PSYCHOPHYSICS• What is Psychophysics?• Classical Psychophysics• Thresholds• Signal Detection Theory• Psychophysical Laws

What is Psychophysics?• Study of the relationship between physical

properties and psychological experience

Classical Psychophysics

• Assumption is that there is an absolute threshold - point above which a stimulus can always be detected and below which it can never be detected

Method of Limits

• Increase the intensity until the observer reports detection (ascending series)

• Decrease the intensity until the observer no longer reports detection (descending series)

• Look for an average over a number of series

Method of Adjustment

• Observer adjusts intensity until the stimulus is just detectable

• Can be done quickly, but results tend to be variable

Method of Constant Stimuli

• A range of intensities is selected and presented in random order; observer responds “Yes” or “No”

• Include “catch trials” on which no stimulus is presented

Do Absolute Thresholds exist?

• Gradual change from no detection to detection

• By convention, the threshold is the stimulus intensity at which the stimulus is detected 50% of the time

What Classical Psychophysics predicts:

What usually happens:

Difference Threshold

• Smallest difference between stimuli that can be detected

• Standard stimulus is compared to comparison stimulus

Signal Detection Theory

• How can we do psychophysics without absolute thresholds?

• Assumes that responses are a product of making decisions about signals embedded in noise

What Determines Responses in SDT?

• sensitivity - ability to detect stimuli• criterion - strategy for deciding how to

respond

What Affects Sensitivity?

• how good an observer you are• signal to noise ratio

What Affects the Criterion?

• payoff for responses• probability that the stimulus will occur

Psychophysics the SDT Way• Present trials with the stimulus present and

with the stimulus absent• Observer can respond yes or no on each trial• Sensitivity and criterion can be measured

Psychophysics the SDT Way

Hit False Alarm

Miss Correct Rejection

Stimulus Present?Yes No

Measuring Sensitivity

• When sensitivity is higher, there should be more hits and fewer false alarms

• d-prime (d’) is the standardized difference between the signal-present and signal-absent distributions

Sensory Activity

Signal Present Distribution

Sensory Activity

Signal Absent Distribution

Sensory Activity

Absent Present

Sensory Activity

d’

Sensory Activity

small d’

Sensory Activity

large d’

Measuring the Criterion

• The criterion () is the level of intensity at which the observer chooses to switch from “No” to “Yes” responses

Sensory Activity

Respond “No” Respond “Yes”

Criterion

Sensory Activity

Lax Criterion

Sensory Activity

Strict Criterion

Weber’s Law

• Describes how much of a physical difference is needed between two stimuli before people can just tell the difference (difference threshold or “just noticeable difference”)

• Works very well except at extremely low or high intensities

Weber’s Law

K= DL

SK = Weber fraction

DL = difference threshold

S = intensity of standard stimulus

Example of Weber’s Law

• If a standard stimulus weighs 10 pounds, how much different would another stimulus have to be for the difference to be noticeable?

K = .02 for weight

S = 10 pounds

.02 = DL

10

DL = .02(10) = .20 pounds

Stevens’ Power Law

• Describes relationship between physical and psychological intensity

• Sensory magnitude is an exponential function of physical intensity

• Exponent >1: Response Expansion• Exponent <1: Response Compression