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Module 1.6a

Why, then, if we stare at an object without flinching, does it not vanish from sight?
Because, unnoticed by us, our eyes are always moving. This continual flitting from one spot
to another ensures that stimulation of the eyes’ receptors continually changes (Figure 1.6-3).

Figure 1.6-3
The jumpy eye
Our gaze jumps from one spot
to another every one-third of
a second or so. Eye-tracking
equipment recorded a person’s
eye jumps while looking at this
photograph of Edinburgh’s
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
Princes Street Gardens
(Henderson, 2007). The circles
represent visual fixations, and
the numbers indicate the time
Copyright © Bedford, Freeman & Worth Publishers.
of fixation in milliseconds
(300 milliseconds = 3/10ths of a
second).
© John M. Henderson

®
AP Science Practice
What if we actually could stop our eyes from moving? Would sights seem to vanish,
as odors do? To find out, psychologists have devised ingenious instruments that maintain Research
a constant image on the eye’s inner surface. Imagine that we have fitted a volunteer, Mary, The ability to track and measure
with such an instrument — a miniature projector mounted on a contact lens. When Mary’s eye movements helped advance
the field of visual sensation.
eye moves, the image from the projector moves as well. So everywhere that Mary looks, the Investigators are now able to
scene is sure to go. Can you guess the weird result? (See Figure 1.6-4.) obtain concrete evidence of the
way our eyes naturally avoid
sensory adaptation. It is often
“Stabilized images on the retina,” by R. M. Pritchard. Copyright © 1961 Scientific American, Inc. All evolve.
advances in technology or
equipment that help theories

Rights Reserved.

®
(a) (b) AP Science Practice
Research
Figure 1.6-4 Remember that there are strict
Sensory adaptation: Now you see it, now you don’t! guidelines for the ethical treatment
(a) A projector mounted on a contact lens makes the projected image move with the of human participants. So while
the study presented here might
eye. (b) Initially, the person sees the stabilized image. But thanks to sensory adaptation, seem uncomfortable, the volun-
her eye soon becomes accustomed to the unchanging stimulus. Rather than the full teers provide informed consent.
image, she begins to see fragments fading and reappearing.

Although sensory adaptation reduces our sensitivity, it offers an important benefit: free-
dom to focus on informative changes in our environment. Technology companies under-
stand the attention-grabbing power of changing stimulation: Our phone’s notifications are
hard to ignore. If we’re performing other tasks, these intrusions can harm our performance
(Stothart et al., 2015).

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