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

Why do we perceive?

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

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)

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

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 Methods 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 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

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 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

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

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)

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

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

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.