Chapter 11

Question 1

Define threshold. Why is it difficult to measure?

Answer 1

Minimum quantity of a stimulus that can be detected

Humans are not perfect observers, answer may vary for the same patient

Question 2

What is a psychometric function?

Answer 2

AKA frequency of seeing (FOS) curve

Range of stimulus intensities tested, percentage of stimulus detected is plotted as a function of stimulus intensity

Question 3

Compare and contrast the psychometric function for an ideal vs real observer.

Answer 3

Ideal: below the threshold intensity the stimulus is never seen, above this intensity it is always seen

Real: as intensity of stimulus increases, probability of seeing the stimulus increases, there is no clear defines intensity below which stimulus is never seen and above which it is always seen

Question 4

What is threshold define as in real scenarios.

Answer 4

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

Question 5

What can affect the perception of a stimulus by the visual system?

Answer 5

Stimulus results in neural activity
Random neural noise can affect the stimulus therefore making the threshold variable
May be present in diseased visual system that have more noise (affect VF measurements, peripheral)
(Attention, motivation, and fatigue can also affect threshold)

Question 6

Describe the methods of ascending limits for threshold measurements. What can negatively affect these results?

Answer 6

Stimulus is initially NOT visible b/c it is below threshold
Trials of stimuli are increased until it is visible (several trials may be averaged together)
Useful for dark adaptometry b/c the state of retinal adaptation is affected minimally by stimulus

May have observer anticipation of stimulus, can help to begin each trial at a different intensity

Question 7

Describe the method of descending limits for measuring threshold. What is it used to measure clinically? What can it be affected by?

Answer 7

Reverse of ascending
Trial begins with clearly visible stimulus, visibility is decreased systemically until it can no longer be seen
Commonly used to determine visual acuity (big letters to small, threshold MAR is when letters can no longer be resolved)

Affected by observer anticipation, doctor can ensure that patient at least understands the test since they start off by being about to see optotype

Question 8

Describe forced choice methodology. Who should be setting the threshold criterion?

Answer 8

Encouraging the patient to continue to read down the chart even when they say they can no longer see it

Many patients stop early out of fear that they will be wrong or they just give up (stop once 4 or more mistakes are made)

Practitioner to ensure reliable consistent results (even with bias)

Question 9

Describe the staircase method of determining threshold.

Answer 9

Combo of ascending and descending
Start with stimulus below threshold and gradually increase until it is seen, reverse and decrease visibility until stimulus cannot be seen, reverse ascending, repeat.

Threshold is taken to be the stimulus intensity at one of the reversals, or average of the last few reversals

Question 10

Describe the method of constant stimuli for threshold measurement. How can we determine the reliability of the data?

Answer 10

Stimulus visibility is varied randomly from presentation to presentation (blank trials included), FOS curve is plotted based on responses

Patient expectations are kept constant throughout

Number of false positives (report yes during blank)= lax threshold criterion (high false positive)

Number of false negative (report no during stimulus)

Question 11

Describe the method of adjustment for threshold measurement.

Answer 11

the subject adjusts the stimulus intensity until it is barely visible (or invisible), relatively quick threshold determination

Low repeatability

Question 12

Describe strict vs lax threshold criterion. How is this resolved?

Answer 12

Strict threshold criterion- donot report unless sure, high threshold low sensitivity

Lax criterion- low threshold, high sensitivity

Vary from observer to observer or trial to trial

Forced choice method

Question 13

Describe the forced choice method used in threshold determination. What point on the curve shows threshold?

Answer 13

Forcing a choice between several alternatives, one of which contains the stimulus

graph starts at probability of getting the answer correct (50% for 2 choices)
Threshold is assumed to be halfway between chance (lowest) and perfect (100%)

Question 14

How does more choices affect results of forced choice? How do threshold measurements for this method compare to other methods?

Answer 14

More choices create a steeper curve with less chance of error or estimation
Forces choice usually results in lower threshold, patients do well

Question 15

What method of threshold measurement is commonly used for infants?

Answer 15

Forces choice

Question 16

Describe the signal detection theory.

Answer 16

Predicts the effects of factors that may affect threshold determination (decision criteria, attention, motivation, neural noise)
Observers’ task is to differentiate signal and noise from background noise alone

Question 17

Describe the curve for signal detection theory.

Answer 17

As signal becomes larger, N and S become further, detect ability increases
Weak stimulus, N and N and S distributions overlap, (hard to distinguish stimulus, very high stimulus

Question 18

Describe the effect of observer criterion.

Answer 18

Uses neural activation to measure threshold vs perception

Hit: stimulus results in neural activity that exceed the threshold criterion

Miss: activity does not exceed the criterion

Question 19

Describe how we can use the payoff matrix to control a subject’s criterion

Answer 19

Pay her $5 every time she correctly detects the stimulus (a hit) and fine her $25 every time she incorrectly reports seeing a stimulus that isn’t there (false positive)?:strict criterion that results in relatively few false positives and hits and many misses and correct rejects

pay her $25 every time she correctly detects the stimulus (a hit) and fine her $5 every time she incorrectly reports seeing a stimulus that isn’t there (false positive)- lax criterion that results in a relatively high number of false positives and hits and relatively few misses and correct rejects

Question 20

What does a receiver operating characteristic curve (ROC) show?

Answer 20

Probability of a hit as a function of the probability of a false positive for a different threshold criteria
Lax- high probability for hit and false positive
Strict- low probability of hit and false positive

Question 21

Describe Weber’s Law.

Answer 21

Recognizing an increment on a background (recognize combine stimulus and background from background itself

threshold increment is sometimes referred to as a just noticeable difference (JND) or difference limen (DL)

Question 22

What happens to JND as the background intensity increases?

Answer 22

JND also increases
Ratio remains constant

Question 23

Describe lightness constancy.

Answer 23

Contrast remains constant, appearance remains the same regardless of light that is reflected from it

Question 24

Describe simultaneous contrast.

Answer 24

Brightness depends on the background (consistent with Weber’s law)

Question 25

How can we control for subject bias when using the methods of constant stimuli?

Answer 25

Use blank trials to determine false positives or using suprathreshold to determine false negatives