Detection Thresholds Flashcards by Aayan Rafiq

What do we mean by detection thresholds?

The ability or not having the ability to detect the presence of light

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Give the 4 distinct stages of perception

Detection – is there something there?
Identification – what is there?
Discrimination – is that the same as that?
Scale – how similar are two things?

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How do we see?

-Sun emits radiation

-Some of the radiation is gathered, some is absorbed by objects in the environment and some is reflected off objects into the eye

-Photoreceptors transduce the light into an electrical signal which gets sent to the brain for further processing

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What is the light sources emission spectrum referring to?

Refers to the fact that different light sources emit different wavelengths of light

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What occurs as a result of different light sources emit different wavelengths of light?

Different wavelengths of light will enter our eye

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What range is the human eye sensitive to?

400 - 720nm

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What determines what the visible spectrum is for us?

Chromophores within our photoreceptors

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How does the brain see colour?

Discounts the illuminant

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Which structures in the eye provide most of the retinas protection damaging UVC, UVB and UVA radiation?

Cornea and lens

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How does the cornea and lens protect the retina from UVC, UVB and UVA radiation?

Cornea and lens absorb the damaging UV radiation

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What happens to the absorption characteristics of structures in the eye as we age?

Absorption of structures decreases as we age

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What 4 things is retinal image quality dependant on?

-Refractive power of the eye

-Transparency of the media

-Amount of light

-Image focus

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How are spherical aberrations generated?

Generated when light rays passing through the edge of a refracting surface (lens) are brought into focus in front of those that pass through the centre of the lens

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How are chromatic aberrations generated?

Generated by shorter wavelengths being refracted more than longer wavelengths

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How does the eye minimise spherical aberrations?

-Refractive index of the lens is different at the edge compared to the centre (gradient index)

-Orientation/curvature of the cornea/lens helps

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How does the eye minimise chromatic aberrations?

Yellow pigment at the fovea absorbs blue light

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What does the Rayleigh Criterion state?

In a diffraction limited optical system (only problem with the optical system is diffraction) two-point sources of light can be distinguished (resolved) if the central maxima of the two point sources are at least as far apart as the first minima of either point source

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Define what an absolute threshold is

The minimum amount of stimulus energy needed to elicit a detection response from an individual

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Describe The Method of Limits as a way to calculate the detection threshold

-Stimuli are presented in either ascending (dark to light) or descending (light to dark) order

-The observer reports whether they can see the stimulus or not

-Measurement is repeated a number of times, averaged, and a psychometric function generated

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Give the advantages of The Method of Limits as a way to calculate the detection threshold

-Quick
-Easy to set up

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Give the disadvantages of The Method of Limits as a way to calculate the detection threshold

Subjects often ‘anticipate’ the threshold if each trial starts at the same level (may bias the results)

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Describe The Staircase Method as a way to calculate the detection threshold

-Stimulus intensity is increased

-When the subject reports seeing the stimulus the direction is reversed (ascending becomes descending or descending becomes ascending) and the visibility of the stimulus is reduced until the subject reports that it disappears

-Staircase is then reversed again and the stimulus intensity is increased until it is once again visible

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What is the threshold frequently defined as in The Staircase Method as a way to calculate the detection threshold?

Defined as having occurred after three or four reversals

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Give the advantages of The Staircase Method as a way to calculate the detection threshold

-Quick
-Reliable method
-More time is spent measuring around the persons threshold

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Give the disadvantages of The Staircase Method as a way to calculate the detection threshold

Px's may predict what's going to come next

Describe The Method of Constant Stimuli as a way to calculate the detection threshold

-A fixed or constant set of stimuli are chosen before the experiment -Stimuli are presented randomly -For each stimulus value the subject reports whether they see or don’t see the stimulus -Each stimulus is presented multiple times

When is The Method of Constant Stimuli used to calculate the detection threshold?

Used when you have more time and when you’re not exactly sure where the threshold is going to be

Give the advantage of The Method of Constant Stimuli as a way to calculate the detection threshold

Subjects cannot anticipate the visibility of each stimulus

Give the disadvantage of The Method of Constant Stimuli as a way to calculate the detection threshold

Very time consuming

Describe The Method of Adjustment as a way to calculate the detection threshold

-Subject adjusts the independent variable (for example the intensity of the light) using a knob until the threshold is reached -Usually done from subthreshold to threshold (ascending intensities)

Give the advantage of The Method Adjustment as a way to calculate the detection threshold

Quick

Give the disadvantage of The Method Adjustment as a way to calculate the detection threshold

Suffers from variations in the subject’s threshold criterion (relaxed vs 100% certain)

What do thresholds depend on?

Decision criteria of the Px

What does it mean if a Px has a conservative decision criteria?

-Px is very careful in deciding whether they see a light or not -They will only respond “yes” if they are 100% sure (these Px's may not respond to all lights)

What does it mean if a Px has a liberal decision criteria?

-Px is much more relaxed about whether they have seen the light or not -If they think that they might have seen the light then they would respond “yes” (these Px's will most likely respond to all lights)

What does signal detection theory assume about an observers visual system?

Assumes that within the observers visual system there is some randomly fluctuating noise (N)

When an individual is having their detection threshold measured, what is the task?

The task for an individual is to tell the difference between this internal noise (N) alone when no stimulus signal is present from times when the stimulus signal is present and is combined with this internal noise (N+S)

If the noise within an individual is constant, what can the individual do?

Tune the noise out

If activity > threshold, what will the Px report?

Report that they see a light

If no signal is present and noise is a constant level below the threshold, is activity < or > threshold?

Activity < Threshold

If signal is present and noise is a constant level, is activity < or > threshold?

Activity > Threshold

If the internal noise within a Px is loud, what can this produce a lot of when determining the detection threshold?

False alarms

What are false alarms?

Subject is pressing the button when there is no light present

What do false alarms indicate?

Indicate the presence of something which is not there

Why would the Px not see the light even though it is present if there is a signal and noise present?

Internal noise has obscured the presence of the light

What does having a conservative decision criteria lead to in terms of misses and false alarms?

-Increase in the number of misses -Reduction in the number of false alarms

What does having a liberal decision criteria lead to in terms of misses and false alarms?

-Reduction in the number of misses -Increase in the number of false alarms

If the signal is present and the response is yes, what is this called?

Hit (True positive)

If the signal is absent and the response is yes, what is this called?

False alarm (False positive)

If the signal is present and the response is no, what is this called?

Miss (False negative)

If the signal is absent and the response is no, what is this called?

Correct rejection (True negative)

Describe Forced Choice Procedures as a way to calculate the detection threshold

-Subject is forced to choose from a number of alternative choices (one of which contains the stimulus) -% correct for the various stimuli intensities can then be used to construct a psychometric function to determine threshold

Give the advantage of Forced Choice Procedures as a way to calculate the detection threshold

Subjective criteria can be minimised (Px has to choose)

What does 2AFC mean?

Two-alternative forced choice (2AFC) describes a subject choosing between two alternatives

Calculate and explain what the threshold will be set to as a % for a 2AFC procedure

-There is already a 50% chance of a Px guessing the correct response with 2AFC as there is only 2 options for the Px to choose from -Threshold will be set in between 50% (minimum) and 100% - 100% + 50% = 150% - 150%/2 = 75% -Threshold will be set at 75%

What other types of Forced Choice Procedures are there?

-4AFC -6AFC

Calculate and explain what the threshold will be set to as a % for a 4AFC procedure

-There is already a 25% chance of a Px guessing the correct response with 2AFC as there is only 4 options for the Px to choose from -Threshold will be set in between 25% (minimum) and 100% - 100% + 25% = 125% - 125%/2 = 62.5% -Threshold will be set at 62.5%

Calculate and explain what the threshold will be set to as a % for a 6AFC procedure

-There is already a 16.67% chance of a Px guessing the correct response with 6AFC as there is only 6 options for the Px to choose from -Threshold will be set in between 16.67% (minimum) and 100% - 100% + 16.67% = 116.67% - 116.67%/2 = 58.3% -Threshold will be set at 58.3%

Describe Advanced Adaptive Methods as a way to calculate the detection threshold

-Involve presenting signals based on the subjects previous responses -3 correct responses, intensity decreased by one step -1 incorrect response, intensity is increased by one step -Session ends when a narrow range of stimuli is reached -Threshold is taken as the average of the intensity levels within the period of stable tracking

What are Parameter Estimation by Sequential Testing (PEST) techniques?

-Similar to Advanced Adaptive Methods -Large changes in intensity -Intensity is progressively halved each time until a narrow range of stimuli is reached

What does the sensitivity of test mean?

Proportion/ratio of hits (true positives) to the number of hits + misses (true positives + false negatives)

Give the formula to calculate sensitivity

Sensitivity = Hits / (Hits + Misses) OR True Positives / (True Positives + False Negatives)

What can the maximum value of sensitivity be?

What will a very sensitive test have?

Very few misses BUT it may have a lot of false alarms

What does the specificity of test mean?

Proportion/ratio of correct rejects (true negatives) to the number of correct rejects (true negatives) + false alarms (false positives)

Give the formula to calculate specificity

Specificity = Correct rejects / (Correct rejects +False alarms) OR = True Negatives / (True Negatives + False Positives)

What will a very specific test have?

Very few false alarms HOWEVER it may have a high number of misses

Detection Thresholds Flashcards

(67 cards)