Psych 480: Fundamentals of
Perception and Sensation
Dr. Keith S. Jones
Why do we perceive?
Realism
vs.
The Ecological
approach
Idealism
Information
Processing
approaches
Gibson
Elaboration?
Yes
Constructivist
approaches
Helmholtz
Rock
No
Computational
approaches
Marr
A general IP model
Distal
Stimulus
(a lot can go wrong here)
Proximal
Stimulus
(if elaborative)
Transduction
“Processing”
Percept
(if elaborative)
Cognition
Psychophysics
Distal
Stimulus
Proximal
Stimulus
How do these relate to one another?
(if elaborative)
Transduction
“Processing”
Percept
(if elaborative)
Cognition
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?
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/
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
Example
• Fechner’s law
• Perceived
magnitude =
K log Intensity
• Assume
– absolute threshold =
50 units
– difference threshold
= 10%.
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)
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)
Stevens’ attempt
• Methods
– Magnitude estimation
– Magnitude production
– Cross-modality matching
• Steven’s Power Law
– Perceived magnitude
= K Intensity power
Exponent = .5
brightness
Exponent = 1
line length
Exponent = 3.5
electric shock
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.
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.
“New” psychophysics
Signal Detection Theory
ß
If likelihood ratio (Psn/Pn) > ß,
then person says “Yes”,
else he or she says “No”
http://acad.cgu.edu/wise/sdt/sdt.html
Signal Detection Theory
Signal Detection Theory
Correct
Rejection
False
Alarm
Hit
Miss
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)
How it works ...
d’ = zp(fa) - zp(hit)
ß
Zhit = -.5
Zfa = 2.5
How it works ...
ß = height(hit)/ height(fa)
ß
Height = .34
Height = .05
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
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.