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Sensory adaptation even influences how we perceive emotions. By creating a 50-50 morphed
blend of an angry face and a scared face, researchers showed that our visual system adapts to
a static facial expression by becoming less responsive to it (Butler et al., 2008; Figure 1.6-5). The
effect is created by our brain, not by our retinas. We know this because the illusion also works
when we view either side image with one eye, and the center image with the other eye.

Figure 1.6-5
Emotion adaptation
Gaze at the angry face on the
left for 20 to 30 seconds, then
look at the center face (looks
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
scared, yes?). Then gaze at the
scared face on the right for 20 to
30 seconds, before returning to
the center face (now looks angry,
yes?). (From Butler et al., 2008.)
Factors contributing to the adaptation after effects
of facial expression, Andrea Butler, lpek Oruc,
Christopher J. Fox, Jason J.S. Barton, Brain Research,
29 January 2008.
The point to remember: Our sensory system is alert to novelty. Bore it with repetition and
it frees our attention for more important things. We perceive the world not exactly as it is, but
as it is useful for us to perceive it.
Next up, let’s consider some marvels that enable our seeing, hearing, and in other ways
experiencing our worlds.

®
AP Science Practice Check Your Understanding
Apply the Concept
Examine the Concept Copyright © Bedford, Freeman & Worth Publishers.
▶ ▶Explain sensory adaptation. ▶ ▶In the last day, what types of sensory adaptation have you
experienced?
▶ ▶Why is it that after wearing shoes for a while, you cease to notice
them (until questions like this draw your attention back to them)?
Answers to the Examine the Concept questions can be found in Appendix C at the end of the book.

Module 1.6a REVIEW

1.6-1 Which three steps are basic to all of our we will detect a faint stimulus amid background noise.
sensory systems? Individual absolute thresholds vary, depending on the
strength of the signal as well as on our experience, expec-
• Our senses (1) receive sensory stimulation (often using tations, motivation, and alertness.
specialized receptor cells), (2) transform that stimulation • Our difference threshold (also called the just noticeable difference
into neural impulses, and (3) deliver the neural informa- [jnd]) is the minimum stimulus difference we can discern be-
tion to the brain. Transduction is the process of converting tween two stimuli 50 percent of the time. Weber’s law states
one form of energy into another. that two stimuli must differ by a constant minimum percent-
age (not by a constant amount) to be perceived as different.
1.6-2 How do absolute thresholds and difference
thresholds differ? 1.6-3 What is the function of sensory adaptation?

• Our absolute threshold for any stimulus is the minimum • Sensory adaptation (our diminished sensitivity to routine
stimulation necessary for us to detect it 50 percent of odors, sights, sounds, and touches) focuses our attention
the time. Signal detection theory predicts how and when on informative changes in our environment.

122 Unit 1 Biological Bases of Behavior

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