Psychophysics

Question 1

Difference Threshold

Answer 1

The smallest change in a stimulus that can be

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

Question 2

Absolute Threshold

Answer 2

The minimum intensity of a stimulus that can be

detected

Question 3

Weber’s law

Answer 3

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
This constant ratio of JND and the intensity of a
reference stimulus (i.e., / ) is called the
Weber fraction for that stimulus dimension

Question 4

Fechner’s law

Answer 4

Fechner built upon Weber’s findings
– 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

Question 5

Implications of Fechner’s Law

Answer 5

• Fechner’s law relates internal experience
(psyche) and physical environment (physics)
– Psyche + physics → Psychophysics
• Fechner’s law is about the absolute, not relative,
intensity of a stimulus
– Turning the focus of research from difference
thresholds to absolute thresholds
• 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 6

How do we measure thresholds?

Answer 6

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
Thus, procedures have been developed to
measure thresholds as precisely as possible
– Method of constant stimuli
– Method of limits
– Staircase procedures
• Each method has its advantages and
disadvantages

Question 7

Method of Constant Stimuli

Answer 7

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

Question 8

Method of constant stimuli advantages

Answer 8

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

Question 9

Method of constant stimuli disadvantages

Answer 9

– 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 10

Method of limits

Answer 10

This method measures the threshold without
determining the shape of the psychometric
function
• The method of limits uses ascending and
descending series of trials
– Descending series
• Present the stimulus at a suprathreshold level
• Decrease stimulus intensity in small steps until participants
can no longer detect the stimulus
– Ascending series
• Present stimulus at a subthreshold level
• Increase stimulus intensity in small steps until participants
can detect the stimulus
The threshold is the average of the limits from
each of the ascending and descending runs

Question 11

Method of limits advantages

Answer 11

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

Question 12

Method of limits disadvantages

Answer 12

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 13

Staircase procedures

Answer 13

Staircase procedures are designed to overcome
these problems
– 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 14

Staircase procedures advantages

Answer 14

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

Question 15

Staircase procedures disadvantages

Answer 15

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

Question 16

Signal Detection

Answer 16

We can never perceive stimuli under the 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
• On some signal trials the level of activity will be
above the criterion, leading to a correct “yes”
response (hit)
• On other signal trials the level of activity may be
below the criterion, leading to an incorrect “no”
response (miss)
• On some catch trials the level of activity may be
above the criterion, leading to an incorrect “yes”
response (false alarm)
• On other catch trials the level of activity will be
below the criterion, leading to a correct “no”
response (correct rejection)