Sensation and Perception - psychophysics.ppt © 2001 Laura Snodgrass, Ph.D.

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