Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 83

change their response bias, such as making clear to them the costs of false alarms
and misses (see Wickens et al. 2014 and Wickens 2002 for more complete lists).
4.3 The Physiology of Vision
In the next few sections we examine the details of vision. We start by offering a high-
level description of the physical structure of the eye. This should help you under-
stand the basic idea of how vision works and what some of its important limitations
are. Unlike a camera snapshot, for example, the eye does not capture everything in a
scene equally, but selectively picks out salient objects and features from the current
context, and focuses on them so they can be processed in more detail.
4.3.1 Overview of Vision
For normally sighted people, vision is by far the most widely used sense. Vision is
important in everyday work because it allows us to use interfaces like those shown
in Fig. 4.2 . Understanding the basics of how human vision works, including its
strengths and weaknesses, will help you to design systems that more closely match
your user's visual capabilities.
4.3.2 The Basic Structure of the Eye
Figure 4.3 shows the basic structure of the eye. The important physiological
features that you need to be aware of are the lens, the retina, the rod and cone cells
(the sensory receptors in the eye which respond to different light waves), the fovea,
and the optic nerve.
When you look at an object, an image of it is projected onto the eye. The angle
that is subtended by that object at the eye is described as the visual angle. The
ability to discriminate between two objects that are close together is described as
visual acuity. This is usually expressed in terms of the minimum visual angle that
can be resolved, for which the standard is normally a gap subtending 1 min of arc
(1/60 of a degree). Acuity is usually expressed as a fraction which expresses the
ratio of the standard distance used in eye-tests (20 ft or 6 m) to the distance at
which a gap subtends 1 min of arc. So when you hear someone described as having
20/20 (or 6/6) vision, the numbers refer to their visual acuity.
The light coming from an object is focused by the lens and projected onto the
retina at the back of the eye. Muscles attached to the sides of the lens contract in
order to thicken the lens and bend the light more to achieve a clear focus when the