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

built upon Weber’s findings; 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 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

Implications of Fechner’s law

Answer 8

• relates internal experience (psyche) and physical environment (physics) - psyche + physics = psychophysics
• is about the absolute, not relative, intensity of stimulus - turning focus from difference to absolute thresholds
• our psychological experience of the intensity of a stimulus tends to change less quickly than the actual change in stimulus intensity

Question 9

example thresholds

Answer 9

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 10

How do we measure thresholds?

Answer 10

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

Question 11

method of constant stimuli

Answer 11

• 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 12

psychometric function of method of constant stimuli

Answer 12

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 13

advantages of method of constant stimuli

Answer 13

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

Question 14

disadvantages of method of constant stimuli

Answer 14

• 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 15

Method of limits

Answer 15

• 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 16

advantages of method of limits

Answer 16

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

Question 17

disadvantages of method of limits

Answer 17

• 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 18

Staircase procedures

Answer 18

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 19

advantages of staircase procedures

Answer 19

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

Question 20

psychometric function of staircase procedures

Answer 20

• 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 21

disadvantages of staircase procedures

Answer 21

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