Week 2 - Psychophysics and signal detection theory

Question 1

Thresholds

Answer 1

• difference threshold
• absolute threshold

Question 2

Difference thresholds

Answer 2

The smallest change in a stimulus that can be detected (JND - just noticeable difference)

Question 3

Absolute threshold

Answer 3

the minimum intensity of a stimulus that can be detected

Question 4

Weber’s law

Answer 4

the size of the JND is a function of the magnitude of a reference stimulus.
e.g. if a weight has to be 41g before it can be discriminated from a 40g reference weigh (JND = 1g), then the JND would be 10 g for a 400g reference weight

Question 5

Weber’s law equation

Answer 5

ΔI/I=K
ΔI: size of detectable difference
I: physical intensity of a reference stimulus
K: constant

Question 6

Fechner’s law

Answer 6

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 7

Fechner’s law equation

Answer 7

ΔS = k x ΔI/I
S: Intensity of sensation of a stimulus
k: Constant
I: Physical intensity of the stimulus

Question 8

example thresholds

Answer 8

sight a candle flame: 48km on a clear dark night
hear mechanical watch ticking: 6m in noise free environment
taste teaspoon of sugar dissolved in 7.6L 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 7cm

Question 9

How do we measure thresholds?

Answer 9

• method of constant stimuli
• method of limits
• staircase procedures

Question 10

method of constant stimuli

Answer 10

• construct 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 11

psychometric function of method of constant stimuli

Answer 11

don’t observe a clear-cut discontinuity between detectable and undetectable stimuli in this psychometric function - usually curved, not two levels of can and cannot detect

Question 12

advantages of method of constant stimuli

Answer 12

• allows the shape of the psychometric function to be established
• provides an accurate estimate of threshold

Question 13

disadvantages of method of constant stimuli

Answer 13

• require pre-testing to roughly estimate the threshold
• wastes a lot of trials which lie far from the threshold (time consuming)
• difficult to measure changes in threshold over brief time periods

Question 14

Method of limits

Answer 14

• measures the threshold without determining the shape of the psychometric function - average of limits from both runs
• uses ascending and descending series of trials
  descending: present stimulus at a suprathreshold level and decrease stimulus intensity in small steps until participants can no longer detect the stimulus
  ascending: present stimulus at a subthreshold level and increase the stimulus intensity in small steps until participants can detect the stimulus

Question 15

advantages of method of limits

Answer 15

• more efficient than method of constant stimuli
• reasonably accurate in determining the threshold

Question 16

disadvantages of method of limits

Answer 16

• many trials are still wasted as they are presented at intensities away from the threshold
• participant may habituate (get used to giving response) and overshoot the true threshold
• overall shape of psychometric function cannot be derived

Question 17

Staircase procedures

Answer 17

designed to overcome problems of method of constant stimuli and method limits
- use ascending and descending runs and base successive runs on previous outcome
- stimulus presented either above or below threshold and intensity changed in small steps until change in response occurs
- direction of change is reversed when change in response occurs
- procedure is terminated after criterion number of reversals
- threshold is taken as the average of these reversal intensities

Question 18

advantages of staircase procedures

Answer 18

• even more efficient than the method of limits
• can be modified in a number of different ways to overcome other limitations

Question 19

psychometric function of staircase procedures

Answer 19

• standard procedure yields an estimate of 50% threshold
• by requiring two yes responses before the stimulus intensity is decreased, this procedure can estimate the 70% threshold
• can be used to figure out the overall shape of the psychometric function
• habituation can be addressed by running multiple series of trials simultaneously

Question 20

disadvantages of staircase procedures

Answer 20

estimation of the threshold tends to require more complex calculations, making it less intuitive

Question 21

Signal detection

Answer 21

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