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Difference Thresholds
ENGAGE 1.6-2 To function effectively, we need absolute thresholds low enough to allow us to detect
important sights, sounds, textures, tastes, and smells. We also need to detect small
(10 minutes) Have students try a differences among stimuli. A musician must detect minute discrepancies when tun-
ing an instrument. Students in the hallway will detect the sound of their friends’ voices
precise weight discrimination task by amid all the other voices. Even after 2 years living in Scotland, all lamb baa s sounded alike to
using Student Activity: Understanding my [DM’s] ears — but not to the lambs’ mothers. After shearing, I observed, each ewe would
Weber’s Law. This quick and easy streak directly to the baa of her lamb amid the chorus of other distressed lambs.

The difference threshold (or the just noticeable difference [jnd] ) is the minimum stimu-
activity provides a nice demonstration Eric Isselée/Shutterstock lus difference a person can detect half the time . That detectable difference increases with the

of that law. size of the stimulus. If we listen to our music at 40 decibels, we might barely detect an added
5 decibels (the jnd). But if we increase
M1.6a: Understanding the volume to 110 decibels, we proba-
Weber’s Law difference threshold the bly won’t detect an additional 5-decibel
change.
minimum difference between
two stimuli required for In the late 1800s, German physician
detection 50 percent of the time. Ernst Weber described a principle so
ENGAGE 1.6-2 We experience the difference simple and so widely applicable that we
threshold as a just noticeable
still refer to it as Weber’s law : For an

difference (jnd). average person to perceive a difference,
(10 minutes) To demonstrate Weber’s Weber’s law the principle that, two stimuli must differ by a constant
law, students will need three quar- to be perceived as different, two minimum percentage (not a constant
stimuli must differ by a constant
ters, two envelopes, and a pair of minimum percentage (rather amount ). The exact percentage varies,
depending on the stimulus. Two lights,
shoes. Have them place one quarter than a constant amount). for example, must differ in intensity by
in one envelope and the remaining 8 percent. Two objects must differ in
two quarters in the other envelope. The difference threshold In this copy of the Twenty- weight by 2 percent. And two tones must

Third Psalm, each line of the typeface increases in size
differ in frequency by only 0.3 percent
When they lift each envelope, they SPOTLIGHT ON: slightly. How many lines are required for you to experience ( Teghtsoonian, 1971 ).
can easily determine which is heavier. Ernst Weber a just noticeable difference?
Now have them put each envelope in
a shoe. When they lift the shoes, one AP Science Practice Check Your Understanding
®
at a time, the weight difference will be
imperceptible. Examine the Concept Apply the Concept
▶ Explain the difference between bottom-up and top-down ▶ Using sound as your example, explain how these concepts
processing. differ: absolute threshold, subliminal stimulation , and difference
Information from Coren, S., Ward, L. M., & Enns, ▶ Explain the basic steps of transduction . threshold.
J. T. (2003). Sensation and perception (6th ed.). Answers to the Examine the Concept questions can be found in Appendix C at the end of the book.

Wiley. Copyright © Bedford, Freeman & Worth Publishers.
TEACH 1.6-3 Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
Sensory Adaptation
Teaching Tip 1.6-3 What is the function of sensory adaptation?

1.6-3
What is the function of sensory adaptation?
A fun way to introduce sensory adap- It’s one of life’s little curiosities: You may not notice a fan’s noise until it’s turned off. The
tation is to ask students to feel differ- same is true for odors. Sitting down on the bus, you are struck by your seatmate’s heavy per-
ent items they wear on a daily basis, sensory adaptation diminished fume. You wonder how she endures it, but within minutes you no longer notice. Sensory
adaptation has come to your rescue. When constantly exposed to an unchanging stimulus,
such as a headband, glasses, socks, sensitivity as a consequence of we become less aware of it because our nerve cells fire less frequently. (To experience sensory
constant stimulation.
jewelry, etc. Be descriptive as you adaptation, put a rubber band on your wrist. You will feel it — but only for a few moments.)
ask students about these items they
are wearing: Do you feel your socks 120 Unit 1 Biological Bases of Behavior
gripping your ankles? Are your glasses
pinching your nose? Students become
squirmy as they “feel” these items 03_myersAPpsychology4e_28116_ch01_002_163.indd 120 15/12/23 9:25 AM
rather intensely! Use this exercise to
emphasize the importance of sensory
adaptation.

120 Unit 1 Biological Bases of Behavior

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