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Perception block 11 > Psychophysical Methods > Flashcards

Psychophysical Methods Flashcards

1
Q

The study of the relationship between the physical stimuli and perceptual responses

A

Psychophysics

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2
Q

The minimum quantity of a stimulus that can be detected

A

Threshold
-the observer is required to detect a light (stimulus) flashed a surrounding background, similar to a visual field test. The test is repeated for a range of stimulus intensities, from dim to intense

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3
Q

The percentage of stimuli detected is plotted as a function of stimulus intensity to produce a

A

Frequency of seeing (FOS) curve, also referred to as a psychometric function

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4
Q

Manifests an unambiguous threshold, below the threshold intensity stimulus is never seen, and above this intensity is always seen

A

An ideal observer

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5
Q

As the intensity increases of the stimulus is increased, the probability of seeing the stimulus increases, there is no clearly defined intensity below which the stimulus is never seen and above which it is always seen

A

A real observer

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6
Q

There are no perfect observers, therefore threshold is based on

A

Theoretical considerations

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7
Q

What is threshold defined as

A

The intensity that results in detection of the stimulus on 50% of the presentations

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8
Q

A diseased individual is _____ than a healthier one, making the FOS curve less steep (more flat)

A

Noisier

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9
Q

From several methods to measure a threshold, the most suitable method is determined by the ____ of the experiment or clinical procedure

A

Nature

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10
Q

What are the methods of threshold determination

A 
Method of ascending limits 
Method os descending limits 
Staircase method 
Method of constant stimuli 
Method of adjustment 
Forced choice method
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11
Q

The stimulus is initially not visible, the stimulus intensity is increases systematically until it is visible, several trials may be performed, and the results are averaged to obtain a threshold

A

Method of ascending limits

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12
Q

Advantages of method of ascending limits

A

Dark adaptometry-becuse the state of retinal adaptation is affected minimally by the stimulus

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13
Q

Disadvantages of method ascending limits

A

Observer anticipation

Mitigation of potential bias-begin each trial at a different intentisty

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14
Q

Reverse of method of ascending limits, the stimulus is clearly visible at the beginning, the stimulus intensity is decreases systematically until it is no longer seen. Several trials may be performed, and the results are averaged to obtain a threshold

A

Method of descending limits

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15
Q

Advantages of the method of descending limits

A

Determination fo VA: s Ellen eye chart optotypes become progressively smaller from top to bottom. Threshold MAR is determined when the optotypes are too small to be resolved

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16
Q

Disadvantages of method descending limits

A

Observer anticipation

Mitigation of potential bias-begin each trial at a different intensity

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17
Q

Combination of ascending and descending limits. Initially the stimulus is below threshold, and increases systematically until visibility. At this point, the shape is reversed. The stimulus intensity is decreased systematically until it is no longer seen. Either on particular reversal or average of few reversals are used to determine threshold

A

Staircase method of determination of threshold

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18
Q

Advantages of staircase method of determination go threshold

A

Quick and reliable
Psychophysical experimentation
Automated VF testing

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19
Q

Stimulus visibility is varied randomaly from presentation to presentation

A

Method of constant stimuli

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20
Q

FOS curve is plotted based on the subjects repsonses

A

Method of constant stimuli

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21
Q

‘Yes-no’ procedure: observer is asked whether the stimulus is seen or not

A

Method of constant stimuli

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22
Q

Blank trials-when no stimulus is presented, even though observer is asked

A

Method of constant stimuli

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23
Q

When no stimulus is presented, even though observer is asked

A

Blank trials

24
Q

The number of times the observer reports seeing the stimulus during a blank trials

A

False positive (false alarm)

25
Q

What method uses false positives

A

Method of constant stimulus

26
Q

When the stimulus is clearly above threshold and the subject reports that such a stimulus is not seen, this response is a

A

False negative

27
Q

What method uses a false negative

A

Method of constant stimuli

28
Q

Advantages of method of constant stimuli

A

Maintains oberservers expectations at the same level

29
Q

Disadvantages of method of constant stimuli

A

Time consuming
Not prescribed from clinical applications
False-positive and false negative affect FOC. FOC curves has to be compensated or adjusted for false -(+/-)ve rates
-mitigation: forced choice method

30
Q

The subject adjusts the stimulus until it is barely visible (or invisible)

A

Method of adjustment

31
Q

Advantages of method of adjustment

A

Allows for a relatively quick threshold determination

32
Q

Disadvantages of method of adjustment

A

Repeatability is not as high as the other methods discussed

33
Q

Observer is asked whether the stimulus is seen or not

A

Yes no procedure

34
Q

When no stimulus is prestend, even though observer is asked

A

Blank trial

35
Q

Observers criterion of forced choice method

A
  1. Not all observers use the same criteria when deciding whether to not they see a stimulus
  2. The threshold criteria may vary trial to trial for the same observer
  3. An observer may use one set of criteria for one type of stimulus and another set for a different stimulus
36
Q

Strict threshold criterion in forced choice

A

Do not report seeing a stimulus until they are absolutely certain they see it.

37
Q

What does strict threshold criterion result in

A

Relatively high threshold (low sensitivity)

38
Q

Lax threshold criterion in forced choice

A

Do report seeing a stimulus even though they may have a great deal of uncertainty regarding their decision

39
Q

What does lax threshold result in. In forced choice

A

Relatively low threshold (high sensitivity)

40
Q

What causes the threshold criteria to be potentially complicated for interpretation of experimental results in forced choice?

A

Observer criterion

41
Q

In this methodology, the effects of the observers criteria are minimized by forcing a choice between several alternatives, one of which contains the stimulus

A

Forced choice method

42
Q

Forced choice method: steps

A
  • stimulus randomly presented in one of the two windows
  • the other window does not contain a stimulus-it is blank
  • the observer is forced to choose which window contains the stimulus
  • I cannot see the stimulus is not acceptable
  • the role of the observers threshold criteria is reduced
43
Q

What is a perfect example of forced choice method

A

Vector vision CSV chart

-two force choice method

44
Q

If the experiment forces the observer to choose between two alternatives, it is referred to as

A

Two-alternative forced choice (2AFC) experiment

45
Q

A psychometric function for a 2AFC experiment

A
  • The lowest percentage correct is 50%, not 0, because the observer is expected to guess correctly 50% of the time
  • threshold is typically assumed to be the point midway between chance performace (50) and perfect performance (100)
  • the threshold for a 2AFC experiment is 75
46
Q

Stimulus in which the stimulus is randomly presented in one of four windows, the observer is forced to choose the correct window

A

Four alternative forced choice (4AFC) method

47
Q

4AFC experiment

A
  • the psychometric function for this experiment shoes chance performance of 25%
  • threshold is taken as the point midway between chance performance and 100%, which is 62.5%
48
Q

Why is 4AFC better than 2 AFC

A

Because the 4AFC psychometric function is steeper, the threshold can be ascertained with more confidence

49
Q

Advantages of 4AFC method

A
  • Compared to other psychophysical methods, forced choice cynically results in lower thresholds
  • when observer are forced to guess, they often do remarkably well despite claims they cannot see the stimulus
  • used effectively to determine VA and other visual capabilities of infants
50
Q

What is the chance performance for a 2AFC

A

50%

51
Q

What is the steepness of the curve for a 2AFC

A

Not steep

52
Q

What is the threshold for a 2AFC?

A

75%

53
Q

What is the chance performace for a 4AFC

A

25%

54
Q

What is the threshold of a 4AFC

A

62.5 %

55
Q

Steepness of 4AFC

A

Steeper than 2AFC, better

Perception block 11 (8 decks)

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