be provided may substitute auditory (e.g., speech) or tactile (e.g., Braille) for the VDT output or
visual cues for sounds or speech. In the case of low vision, the standard size, contrast, and spacing of
the displayed information is inadequate. For individuals who are blind, alternative computer outputs
based on either auditory (hearing) or tactile (feeling) modes are used. Persons who are deaf or hard of
hearing may also experience difficulties in recognizing auditory computer outputs. Adaptations that
facilitate some of these functions are included in Accessibility Options ∗ in Windows. These include
ShowSounds, which displays captions for speech and sounds, and SoundSentry, which generates a
visual warning when the system generates a sound.
Alternatives to Visual Input for Individuals Who Have Low Vision. Individuals who have low
vision require alternatives to the standard computer screen. Commercial screen magnifiers are used
to compensate for several of these problems. The most common adaptation is screen-magnifying
software that enlarges a portion of the screen. The unmagnified screen is called the physical screen.
Screen magnifiers have three basic modes of operation. These are lens magnification, part-screen
magnification, and full-screen magnification (Blenkhorn et at., 2002). At any one time the user has
access to only the portion of the physical screen that appears in this magnified viewing window. Lens
magnification is analogous to holding a handheld magnifying lens over a part of the screen. The
screen magnification program takes one section of the physical screen and enlarges it. This means
that the magnification window must move to show the portion of the physical screen in which the
changes are occurring. Part-screen magnification displays a magnified portion of the screen referred
to as the focus of the screen in a separate window usually at the top or bottom of the screen
(Blenkhorn et al., 2002). Typical foci are the location of the mouse pointer, the location of the text-
entry cursor, a highlighted item (e.g., an item in a pull-down menu), or a currently active dialog box.
Screen readers automatically track the focus and enlarge the relevant portion of the screen.
Adaptations that allow persons with low vision to access the computer screen are available in several
commercial forms. Lazzaro (1999) describes several potential methods of achieving computer access

∗ Microsoft, Seattle, Wash.