AP Psychology
Name___________________________________
Psychophysics
Learning Targets:
 Explain how stimulus intensity is related to absolute threshold and just noticeable difference (JND)
 Analyze how Weber’s law is related to the concept of just noticeable difference and how these concepts relate
to psychophysics
Detection vs. Difference
Detection: The detection question is concerned with the limits on our ability to detect very faint signals
– How intense does a light have to be for us to see it?
– How intense does a sound have to be for us to hear it?
• Absolute threshold is the minimum amount of energy in a sensory stimulus that is detected 50% of the
time
• Subliminal stimulus is one that is detected only up to 49% of the time
***In an absolute threshold, an individual is moving from not being able to detect a stimulus at all to being
able to detect something.
Difference: The difference question is concerned with limits on our detection abilities, but in this case with our
ability to detect very small differences between stimuli
– What is the smallest difference in brightness between two lights that we can see?
– What is the smallest difference in loudness between two sounds that we can hear?
•
A difference threshold (also called a just noticeable difference, or jnd) is the minimum difference
between two stimuli that is detected 50% of the time
***In a difference threshold, the individual already has a stimulus present, but they are able to identify that
a change has occurred.
• Weber’s Law says that for each type of sensory judgment, the difference threshold is a constant
fraction of the standard stimulus value used to measure it
________________________________________________________________________________________
Directions: Read the following statements and determine which of the following terms is being
referenced: absolute threshold, just noticeable difference, and Weber’s Law.
1. Janice picks up her gym bag only to notice that it feels heavier than before, she is not sure because
she can hardly detect a chance but investigates further, and she finds that her son has added a
matchbox car to her workout gear.
2. Knute has told his son to go and complete his homework in his bedroom and not to use any technology,
ten minutes after his son goes upstairs Knute believes that he hears the faint buzz of the television.
3. Your friend hates carrying her books around school, so she has been secretly placing her things in your
backpack during English class. You have not noticed when she has placed pencils or her folder in your
backpack but now that she has put her AP Psychology textbook in your bag, you immediately notice a
difference.
4. The server has placed one sugar cube in your tea as you requested, and you can just detect a hint of
sweetness.
5. Had he added another sugar cube when your back was turned, you would have noticed that your tea
was sweeter.
6. Harold is training for the Olympics in the category of weight lifting, he does not notice when his trainer
adds 10 more pounds to his 300-pound deadlift, but does notice when twenty pounds is added.
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AP Psychology
Name___________________________________
7. Gustav Fechner described being able to identify the minimal change necessary for one to detect that
something has changed.
8. Briefly describe the difference between an absolute and a difference threshold.
9. If a stimulus falls below one’s absolute threshold, it is said to be SUBLIMINAL, the minimum stimulus
need to reach one’s absolute threshold is ____.
10. Each morning you listen for the sound of a cardinal at your bedroom window, as you get ready for
school you hear a very faint sound you instantly identify as you friend the cardinal.
KEY
1. Janice picks up her gym bag only to notice that it feels heavier than before, she is not sure because
she can hardly detect a chance but investigates further, and she finds that her son has added a
matchbox car to her workout gear.
JND
2. Knute has told his son to go and complete his homework in his bedroom and not to use any technology,
ten minutes after his son goes upstairs Knute believes that he hears the faint buzz of the television.
ABSOLUTE THRESHOLD
3. Your friend hates carrying her books around school, so she has been secretly placing her things in your
backpack during English class. You have not noticed when she has placed pencils or her folder in your
backpack but now that she has put her AP Psychology textbook in your bag, you immediately notice a
difference.
WEBER’S LAW
4. The server has placed one sugar cube in your tea as you requested, and you can just detect a hint of
sweetness.
JND
5. Had he added another sugar cube when your back was turned, you would have noticed that your tea
was sweeter.
JND
6. Harold is training for the Olympics in the category of weight lifting, he does not notice when his trainer
adds 10 more pounds to his 300-pound deadlift, but does notice when twenty pounds is added.
WEBER’S LAW
2
AP Psychology
Name___________________________________
7. Gustav Fechner described being able to identify the minimal change necessary for one to detect that
something has changed.
JND
8. Briefly describe the difference between an absolute and a difference threshold.
In an absolute threshold, an individual is moving from not being able to detect a stimulus at all
to being able to detect something. In a difference threshold, the individual already has a
stimulus present, but they are able to identify that a change has occurred.
9. If a stimulus falls below one’s absolute threshold, it is said to be SUBLIMINAL, the minimum stimulus
need to reach one’s absolute threshold is 50%.
10. Each morning you listen for the sound of a cardinal at your bedroom window, as you get ready for
school you hear a very faint sound you instantly identify as you friend the cardinal.
ABSOLUTE THRESHOLD
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