3. Perception Psychophysics

Question 1

What is psychophysics?

Answer 1

• relation between subjective perceptual experience and the objective physical stimuli that give rise to the experience
• develops methods and techniques

Question 2

What are the three specific questions asked in psychophysics?

Answer 2

1. what is the smallest difference between two stimuli we can detect?
2. how is perceived magnitude related to stimulus intensity?
3. what are the absolute limits of perception?

Question 3

What is just-noticeable difference? (JND)

Answer 3

• what the smallest difference between two stimuli is that we can detect

Question 4

What is Webers law?

Answer 4

• when the standard is more intense, discriminating differences is more difficult

Question 5

What is the formula for Webers law?

Answer 5

△P / P = k

△P= just-noticeable difference (JND)
P = intensity of standard
K = % change in comparison for stimuli to be recognised

Question 6

How are the JND (△P) and stimulus intensity related?

Answer 6

• JND is a constant proportion

Question 7

What are the limits of perception?

Answer 7

• what is the quietest sound we can hear?
• what is the faintest light we can see?
• what is the lightest touch we can feel?

our sensory system allows us to experience only a limited part of our physical environment: stops us being overwhelmed

Question 8

What are the methods of finding the thresholds?

Answer 8

• method of constant stimuli (requires fitting a psychometric function)
• method of limits
• method of adjustment

Question 9

How does assessing the method of constant stimuli help find a threshold?

Answer 9

• stimuli are pre-selected by the experimenter and run in a random order
• psychometric function is fit to results

Question 10

How does assessing the method of limits help find a threshold?

Answer 10

• step-up/step-down procedures
• stop at crossover points
• human error, so we take average to be the threshold

Question 11

How does assessing the method of adjustment help find a threshold?

Answer 11

• PPs adjust stimulus intensity themselves: until they can tell it’s there
• aim is for a perceptual threshold
• this can be repeated with different starting intensities

Question 12

How do we decide which method to use when finding a threshold?

Answer 12

depends on the goal of the study, desired accuracy and available time
- method of constant stimuli is the most precise, but slowest as its boring and hard to stay engaged
- method of limits is faster as the stimuli is chosen to suit the PPs threshold, but order effects may be present
- method of adjustment is quickest, but more dependant on PP cooperation

Question 13

What are potential issues with psychophysics?

Answer 13

• response reliability
• response consistency
• are they true reports?

Question 14

What is bias influenced by?

Answer 14

• the costs and benefits of the response outcome

Question 15

How can we estimate magnitude?

Answer 15

• present another stimulus and ask PP to rate magnitude with respect to the standard given at the beginning

Question 16

What is stevens power law?

Answer 16

P=KS^n

P= perceived magnitude
S = stimulus intensity
K, n = constants (specific to percept under investigation)