the change are important. Changes that take place slowly over a long period
of time are not as readily perceived as changes that occur quickly.
(2) Fatigue: The fatigue associated with constantly or repeatedly
viewing an object (stimuli) decreases the sensitivity of the eye to change.
(3) Contrast: The ability of the eye to detect change is dependent
upon the differences (contrasts) of the stimuli. The greater the
difference, the easier to perceive change. For example, it is easier to
accurately perceive movement when flying over mountainous terrain with
differing types of vegetation and contour than it is to perceive movement
when flying over a desert with little or no contrast.
(4) Expectancy: People tend to see what they expect to see,
especially when there is little contrast between what is seen and what is
expected. For example, when navigating, a small lake may be perceived to be
the larger lake you expected to find along the flight path.
b. Perceptual Conflict. Because the strength of a stimulus tends to
dominate the entire visual perception, an object with a strong stimulus may
prevent the viewer from perceiving an object with a weaker stimulus. For
example, the presence of a large mountain may cause a failure to notice the
small hill in front of it. There are two other areas which may create a
perceptual conflict with which aviators must be familiar.
(1) Relative motion: Perceptions of movement are influenced by the
distance to and the movement of the object being viewed. When flying in
formation, a decrease in the speed of one aircraft may be perceived by the
occupants of the second aircraft as an increase in their speed.
(2) Past Experience: If your past experience (stimulus) of an object
is different than a new stimulus for the same object you may fail to
recognize it under the new stimulus. For example, if your past experience
has been that dirt roads are either brown or grey, you may fail to recognize
a dirt road that is red in color.
c. Factors of a Visual Perception. There are two major factors
effecting visual perception. They are visual acuity and depth perception.
(1) Visual acuity: Visual acuity is the ability to see clearly,
sharply and precisely. The degree of acuity is dependent upon the type of
vision used (peripheral and central). Peripheral vision is used extensively
at night and has limitations as covered in Lesson 2. The perceptual
limitations of central vision are time, accommodation and color.
(a) Time. The brain requires time to interpret what the eye
transmits to it. An example would be a mirage in the desert. It takes
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