Transcript Psychophysics Outline

Psychophysics Outline
• Classical Psychophysics
– definition
– psychometric function
– sensory scales
• zero point = absolute threshold
– step function versus ogive
– sources of variability
• units = difference threshold
– jnd
– psychophysical laws
Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.
Outline
• Signal Detection Theory
– why an improvement
– d’ and B
– factor affecting d’ and B
– ROC curves
• comparing perceivers
Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.
Classical Psychophysics
• Purpose:
To derive the mathematical relationship between the
experiential aspect of perception and the physical
characteristics of the stimulus
• Psychometric Function
– formula that relates subjective experience to the
physical stimulus
– cannot use verbal descriptions
Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.
Sensory Scales
• Zero point = absolute threshold
– theoretically the minimal detectable energy
– sensitivity is the inverse of threshold
– should get a step function
– really get an ogive
Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.
Sources Of Variability That Produce The Ogive
• Imprecise equipment
• Perceiver
– learning and adaptation
– drugs, fatigue, boredom, hunger
– responses are difficult
• self-confidence
• bias to say yes
– transients (noise) in the nervous system
Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.
Sensory Scales
• Absolute threshold
– stimulus that is detected on 50% of the trials
• Units = difference threshold
– smallest detectable change in perception
– the change that is detected on 50% of the trials
– also called the “jnd” just noticeable difference
Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.
Psychophysical Laws
• History
– Bernoulli (1738), the psychological worth of
money
• Weber, Fechner, Stevens
– variations on mathematical functions that all
describe the same relationship
– the amount of stimulus needed to notice a
change depends on what you already have
– the more stimulus already present the larger
the change needed to notice a difference
Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.