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Difference Thresholds
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
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
amid all the other voices. Even after 2 years living in Scotland, all lamb baa s sounded alike to
my [DM’s] ears — but not to the lambs’ mothers. After shearing, I observed, each ewe would
Eric Isselée/Shutterstock lus difference a person can detect half the time . That detectable difference increases with the

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-

size of the stimulus. If we listen to our music at 40 decibels, we might barely detect an added
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
5 decibels (the jnd). But if we increase
the volume to 110 decibels, we proba-
bly won’t detect an additional 5-decibel
Copyright © Bedford, Freeman & Worth Publishers.
difference threshold the 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
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,
Weber’s law the principle that, two stimuli must differ by a constant
to be perceived as different, two minimum percentage (not a constant
stimuli must differ by a constant
minimum percentage (rather amount ). The exact percentage varies,
than a constant amount). depending on the stimulus. Two lights,
for example, must differ in intensity by
8 percent. Two objects must differ in

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
SPOTLIGHT ON: slightly. How many lines are required for you to experience ( Teghtsoonian, 1971 ).
Ernst Weber a just noticeable difference?

®
AP Science Practice
Check Your Understanding
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
▶ Explain the basic steps of transduction . threshold.

Answers to the Examine the Concept questions can be found in Appendix C at the end of the book.

Sensory Adaptation

1.6-3 What is the function of sensory adaptation?
1.6-3 What is the function of sensory adaptation?
It’s one of life’s little curiosities: You may not notice a fan’s noise until it’s turned off. The
same is true for odors. Sitting down on the bus, you are struck by your seatmate’s heavy per-
fume. You wonder how she endures it, but within minutes you no longer notice. Sensory
sensory adaptation diminished
sensitivity as a consequence of adaptation has come to your rescue. When constantly exposed to an unchanging stimulus,
constant stimulation. we become less aware of it because our nerve cells fire less frequently. (To experience sensory
adaptation, put a rubber band on your wrist. You will feel it — but only for a few moments.)

120 Unit 1 Biological Bases of Behavior

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