3. psychophysics

Question 1

define threshold difference

Answer 1

the smallest change in a stimulus that can be detected

a.k.a. JND - Just Noticeable Difference

Question 2

Define absolute threshold

Answer 2

The minimum intensity of a stimulus that can be detected

Question 3

what did weber define in Weber’s law

Answer 3

that 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 form a 40g reference weight (JND=1g)
> then the JND would be 10g for a 400g reference weight

Question 4

what did fechner say in building upon webers findings

Answer 4

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

What are the implications of fechners law

Answer 5

> relates to internal experiences (psyche) and physical environment (physics)
> Psyche + Physics = Psychophysics

> is about the absolute , not relative intensity o fa stimulus
> turning the focus of research from difference threshold to absolute threshold

> asserts that our psychological experience of the intensity of a stimulus tends to change less quickly that the actual change in stimulus intensity

Question 6

what procedures have been developed to measure thresholds

Answer 6

method of constant stimuli

method of limits

Staircase procedures

Question 7

describe the process of:

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 proportions of detections occurring at each stimulus magnitude

> the threshold is said to be at a magnitude at which the stimulus is detected at a certain proportion of the time (e.g. 50% of times difference is reported present)

Question 8

advantages of

Method of constant stimuli

Answer 8

allows the shape of the psychometric function to be established

provides an accurate estimate of threshold

Question 9

disadvantages of

Method of constant stimuli

Answer 9

requires pretesting to roughy estimate the threshold

wastes a lot of trials which lie jar from the threshold
> time consuming

is difficult to measure changes in threshold over brief time periods

Question 10

describe the method of limits

Answer 10

measures threshold without determining the shape of the psychometric function

uses ascending and descending series of trials

threshold is the average of the limits form each of the ascending and descending runs

Question 11

what is a descending series in method of limits

Answer 11

resent the stimulus as a suprathreshold level

decreases stimulus intensity in small steps until participant can no longer detect the stimulus

Question 12

what is an ascending series in method of limits

Answer 12

presents stimulus at a substhreshold level

Increases stimulus intensity in small steps until participant can detect stimulus

Question 13

advantages of

Method of limits

Answer 13

more efficient (quicker) than constant stimuli

still reasonably accurate in determining the threshold

Question 14

disadvantages o f

method of limits

Answer 14

many trials wasted as they jre presented at intensities away from the threshold

Participants may habituate (get used to giving yes or no response) and over shoot the true threshold

Overall shape of psychometric function cannot be derived

Question 15

how does staircase procedures work

Answer 15

• link series of ascending and descending runs so that successive runs are based on outcomes of previous run
• stimulus resented above or below threshold and intensity is changed until reversal (change in response) occurs
• direction of change is reversed when another reversal in response occurs
• procedure is terminated after a criterion number of reversals
• threshold is taken as the average of these reversal intensities

Question 16

advantages of

Stair case procedures

Answer 16

even more efficient than method of limits

can be modified in a number of different ways of overcome other limitations

Question 17

staircase procedure:

the standard procedure yields an estimate of __ threshold

production of 70% threshold by requiring 2 yes’s before reversal means:

Answer 17

50%

staircase procedure can be used to figure out the overall shape of the psychometric function

Question 18

what is over come by the staircase procedure

Answer 18

issue of habituation by running multiple series of trials simultaneously

Question 19

disadvantages of :

Staircase procedures

Answer 19

estimation of the threshold tends to require more complex calculations (especially when the procedure is modifies)

Making it less intuitive

Question 20

signal detection

what criteria is used when we unconsciously decide that we have perceived the signal

Answer 20

noise Vs Signal+noise

> signal present, response yes = hit

> signal present, response no = miss

> signal absent, response yes = false alarm

> signal absent response no, correct rejection

Question 21

what tells us how sensitive an observer is to a particular stimulus

Answer 21

the separation between the signal +noise and noise distribution

this measure of sensitivity called d-prime (d’)

Question 22

How is d’ estimated in signal detection

Answer 22

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 rejection s) tells us the location of the criterion relative to the noise distribution

convert these proportions to z scores
> these scores tell is the distance from the criterion to each distribution mean

d’ is the sum of these distances
> d’ = Z (FA) - Z(HIT)

Question 23

d’ is a measure of…

Answer 23

sensitivity which is independent of response bias in signal detection

> meaning it is possible to get the same d’ from a range of different response patterns

Question 24

what is an ROC curve

Answer 24

received operating characteristic curve

this curve shows the range of hit and false alarm rates that yield the same sensitivity (d’)