2. Psychophysics

Question 1

What are the questions asked in relation to thresholds?

Answer 1

– What is the smallest change in a stimulus that can be detected?
– What is the smallest/largest magnitude of stimuli for
each sensory modality?

Question 2

What are the questions asked in relation to signal detection?

Answer 2

– How do we detect a signal out of noise?

Question 3

What are difference thresholds?

Answer 3

The smallest change in a stimulus that can be

detected (a.k.a. JND–Just Noticeable Difference)

Question 4

what is the absolute threshold?

Answer 4

The minimum intensity of a stimulus that can be

detected

Question 5

What is Weber’s Law?

Answer 5

Weber found that the size of the JND is a
function of the magnitude of a reference
stimulus
– For example, if a weight has to be 41 g
before it can be discriminated from a 40 g
reference weight (JND = 1 g), then the JND
would be 10 g for a 400 g reference weight

Question 6

What is the formula for Webers law?

Answer 6

JND / Physical intensity of a reference stimuli = Constant

This constant ratio of JND and the intensity of a
reference stimulus is called the Weber fraction for that stimulus dimension

Question 7

What did Fechner’s law build on?

Answer 7

WEBER’S FINDINGS

Question 8

what was fechner’s idea?

Answer 8

if a Weber fraction is constant for a given stimulus dimension, then the mind might use the Weber fraction
as a unit for perceiving that stimulus dimension

Question 9

What does Fechner’s law relate to?

Answer 9

intenral experience (psyche) and physical environment (physics)

Psyche + physics –> Psychophysics

Question 10

What is fechner’s law about?

Answer 10

Fechner’s law is about the absolute, not relative,
intensity of a stimulus
– Turning the focus of research from difference
thresholds to absolute thresholds

Question 11

What does Fechner’s law asset about our psychology experience of the intensity of a stimulus?

Answer 11

Fechner’s law asserts that our psychological
experience of the intensity of a stimulus tends to
change less quickly than the actual change in
stimulus intensity

Question 12

What did Galanter (1962) suggest is the reason why we suck at measuring thresholds?

Answer 12

It is not always easy to measure thresholds in
part because we are so good as perceivers
– For example, we can (Galanter, 1962):
• sight a candle flame from a distance of 48 km on a clear dark
night
• hear a mechanical watch ticking at a distance of 6 m in a
noise-free environment
• taste a teaspoon of sugar dissolved in 7.6 liters of water
• smell one drop of perfume diffused through three rooms
• feel the wing of a fly dropped on our cheek from a height of 7
cm

Question 13

What are the procedures that have allowed us to measure thresholds as precisely as possible?

Answer 13

– Method of constant stimuli
– Method of limits
– Staircase procedures

Each method has its advantages and disadvantages

Question 14

What is the method of constant stimuli?

Answer 14

• Construct a set of stimuli with magnitudes ranging from
above to below the presumed threshold value
• Present these stimuli a number of times in a random
order
• Participants respond whether or not they detect the
stimulus on each trial
• Plot the proportion of detections occurring at each
stimulus magnitude
• The threshold is taken as the magnitude at which the
stimulus is detected a criterion proportion of the time
(e.g., 50%)

Question 15

what dont we observe when using the method of constant stimuli?

Answer 15

Typically, we don’t observe a clear cut discontinuity between detectable and
undetectable stimuli in this psychometric
function

Question 16

what are the advantages of the method of constant stimuli?

Answer 16

– Allows the shape of the psychometric function to be
established
– Provides an accurate estimate of threshold

Question 17

what are the disadvantages of the method of constant stimuli?

Answer 17

– Requires pre-testing to roughly estimate the threshold
– Wastes a lot of trials which lie far from the threshold
(making this method time-consuming)
– It is difficult to measure changes in threshold over
brief time periods with this method

Question 18

what does the method of limits measure?

Answer 18

This method measures the threshold without
determining the shape of the psychometric
function

Question 19

What does the method of limits require the use of?

Answer 19

The method of limits uses ascending and

descending series of trials

Question 20

What are descending series in the method of limits?

Answer 20

• Present the stimulus at a suprathreshold level
• Decrease stimulus intensity in small steps until participants
can no longer detect the stimulus

Question 21

What are ascending series in the method of limits?

Answer 21

• Present stimulus at a subthreshold level
• Increase stimulus intensity in small steps until participants
can detect the stimulus

Question 22

what is the theshold in the method of limits?

Answer 22

The threshold is the average of the limits from each of the ascending and descending runs

Question 23

What are the advantages of the method of limits?

Answer 23

– More efficient (i.e., quicker) than the method of
constant stimuli
– Still reasonably accurate in determining the threshold

Question 24

What are the disadvantages of the method of limits?

Answer 24

– Many trials are still “wasted” as they are presented at
intensities away from the threshold
– Participant may habituate (get used to giving a “yes”
or “no” response) and thus overshoot the true
threshold
– The overall shape of the psychometric function
cannot be derived

Question 25

what is the staircase procedure designed to overcome?

Answer 25

Staircase procedures are designed to overcome

the disavantages of the method of limits and the method of constant stimuli?

Question 26

What is the process of the staircase procedure?

Answer 26

– They involve linked series of ascending and
descending runs with each successive run being
based on the outcome of the preceding run
– The stimulus is presented either above or below
threshold and the intensity is changed in small steps
until a reversal (change in response) occurs
– The direction of change is then reversed when
another reversal in response occurs
– The procedure is terminated after a criterion number
of reversals
– The threshold is taken as the average of these
reversal intensities

Question 27

what are the advantages of the staircase procedure?

Answer 27

– Even more efficient than the method of limits
– Can be modified in a number of different ways to
overcome other limitations

Question 28

what are the disadvantages of the staircase procedure?

Answer 28

Estimation of the threshold tends to require more
complex calculations (especially when the procedure is modified), making it less intuitive

Question 29

What estimate of threshold does the standard procedure of the staircase procedures provide??

Answer 29

The standard procedure yields an estimate of the 50% threshold

Question 30

How can the staircase standard procedure estimate the 70% threshold?

Answer 30

By requiring two “yes”
responses before the
stimulus intensity is
decreased, this
procedure can estimate
the 70% threshold

Question 31

What can the staircase procedures be used to figure out?

Answer 31

staircase procedures can be used to figure

out the overall shape of the psychometric function

Question 32

How can the issue of habituation be addressed??

Answer 32

The issue of habituation can be addressed by

running multiple series of trials simultaneously

Question 33

why can we never perceive stimuli under a perfect condition?

Answer 33

There is always some noise, even when there are no
stimuli in the environment and we can never know whether we are perceiving the true stimuli (signal) or the noise. As a result, often, what we do is to use a certain criterion with which we (unconsciously) decide
that we have perceived the signal

Question 34

When is there a hit in signal detection?

Answer 34

On some signal trials the level of activity will be
above the criterion, leading to a correct “yes”
response (hit)

Question 35

When is there a miss in signal detection?

Answer 35

On other signal trials the level of activity may be
below the criterion, leading to an incorrect “no”
response (miss)

Question 36

When is there a false alarm in signal detection?

Answer 36

On some catch trials the level of activity may be
above the criterion, leading to an incorrect “yes”
response (false alarm)

Question 37

When is there a correct rejection in signal detection?

Answer 37

On other catch trials the level of activity will be

below the criterion, leading to a correct “no” response (correct rejection)

Question 38

What does the separation between the signal+noise and the noise distribution tell us and what is it a measure of?

Answer 38

The separation between the signal+noise and noise
distributions tells us how sensitive an observer is to that
stimulus
– This measure of sensitivity is called d-prime (d’).

Question 39

How do we estimate d prime?

Answer 39

– The proportion of hits (or misses) tells us the location
of the criterion relative to the signal+noise distribution
– The proportion of false alarms (or correct rejections)
tells us the location of the criterion relative to the
noise distribution
– Convert these proportions to z-scores
• These scores tell us the distance from the criterion to each
distribution mean
– d’ is then the “sum” of these distances
• d’ = Z(FA) − Z(HIT)

Question 40

what is d prime a measure of?

Answer 40

d’ is a measure of sensitivity which is
independent of response bias
• In other words, it is possible to get the same d’
from a range of different response patterns

Question 41

Where can we see the d prime effect?

Answer 41

We can see this effect in a receiver operating
characteristic curve (ROC curve). This curve shows the range of hit and false
alarm rates that yield the same sensitivity (d’)

Question 42

what does the ROC curve show?

Answer 42

ROC curves showing the effect of different criteria and

sensitivities on the hit and false alarm rates