2. Psychophysics Flashcards
What are the questions asked in relation to thresholds?
– What is the smallest change in a stimulus that can be detected?
– What is the smallest/largest magnitude of stimuli for
each sensory modality?
What are the questions asked in relation to signal detection?
– How do we detect a signal out of noise?
What are difference thresholds?
The smallest change in a stimulus that can be
detected (a.k.a. JND–Just Noticeable Difference)
what is the absolute threshold?
The minimum intensity of a stimulus that can be
detected
What is Weber’s Law?
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
What is the formula for Webers law?
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
What did Fechner’s law build on?
WEBER’S FINDINGS
what was fechner’s idea?
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
What does Fechner’s law relate to?
intenral experience (psyche) and physical environment (physics)
Psyche + physics –> Psychophysics
What is fechner’s law about?
Fechner’s law is about the absolute, not relative,
intensity of a stimulus
– Turning the focus of research from difference
thresholds to absolute thresholds
What does Fechner’s law asset about our psychology experience of the intensity of a stimulus?
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
What did Galanter (1962) suggest is the reason why we suck at measuring thresholds?
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
What are the procedures that have allowed us to measure thresholds as precisely as possible?
– Method of constant stimuli
– Method of limits
– Staircase procedures
Each method has its advantages and disadvantages
What is the method of constant stimuli?
• 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%)
what dont we observe when using the method of constant stimuli?
Typically, we don’t observe a clear cut discontinuity between detectable and
undetectable stimuli in this psychometric
function
what are the advantages of the method of constant stimuli?
– Allows the shape of the psychometric function to be
established
– Provides an accurate estimate of threshold
what are the disadvantages of the method of constant stimuli?
– 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
what does the method of limits measure?
This method measures the threshold without
determining the shape of the psychometric
function
What does the method of limits require the use of?
The method of limits uses ascending and
descending series of trials
What are descending series in the method of limits?
• Present the stimulus at a suprathreshold level
• Decrease stimulus intensity in small steps until participants
can no longer detect the stimulus
What are ascending series in the method of limits?
• Present stimulus at a subthreshold level
• Increase stimulus intensity in small steps until participants
can detect the stimulus
what is the theshold in the method of limits?
The threshold is the average of the limits from each of the ascending and descending runs
What are the advantages of the method of limits?
– More efficient (i.e., quicker) than the method of
constant stimuli
– Still reasonably accurate in determining the threshold
What are the disadvantages of the method of limits?
– 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
what is the staircase procedure designed to overcome?
Staircase procedures are designed to overcome
the disavantages of the method of limits and the method of constant stimuli?
What is the process of the staircase procedure?
– 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
what are the advantages of the staircase procedure?
– Even more efficient than the method of limits
– Can be modified in a number of different ways to
overcome other limitations
what are the disadvantages of the staircase procedure?
Estimation of the threshold tends to require more complex calculations (especially when the procedure is modified), making it less intuitive
What estimate of threshold does the standard procedure of the staircase procedures provide??
The standard procedure yields an estimate of the 50% threshold
How can the staircase standard procedure estimate the 70% threshold?
By requiring two “yes” responses before the stimulus intensity is decreased, this procedure can estimate the 70% threshold
What can the staircase procedures be used to figure out?
staircase procedures can be used to figure
out the overall shape of the psychometric function
How can the issue of habituation be addressed??
The issue of habituation can be addressed by
running multiple series of trials simultaneously
why can we never perceive stimuli under a perfect condition?
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
When is there a hit in signal detection?
On some signal trials the level of activity will be
above the criterion, leading to a correct “yes”
response (hit)
When is there a miss in signal detection?
On other signal trials the level of activity may be
below the criterion, leading to an incorrect “no”
response (miss)
When is there a false alarm in signal detection?
On some catch trials the level of activity may be
above the criterion, leading to an incorrect “yes”
response (false alarm)
When is there a correct rejection in signal detection?
On other catch trials the level of activity will be
below the criterion, leading to a correct “no” response (correct rejection)
What does the separation between the signal+noise and the noise distribution tell us and what is it a measure of?
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’).
How do we estimate d prime?
– 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)
what is d prime a measure of?
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
Where can we see the d prime effect?
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’)
what does the ROC curve show?
ROC curves showing the effect of different criteria and
sensitivities on the hit and false alarm rates
