1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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