1. Psych 480: Fundamentals of Perception and Sensation
Dr. Keith S. Jones

2. Why do we perceive?

3. Realism Idealism vs. The Ecological approach Information Processing
approaches
Gibson
Elaboration?
Yes No
Constructivist
approaches
Computational
approaches
Helmholtz
Rock
Marr

4. A general IP model Distal Stimulus Proximal Stimulus Transduction
(a lot can go wrong here)
Proximal
Stimulus
(if elaborative)
Transduction
“Processing”
Percept
Cognition
(if elaborative)

5. Psychophysics Distal Stimulus Proximal Stimulus Transduction
How do these relate to one another?
Proximal
Stimulus
(if elaborative)
Transduction
“Processing”
Percept
Cognition
(if elaborative)

6. 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?

7. Early attempts Gustav Fechner (1860) Concepts Methods
Absolute threshold
Difference threshold
aka “JND”
“Weber’s Law”
I/I=K
Methods
Method of constant stimuli
Method of limits
Method of adjustment

8. Fechner’s attempt His measurement system Fechner’s law
Sensation & Perception
09/15/99
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
©1999 Prentice Hall 6

9. Example Fechner’s law Perceived magnitude = K log Intensity Assume
absolute threshold = 50 units
difference threshold = 10%.

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)

11. 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)

12. Stevens’ attempt Methods Steven’s Power Law Magnitude estimation
Sensation & Perception
09/15/99
Exponent = .5
Stevens’ attempt
Methods
Magnitude estimation
Magnitude production
Cross-modality matching
Steven’s Power Law
Perceived magnitude = K Intensity power
brightness
Exponent = 1
line length
Exponent = 3.5
electric shock
©1999 Prentice Hall 6

13. 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.

14. 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,

15. “New” psychophysics Signal Detection Theory ß
If likelihood ratio (Psn/Pn) > ß,
then person says “Yes”,
else he or she says “No”

16. Signal Detection Theory

17. Signal Detection Theory
Correct
Rejection
False
Alarm
Hit
Miss

18. ß = height(hit)/ height(fa)
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)

19. How it works ...
d’ = zp(fa) - zp(hit) ß
Zhit = -.5
Zfa = 2.5

20. How it works ... ß = height(hit)/ height(fa) ß Height = .34

21. Rules of Thumb For d’, if it is For ß, 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

22. Tests of TSD Vary signal probability Pay-off matrices
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.