Psychophysics - Study Guide

How do we respond to stimuli?

• detecting

• perceiving magnitude

• describing

• recognizing

• searching

Absolute threshold

smallest amount of stimulus you can detect

Difference threshold

smallest difference intensity between two stimuli

How do absolute and difference threshold differ?

the number of stimuli

• absolute: 1 stimuli

• difference: 2 stimuli

Classical methods of psychophysics

• method of limits

• adjustment

• constant stimuli

Method of limits

gradually increasing or decreasing the intensity of a stimulus over time until they can be detected

• finding the point at which a stimulus becomes perceptible

• goes in numerical order (increasing: 100, 101, 102… decreasing: 90, 89, 88…)

Method of adjustment

the observer/experimenter adjusts the stimulus intensity in a continuous manner until the observer can just barely detect the stimulus

• the barely detectable intensity is taken as the absolute threshold

• least valid but fastest

Method of constant stimuli

different intensities in random order; where it is detected half the time is threshold

• slowest but most valid

What is the fundamental flaw with all of the classical methods?

they don’t look at false alarms/criterion

SDT: What/where are N distributions?

noise distribution, no signal (nothing present)

left side of SDT

SDT: What/where are S+N distributions?

signal and noise distribution, signal is present

right side of SDT

Criterion

level of evidence required to decide if signal is present (middle line)

Would someone want to be more conservative or liberal?

more liberal; more worried about misses - results in more false alarms than misses

Hits

saying something is there, and there actually is something there

Misses

saying something is not there, when something actually is there

Correct rejections

saying something is not there, and there is actually nothing there

False alarms

saying something is there, when something is actually not there

Hits + ______ = 100%

misses

False alarms + _____ = 100%

correct rejections

Hits and misses are under which distribution?

S+N Distributions

Correct rejections and false alarms are under which distribution?

N distributions

What happens to each 4 outcomes if the criterion shifts more liberal?

• hits will go up

• misses will go down

• false alarms go up

• correct rejections go down

What happens to the 4 outcomes if the criterion shifts more conservative?

• hits will go down

• misses will go up

• false alarms go down

• correct rejections go up

Purpose of ROC Curves

to determine and compare sensitivity levels

How is sensitivity (d’) determined?

by the distance between the peaks of the N and S+N distributions

Who is the most sensitive on the ROC curve?

most bowed line

• in picture: A

Who is more liberal/conservative if sensitivity is the same?

liberal: higher on the line

conservative: lower on the line

• in picture: line B; Aaron is most conservative, Chris is most liberal

What does it mean if there are multiple points/names on one line in an ROC curve?

they have the same sensitivity but different criterion

What happens to the difference threshold as the magnitude of the standard stimulus increases?

the size of the difference threshold increases

What is Webers’s Law?

the just noticeable difference between two stimuli is a constant proportion of the original stimulus intensity

What is Weber’s fraction?

K= DT/S

• K = constant

• DT = difference threshold

• S = standard stimulus

Magnitude estimation

the observer assigns numbers to stimuli that are proportional to perceived magnitude

Response compression

as intensity is increased, the perceptual magnitude increases, but not as rapidly as the actual intensity

• ex: brightness

Response expansion

as intensity is increased, perceptual magnitude increases more than actual intensity

• ex: electric shock

What is the difference between response compression and expansion?

compression = underestimation

expansion = overestimation

Search

some perceptual research uses methods that require the observer to respond as quickly as possible

• ex: visual search; finding “where’s waldo”

Reaction time

the time between presentation of the stimulus and the observer’s response to the stimulus

• measure from time it took to present stimulus until when it was noticed