Psychophysics

Question 1

What is a difference threshold?

Answer 1

the smallest change in a stimulus that can be detected (a.k.a JND - Just Noticible Difference)

Question 2

What is an absolute threshold?

Answer 2

the minimum intensity of a stimulus that can be detected

Question 3

What is weber’s law (difference threshold)?

Answer 3

the size of the JND is a function of the magnitude of a reference stimuli

Question 4

What is the equation for weber’s law (difference threshold)?

Answer 4

= size of detectable difference (JND) divided by the physical intensity of a reference stimuli

Question 5

What is Fechner’s law (absolute threshold)?

Answer 5

• relates internal experience (psyche) and physical environment (physics) - Psyche + physics → Psychophysics
• 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

What are the three methods to measure thresholds?

Answer 6

○ Method of constant stimuli
○ Method of limits
○ Staircase procedures

Question 7

What is the method of constant stimuli?

Answer 7

• Construct a set of stimuli with magnitudes ranging from above to below the presumed threshold value
• Present the stimuli 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. stimuli test for assessment with Ts

Question 8

What are the advantages of the method of constant stimuli?

Answer 8

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

Question 9

What are the disadvantages of the method of constant stimuli?

Answer 9

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

Question 10

What is the method of limits?

Answer 10

This method measures the threshold without determining the shape of the psychometric function
- uses ascending and descending series of trials

Question 11

What is a descending series in the method of limits?

Answer 11

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

Question 12

What is an ascending series in the method of limits?

Answer 12

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

Question 13

What are the disadvantages of the method of limits?

Answer 13

• 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 14

What are the advantages of the method of limits?

Answer 14

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

Question 15

What is the staircase procedures method?

Answer 15

• linked series of ascending and descending runs with each successive run 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 16

What are the advantages of the staircase procedure?

Answer 16

• Can be used to figure out the overall shape of the psychometric function
• Even more efficient than the method of limits
• Can be modified in a number of different ways to overcome other limitations

Question 17

What are the disadvantages of the staircase procedure?

Answer 17

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

Question 18

How can the issue of habitation be addressed?

Answer 18

by running multiple series of trials simultaneously

Question 19

What is signal detection?

Answer 19

The separation between the signal + noise and noise distributions tells us how sensitive an observer is to that stimulus

Question 20

What is d-prime?

Answer 20

d is the measure of sensitivity

Question 21

How do we estimate d?

Answer 21

• 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

Question 22

What are the four possible outcomes of a signal detection task?

Answer 22

HIT - trials the level of activity will be above the criterion, leading to a correct “yes” response (hit)
MISS - trials the level of activity may be below the criterion, leading to an incorrect “no” response (miss)
FALSE ALARM - trials the level of activity may be above the criterion, leading to an incorrect “yes” response (false alarm)
CORRECT REJECTION - trials the level of activity will be below the criterion, leading to a correct “no” response (correct rejection)