Psych 480: Fundamentals of Perception and Sensation
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Transcript of Psych 480: Fundamentals of Perception and Sensation
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Psych 480: Fundamentals of Perception and Sensation
Dr. Keith S. Jones
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Why do we perceive?
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Realism Idealism
Elaboration?
The Ecological approach
Yes No
vs.
Information Processing approaches
Constructivist approaches
Computational approaches
MarrHelmholtz Rock
Gibson
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A general IP model
Distal Stimulus
Proximal Stimulus
Transduction “Processing” Percept
(a lot can go wrong here)
(if elaborative)
Cognition(if elaborative)
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Psychophysics
Distal Stimulus
Proximal Stimulus
Transduction “Processing” Percept
How do these relate to one another?
(if elaborative)
Cognition(if elaborative)
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Activity (2 pts)• How would you measure the
magnitude of a perception?– Describe your measurement technique.– Explain how it accomplishes your goal.– What problems, if any, exist with your
technique?
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Early attempts• Gustav Fechner (1860)
– Concepts• Absolute threshold• Difference threshold
– aka “JND”– “Weber’s Law”
I/I=K– Methods
• Method of constant stimuli• Method of limits• Method of adjustment
http://www.uni-leipzig.de/fechnerday/
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Fechner’s attempt• His measurement system
• Use a person’s absolute and difference thresholds as a counting system.• Assumes that
• All JNDs are subjectively equal• Weber’s law is true
• Fechner’s law• Perceived magnitude = K log Intensity
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Example• Fechner’s law
• Perceived magnitude = K log Intensity
• Assume– absolute threshold =
50 units – difference threshold
= 10%.
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Problems• Weber’s law is NOT always true
– It breaks down at the extremes
• Stevens (1957) argued that JNDs are not subjectively equal.– He argued that they might be for
metathetic dimensions (e.g., pitch, color), but not prothetic dimensions (e.g., loudness, brightness)
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The tower of Babel was never finished because the workers could not reach an understanding on how they should build it; my psychophysical edifice will stand because the workers will never agree on how to tear it down. (Fechner, 1877)
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Stevens’ attempt• Methods
– Magnitude estimation– Magnitude production– Cross-modality matching
• Steven’s Power Law– Perceived magnitude
= K Intensity power
brightness
line length
electric shock
Exponent = .5
Exponent = 1
Exponent = 3.5
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Problems• If you use a category rating scale (i.e., a likert
scale), then data follow a logarithmic function, not a power function.– Stevens argues that these scales are inherently biased
b/c the categories are not subjectively equal.
• There is also evidence to suggest that magnitude procedures are affected by context as well.
• Exponent inconsistency over time.
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The debate rages on ...• Many applied settings have adopted
Stevens’ Power Law as a standard.
• However, from a theoretical standpoint, work is on-going.– Krueger, L.E. (1989). Reconciling Fechner and
Stevens: Toward a unified psychophysical law. Behavioral and Brain Sciences, 12, 251-320.
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“New” psychophysicsSignal Detection Theory
If likelihood ratio (Psn/Pn) > ß, then person says “Yes”,else he or she says “No”
ß
http://acad.cgu.edu/wise/sdt/sdt.html
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Signal Detection Theory
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Signal Detection Theory
CorrectRejection
FalseAlarm
Hit
Miss
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Calculating d’ and ß• Based on the p(hit) and p(false alarm),
you can calculate d’.
d’ = zp(fa) - zp(hit)
• Based on the height of the curves, you can calculate ß.
ß = height(hit)/ height(fa)
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How it works ...
ß
d’ = zp(fa) - zp(hit)
Zfa = 2.5
Zhit = -.5
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How it works ...
ß
ß = height(hit)/ height(fa)
Height = .34
Height = .05
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Rules of Thumb• For d’, if it is
– d’ < 1.5 = difficult– d’ = 1.6 to 2.5 =
moderately difficult– d’ = 2.6 to 3.5 =
moderately easy– d’ > 3.5 = easy
• For ß, – three or less is common– If greater than three,
person is conservative
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Tests of TSD• Vary signal probability
– Should change ß w/o changing d’• Pay-off matrices
– Systematically affect ß by changing operators strategies w/o changing d’
• Vary quality of signal– Should affect d’ w/o affecting ß
• 2nd chance experiment– If person is incorrect, then must be below threshold so
giving a 2nd choice shouldn’t matter. If TSD is correct, then 2nd choice should beat chance b/c people differentially weight different possibilities.