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Psychophysical Assessment of Visual Function
As an O.D. you will measure (assess) vision.How well does the person see?Results depend on how you make the measurement
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You: Any problems with your vision?
Pt: Dont seem to see so well, Doc.
What do you do (how do you learn how well the patient sees)?
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You measure the patients vision.
This course is about the science that stands behind why you measure vision certain ways in the clinic.
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Which chart to use?How many letters per line?How far apart are the letters and lines?How much smaller are the letters on the next line?Which letters to use?How far down the chart must the patient try to read?How score the result?There are many different eye charts The acuity you get will differ depending these factors
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It is a matter of judgment that determines how the visual system is tested and what constitutes normal variation in sensory processes.
The clinician must understand the scientific basis on which these judgments are made and how they can be made in the future as new tests of visual function are developed.Thats what this course is about and why it is clinically relevant
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Dr. Tom Norton606 Worrell [email protected]
Graduate Student Teaching Assistant:Jason Wilson
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Class Mon- 9-10:50Tues, 9-10:50Wed, 11:0011:50Lab on 4 ThursdaysCheck the schedule for your day and time(Schedule will be distributed tomorrow) This week: Group C 1-3; gp A 3-5
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Exams #1 Wed. Jan. 19 (100 pts) Ch 1-3 #2 Tues. Feb. 8 (100 pts) Ch 4-5Final during Final Exam Period (130 pts)(110 pts new, 20 pts cumulative)Labs (4 x 10 pts)Possible pop quizzes (up to 30 pts total)
Total possible points, 370 (up to 400)Letter grade determined @ end of course
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LabsAttend at the assigned day and time (unless you make other arrangements with Dr. Norton in advance)Lab Reports due at Monday class after your labAccurately recording and graphing your data is an important part of your lab grade
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Student-submitted exam questionsA way to control your own future!Procedure: Due several days before exam (email or Word files preferred)Norton reviews, corrects, photocopiesDistributed to class (can use as a study guide)Some of the questions will be used on the exam
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Three main purposes of courseLearn how vision is measured (scientific basis)Basic facts about monocular visual functionWhat is normal?Neural basis of visual functionWhy does the visual system respond as it does?
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TextbookThe Psychophysical Measurement of Visual Function
Norton*, Corliss, Bailey
Richmond Products, Inc
(*TTNs author royalties [$2420.41 so far] donated to the UABSO)
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We will cover 9 ChaptersPrinciples of Psychophysical MeasurementAbsolute Threshold of VisionIntensity DiscriminationAdaptation to Light and DarkSpatial AcuitySpatial VisionTemporal Factors in VisionSkip Chapter 8 (color)8) Postnatal Human Vision DevelopmentThe Aging Visual System
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Overview
At the beginning of each chapter.
Contains a summary of the content of the chapter.
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Declarative section headings summarize the section they precede
In the Method of Constant Stimuli the examiner randomly presents a set of stimuli with fixed, predetermined values
Correct for guessing by incorporating catch trials
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Study Guide
Questions at the end of each chapter intended to help you clarify your knowledge (not as useful as I had hoped)
Lecture overlaps with the book a lot
but questions also come from the book on topics I dont cover in class!
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Glossary intended to help you know what terms mean for examDefinitions given in the text definite full credit if you know them verbatim
Equations must tell what the variables mean
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Equations must tell what the variables meanWhat is the Stevens Power Function?
where Y (psi) is the sensory magnitude, k (kappa) is an arbitrary constant determining the scale unit, F (phi) is the stimulus magnitude, and a (alpha) is an exponent that is characteristic of the stimulus used.
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Graphs The hardest part of this class(because they tend to all look alike) but important because they show the relationship between stimuli and responses
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Graphs how to dissect and learn themWhat is on the X-axis? (& approx. scale)Physical Stimulus on X-axis (Independent Variable)Usual arrangement:
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Graphs how to dissect and learn themWhat is on the X-axis? (& approx. scale)What is on the Y-axis? (& approx. scale) Response on Y-axisWhat you are measuring(Dependent Variable)Usual arrangement:Physical Stimulus on X-axis (Independent Variable)
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Graphs how to dissect and learn themWhat is on the X-axis? (& approx. scale)What is on the Y-axis? (& approx. scale)How plot a data point? Physical Stimulus on X-axis(Independent Variable)Usual arrangement:Response on Y-axisWhat you are measuring(Dependent Variable)
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GraphsWhat is different in each graph in a family of curves?
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Lots of details to learn.
Philosophy: better to have learned and forgotten than to not have learned in the first place.
example
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Joke break
Break the monotony
but remember that the course has a serious purpose, and the exams can be difficult.
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Student Response SystemTest to see if it worksWill use for feedbackWill not look at who responds
Set to room code (23)
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Chapter 1
Principles of Psychophysical MeasurementObjectives:Psychophysical MethodsThresholdConstant StimuliLimitsAdjustmentSignal detection theorySensory Magnitude
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We study visual psychophysics, but there also is auditory psychophysics, somatosensory psychophysics, etc.
Definition:
Psychophysics is the study
of the relationship
between physical stimuli
and perceptual responses
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Why are there so many graphs in this course?Because graphs show relationshipsPhysical Stimulus on X-axis(Independent Variable)Usual arrangement:Response on Y-axisWhat you are measuring(Dependent Variable)
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Two basic types of psychophysical measures
Threshold measures (Do you see it)
2) Sensory Magnitude measures (What does it look like)
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Do you see the light? Physical stimulus light intensityPerceptual response Seeing the light
Threshold measure:
Psychophysics is the study of the relationship between physical stimuli and perceptual responses
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How far down an eye chart can you read? Physical stimulus Letter sizePerceptual response Identifying letters
Threshold measure:
Psychophysics is the study of the relationship between physical stimuli and perceptual responses
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letter size is the stimulusidentifying letters is response
We use psychophysical tools to find the threshold the letter size you can see 50% of the time
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Which is better, 1 or 2? Physical stimulus Lens powerPerceptual response Clarity of the image
Sensory Magnitude:
Psychophysics is the study of the relationship between physical stimuli and perceptual responses
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Why study psychophysics?Psychophysical measurements are fundamental in clinical practiceNeed to know the scientific basis for measuring vision The results you get depend on the way you measure visionNew clinical tools will be developed after you graduate you need the knowledge base to understand how they work and evaluate whether they are useful in your practice.Psychophysics questions have been plentiful on the boards
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Psychophysical measurements
are used for descriptive and for analytical purposes and to follow the course of treatment
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Visual thresholds are the most common psychophysical measurement
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Definition
Threshold is defined as the minimum value of a stimulus required to elicit a perceptual response or an altered perceptual response.
Two types of threshold measurement:
absolute threshold (in vision) is the minimum value of a stimulus required to detect the presence of light under ideal conditions. (Ch. 2)
A difference (or increment) threshold is defined as the minimum change in stimulus value that must be added or subtracted to a stimulus to elicit an altered perceptual response. (Ch. 3)
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The task required of a patient or subject during threshold measurements varies in complexity
detection task (in vision) does the subject or patient see something?
discrimination task (in vision) distinguishing between two stimuli with regard to some stimulus characteristic when each stimulus is visible by itself.
recognition task. providing a name or category of a test object that is visible.
The distinctions among these various types of tasks are not sharp, but are hierarchical.
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Important Stimulus Dimensions
intensity
wavelength
size
exposure duration
frequency
shape
relative locations of elements of the stimulus
cognitive meaning
In addition,(NOT stimulus Dimensions!)
location on the subjects retina
light adaptation of the subjects visual system
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Key in measuring thresholds: Try to keep all dimensions unchanged except the one being measured
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Stimulus Configurations
Spot on an adapting field (increment thresholds)
Bipartite field
Bipartite field with an adapting field
Spatially separated stimuli (difference thresholds)
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There are many possible values of L, But only 1 value (theoretically) for threshold L
(demo)
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Definition
Threshold is defined as the minimum value of a stimulus required to elicit a perceptual response or an altered perceptual response.
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Definition
Threshold is defined as the minimum value of a stimulus required to elicit a perceptual response or an altered perceptual response.
But threshold can vary over time (somewhat)
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Psychophysically measured threshold values vary because of
fluctuations in the stimulus (Ch. 2)
fluctuations in neural activity
fluctuations in alertness or attention
psychological bias
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Fluctuations in neural activity
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For us to see, neural signals must leave the retina and travel to central brain structures.
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In the early retinal cells (photoreceptors, bipolars, horizontal cells, most amacrines), there are only graded potentials (hyperpolarization and depolarization of the cell)In order to send signals out of the retina, action potentials (spikes) must be generated and travel down the ganglion cells axon to the next location (lateral geniculate nucleus, then to visual cortex)
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Graded potentialsThe signal changes from graded potentials (voltage changes) into a digital signal in which the number of action potentials per second (firing rate) carries the visual signal.
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We can eavesdrop on the neurons in the visual pathway with a microelectrode, nestled up against a neuron or its axon and record the responses (number of spikes per second) in response to visual stimuli.B: Action potentials recorded from a single LGN neuron. The same stimulus (a spot of light positioned in the receptive field was presented many 20 times. A: a histogram of the cells responses
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Action potentials recorded from a single LGN neuronNeural fluctuations: the neuron sometimes responds more, sometimes less, to the same stimulus. Also, the neuron has variable background (maintained) activity that makes it hard for the neuron to detect when the stimulus is present.
Responses of a neuron in the lateral geniculate nucleus of an anesthetized cat to three separate presentations of a near-threshold visual stimulus. Each small vertical line represents an action potential produced by the neuron. Each row shows the responses of the neuron in a 3 s period. From 0 until 2.5 s a background luminance was present. The stimulus (a light) was turned on at 2.5 s and turned off at 3.0 s, so the stimulus was on for only 0.5 s. (Unpublished data from D. W. Godwin and T. T. Norton,.)
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This leads us to consider threshold as a probability that a stimulus is detected and to find the stimulus value that is detected 50% of the time (or some other criterion value)
Psychophysically measured threshold values vary because of
fluctuations in the stimulus
fluctuations in neural activity
fluctuations in alertness or attention
psychological bias
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Threshold Determination Methods
Method of Constant Stimuli
Method of Limits
-Staircase
-Tracking
Method of Adjustment
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In the Method of Constant Stimuli the examiner randomly presents a set of stimuli with fixed, predetermined values
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Figure 1-4. Idealized psychometric function for a threshold detection task using the Method of Constant Stimuli. The threshold stimulus value is obtained by drawing a horizontal line from the 50% value on the response axis to the psychometric function and then dropping a vertical line from the function to the test field intensity axis.
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In Class Demo
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Rule: Plot straight lines between data points
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Silliest Plotting ErrorPlot data points from left to right
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Silliest Plotting ErrorPlot data points from left to right
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Most Interesting Curves
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Figure 1-4. Idealized psychometric function for a threshold detection task using the Method of Constant Stimuli. The threshold stimulus value is obtained by drawing a horizontal line from the 50% value on the response axis to the psychometric function and then dropping a vertical line from the function to the test field intensity axis.
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Chart1
0.0146341463
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1
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1
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Whole Class 2010
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Chart3
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Stimulus Value
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Chart2
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Whole Class 2010
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One point (added to the exam #1 grade) to the first person to correctly tell me:
Why might the threshold be lower for the front half of the room?
Hint: think about a stimulus parameter that might change from the front to the back of the room.
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The Importance of Using Straight Lines to Connect Data Points
The data points are the only evidence we have of thresholdWe assume a linear progression from one data point to the nextCan use linear interpolation to determine the threshold accurately
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The Importance of Using Straight Lines to Connect Data Points
A dramatic example: If you measure vision incorrectly, you get an incorrect answer about how well a person sees.
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Another way to mis-estimate threshold
We are looking for the 50% point, not the closest data point, so we use linear interpolation
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We want to measure threshold as accurately as possible. Why be satisfied with 6 when 5.8 is more accurate?
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The Method of Constant Stimuli is the most precise method for determining threshold (the Gold Standard).
But, this method is cumbersome and time-consuming so it is rarely, if ever, used in clinical practice.
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Threshold Determination Methods
Method of Constant Stimuli
Method of Limits
-Staircase
-Tracking
Method of Adjustment
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What is another name for the psychometric function?Threshold lineFrequency-of-seeing curveMethod of Constant StimuliPower function
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In the Method of Limits the examiner sequentially presents a set of stimuli with fixed values
Trial Number (Stimulus Presentation Direction)
Stimulus Value
1
(Ascending)
2
(Descending)
3
(Ascending)
4
(Descending)
5
(Ascending)
1
N
N
N
2
N
N
N
N
3
Y
N
N
Y
4
N
Y
Y
N
Y
5
N
Y
Y
N
6
Y
Y
Y
7
Y
Y
N
8
Y
Y
9
Y
10
Y
Average
Transition
5.5
3.5
3.5
5.5
2.5
4.1
Table 1- 1. Example of subjects responses over five trials using the method of limits.
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In Class Demo
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The Method of Limits is more efficient than the Method of Constant Stimuli because fewer trials are presented.
Two potential problems:
anticipation
perseveration
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Developed during WWII to test bomb detonators
Staircase procedure.
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Staircase procedure.
Stimulus Value
Trial Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
N
3
N
N
N
4
N
Y
N
N
5
Y
Y
N
Y
6
N
Y
7
Y
8
Y
9
Y
Table 1- 2. Example of a subjects responses over 17 trials using the staircase variation on the Method of Limits.
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Staircase procedure.
Stimulus Value
Trial Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
N
3
N
N
N
4
N
Y
N
N
5
Y
Y
N
Y
6
N
Y
7
Y
8
Y
9
Y
Table 1- 2. Example of a subjects responses over 17 trials using the staircase variation on the Method of Limits.
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Staircase procedure.
Stimulus Value
Trial Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
N
3
N
N
N
4
N
Y
N
N
5
Y
Y
N
Y
6
N
Y
7
Y
8
Y
9
Y
Table 1- 2. Example of a subjects responses over 17 trials using the staircase variation on the Method of Limits.
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Staircase procedure.
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(Developed by Nobel Prize-winning auditory physiologist, Georg von Beksy)
Tracking procedure
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Threshold Determination Methods
Method of Constant Stimuli
Method of Limits
-Staircase
-Tracking
Method of Adjustment
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In the Method of Adjustment
the subject controls the stimulus values
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In Class Demo
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Frequency with which LT is seen as equal to L
The distribution of values of LT that a subject decides are equal to L forms a normal distribution if enough trials are used. The mean of the distribution will be very close to L. The threshold is taken as the value of LT that, when added to or subtracted from L gives an LT that is detectable on 50% of the trials. This occurs 0.68 standard deviations above and below the mean.
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The Method of Adjustment is most easily used when the stimulus can be changed in a continuous manner, rather than in steps.
Subjects generally enjoy the Method of Adjustment because they actively participate.
Boredom and inattention are less of a problem with the Method of Adjustment than with the other methods.
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Potential problem with the Method of Adjustment
subjects may use the position of the dial as a cue to where threshold "ought" to be.
This strategy can by foiled by using a dial that has no numbers and has a variable amount of slip.
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Controlling response bias and guessing
Correct for guessing by incorporating catch trials
Establish the guessing rate by forcing the subject to make choices (forced choice technique)
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What do you do if the psychometric function doesnt drop down to 0% Yes responses for low stimulus values?
Assume subject/patient has a bias to guess Yes.
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Correct for guessing by incorporating catch trials where the stimulus is not presented at all. This gives the guessing rate.
-
The catch trial gives the guessing rate. Then subtract the guessing rate from the data to get the True percent of Yes responses
Correct for guessing by incorporating catch trials. In this case no stimulus at all was presented for a value of 0, so this was a catch trial.
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The correction factor is:
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Do not memorize the formula! It isnt used much. Instead, people use the forced choice procedure
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Establish the guessing rate by forcing the subject to make choices
The Forced Choice procedure
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In Class Demo Two-alternative forced choice
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But if there are two alternatives (two-alternative forced-choice) you know the guessing rate is 0.5
The correction factor is still:
-
Figure 1-4. Idealized psychometric function for a threshold detection task using the Method of Constant Stimuli. The threshold stimulus value is obtained by drawing a horizontal line from the 50% value on the response axis to the psychometric function and then dropping a vertical line from the function to the test field intensity axis. Results from yesterdays Method of Constant Stimuli Threshold Measurement
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Chart1
0
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1
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Stimulus Value
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Whole Class, 2009
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Whole Class, 2009
Stimulus Value
Fraction of "Yes" Responses
Front Rows
Stimulus Value
Fraction of "Yes" Responses
Back Rows
Stimulus Value
Fraction of "Yes" Responses
Back Rows
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Chart2
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71
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101
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45600143555555
count45
sum03103395218223225225225225
decimal0.000.010.040.150.420.970.991.001.001.001.00
012345678910
Whole0.000.010.040.150.420.970.991.001.001.001.00
012345678910
front0262053112115115115115115
230.000.020.050.170.460.971.001.001.001.001.00
012345678910
back0141342106108110110110110
220.000.010.040.120.380.960.981.001.001.001.00
Values copied over
012345678910
front
2300000111111
012345678910
back
2100000111111
Sheet1
Stimulus Value
Fraction of "Yes" Responses
Whole Class, 2009
Stimulus Value
Fraction of "Yes" Responses
Front Rows
Stimulus Value
Fraction of "Yes" Responses
Back Rows
Stimulus Value
Fraction of "Yes" Responses
Back Rows
-
Chart4
00
10.0090909091
20.0363636364
30.1181818182
40.3818181818
50.9636363636
60.9818181818
71
81
91
101
Stimulus Value
Fraction of "Yes" Responses
Back Rows
Sheet1
Method of constant Stimuli Class Demo Results - 2009
Stimulus value
PersonRowside012345678910
1100004555555
2100102555555
3100212355555
4100022555555
5100035555555
6200022555555
7200015555555
8201221555555
9200013555555
10200003555555
11200002555555
12201134555555
13300000555555
14300013555555
15300001555555
16300014555555
17300022555555
18300001455555
19400002555555
20400000555555
21400002555555
22400003555555
23400010555555
24400003555555
25400005555555
26400014555555
27500002445555
28500003555555
29500000555555
30500000555555
31500222555555
32500002555555
33500014555555
34500012555555
35500011555555
36500001445555
37600003555555
38600023555555
39600000555555
40601112455555
41600000555555
42600000555555
43600001555555
44600001455555
45600143555555
count45
sum03103395218223225225225225
decimal0.000.010.040.150.420.970.991.001.001.001.00
012345678910
Whole0.000.010.040.150.420.970.991.001.001.001.00
012345678910
front0262053112115115115115115
230.000.020.050.170.460.971.001.001.001.001.00
012345678910
back0141342106108110110110110
220.000.010.040.120.380.960.981.001.001.001.00
Values copied over
012345678910
front
2300000111111
012345678910
back
2100000111111
Sheet1
Stimulus Value
Fraction of "Yes" Responses
Whole Class, 2009
Stimulus Value
Fraction of "Yes" Responses
Front Rows
Stimulus Value
Fraction of "Yes" Responses
Back Rows
-
Results from yesterdays Two-alternative Forced Choice Measurement
-
Intensity Discrimination Lab tomorrowGroups C (1 3) and A (3 5)List posted on bulletin board
Planning to have the lab unless UAB cancels classes due to snow
Snow amount is predicted to be small
-
20095 4 3 2
Chart1
53.33333333336.6666666667
47.619047619-4.7619047619
96.666666666793.3333333333
99.047619047698.0952380952
Obtained percent correct
True percent correct
Stimulus
Percent Correct Responses
Two-alternative Forced-choice in-class Demo2010
Sheet1
Forced Choice in-class Demo
personStimulus
5432
12255
25155
31355
42255
53255
65355
73344
82455
93355
104255
mg112355
124455
131255
144445
151355
163355
172055
184335
192255
202255
212355
222355
234155
242355
253155
265245
273354
283255
293245
302055
313045
322355
333455
343355
352255
361255
371255
384355
394255
400255
412355
423355
43
44
45
sum112100203208
count42424242
mean2.72.44.85.0
percent corr53.347.696.799.0
corrected6.67-4.7693.3398.10
stimulus1234
Obtained percent correct53.347.696.799.0
True percent correct6.67-4.7693.3398.10
Sheet1
Obtained percent correct
True percent correct
Stimulus
Percent Correct Responses
Two-alternative Forced-choice in-class Demo2010
Sheet2
Sheet3
-
5 4 3 2
Chart1
53.33333333336.6666666667
47.619047619-4.7619047619
96.666666666793.3333333333
99.047619047698.0952380952
Obtained percent correct
True percent correct
Stimulus
Percent Correct Responses
Two-alternative Forced-choice in-class Demo2010
Sheet1
Forced Choice in-class Demo
personStimulus
5432
12255
25155
31355
42255
53255
65355
73344
82455
93355
104255
mg112355
124455
131255
144445
151355
163355
172055
184335
192255
202255
212355
222355
234155
242355
253155
265245
273354
283255
293245
302055
313045
322355
333455
343355
352255
361255
371255
384355
394255
400255
412355
423355
43
44
45
sum112100203208
count42424242
mean2.72.44.85.0
percent corr53.347.696.799.0
corrected6.67-4.7693.3398.10
stimulus1234
Obtained percent correct53.347.696.799.0
True percent correct6.67-4.7693.3398.10
Sheet1
Obtained percent correct
True percent correct
Stimulus
Percent Correct Responses
Two-alternative Forced-choice in-class Demo2010
Sheet2
Sheet3
-
5 4 3 2Either way gives the same threshold, but it is easier to use a 75% threshold and not bother to correct
Chart1
53.33333333336.6666666667
47.619047619-4.7619047619
96.666666666793.3333333333
99.047619047698.0952380952
Obtained percent correct
True percent correct
Stimulus
Percent Correct Responses
Two-alternative Forced-choice in-class Demo2010
Sheet1
Forced Choice in-class Demo
personStimulus
5432
12255
25155
31355
42255
53255
65355
73344
82455
93355
104255
mg112355
124455
131255
144445
151355
163355
172055
184335
192255
202255
212355
222355
234155
242355
253155
265245
273354
283255
293245
302055
313045
322355
333455
343355
352255
361255
371255
384355
394255
400255
412355
423355
43
44
45
sum112100203208
count42424242
mean2.72.44.85.0
percent corr53.347.696.799.0
corrected6.67-4.7693.3398.10
stimulus1234
Obtained percent correct53.347.696.799.0
True percent correct6.67-4.7693.3398.10
Sheet1
Obtained percent correct
True percent correct
Stimulus
Percent Correct Responses
Two-alternative Forced-choice in-class Demo2010
Sheet2
Sheet3
-
http://www.opt.uab.edu/nortonpsychophysics/
-
Chapter 1
Principles of Psychophysical MeasurementObjectives:Psychophysical MethodsThresholdConstant StimuliLimitsAdjustmentSignal detection theorySensory Magnitude
-
Using Signal Detection Theory
1) to Understand Threshold Variability and
2) to Control Subject Bias
-
Big point!!
Lesser point
Using Signal Detection Theory
1) to Understand Threshold Variability and
2) to Control Subject Bias
-
Stimuli that are near threshold always are difficult to see! Did I see that, or didnt I?The brain (comprised of neurons) must decide if a stimulus was present against a background of neural noise.
At threshold, neurons must decide whether a stimulus is present against a background of noise
-
Your brain causes perception. Cells in the brain do not respond to light. They respond because they are activated by a chain of cells that start with photoreceptors, which do see light.
-
We can eavesdrop on the neurons in the visual pathway with a microelectrode, nestled up against a neuron or its axon and record the responses (number of spikes per second) in response to visual stimuli.B: Action potentials recorded from a single LGN neuron. The same stimulus (a spot of light positioned in the receptive field was presented many 20 times. A: a histogram of the cells responses
-
The visual system has to decide if a stimulus is present on the fly as events happenIn studying how the visual system responds, we have the luxury of studying neural responses over many repeated trialsUse this information to understand why thresholds can be affected by bias
-
NoiseSignal + Noise
Responses of a neuron in the lateral geniculate nucleus of an anesthetized cat to three presentations of a near-threshold visual stimulus. Each small vertical line represents an action potential produced by the neuron. Each row shows the responses of the neuron in a 3 s period. From 0 until 2.5 s a background luminance was present. The stimulus (a light) was turned on at 2.5 s and turned off at 3.0 s, so the stimulus was on for only 0.5 s. (Unpublished data from D. W. Godwin and T. T. Norton,.)
-
Below is a peristimulus histogram made from the responses to 30 stimulus repetitions like the three lines shown above. We want to compare responses during noise and signal + Noise
Responses of a neuron in the lateral geniculate nucleus of an anesthetized cat to three presentations of a near-threshold visual stimulus. Each small vertical line represents an action potential produced by the neuron. Each row shows the responses of the neuron in a 3 s period. From 0 until 2.5 s a background luminance was present. The stimulus (a light) was turned on at 2.5 s and turned off at 3.0 s, so the stimulus was on for only 0.5 s. (Unpublished data from D. W. Godwin and T. T. Norton,.)
-
We are interested in how many action potentials are generated, over many stimulus presentations, during a 50 msec period when there is no stimulus (maintained discharge) and a 50 msec period when the stimulus is present.Why 50 msec? Arbitrary, but it is about the amount of time the CNS seems to use.
Responses of a neuron in the lateral geniculate nucleus of an anesthetized cat to three presentations of a near-threshold visual stimulus. Each small vertical line represents an action potential produced by the neuron. Each row shows the responses of the neuron in a 3 s period. From 0 until 2.5 s a background luminance was present. The stimulus (a light) was turned on at 2.5 s and turned off at 3.0 s, so the stimulus was on for only 0.5 s. (Unpublished data from D. W. Godwin and T. T. Norton,.)
-
28303215
Making a frequency distribution of neural responses during noise and signal + noise50 ms binsNumber of action potentials in each binnoiseStimulus + noiseDuring noise, 0 spikes occur 1 time, 3 spikes occur 1 time, 2 occur 1 timeDuring signal + noise, 3 spikes occur 1 time, 8 spikes occur 1 time, 15 occur 1 timeDo this across 30 stimulus presentations to get a distribution of the frequency with which a certain number of spikes occurs
-
How can the brain decide if a near-threshold stimulus is present? If a strong stimulus is presented, it produces many more action potentials during the signal + noise than are produced during the noise. But when a stimulus is near threshold, there is overlap between the number of spikes produced during noise and signal + noise
There is no single optimal criterion number of action potentials that the nervous system (such as a cell in the visual cortex) should use to decide whether to respond as though a stimulus was present, or to respond as though a stimulus was not present.
-
One can try various criteria Changing the criterion (the threshold one adopts) affects the pattern of hits, misses, false alarms and correct rejections
The saga of the snake in the grassThis changing threshold is partly responsible for fluctuations in threshold.
-
Imagine the situation faced by a mouse, needing to forage for food, but worrying that a snake might be hanging around and eat the mouse when the mouse goes out to eat
-
Set criterion low, to always detect the snakeIf 6 or more action potentials, decide snake!!
-
Set criterion low, to always detect the snakeIf 6 or more action potentials, decide snake!!Problem: will also see snake some times when it is just the noise of the visual system
-
Out of the four possible outcomes there are two ways to be correct:
by deciding the stimulus is there when it is present (a Hit)
and by deciding that it is not there when it is absent (a Correct Rejection).
There are also two ways to be wrong:
by deciding the stimulus is present when it is absent (a False Alarm)
and by deciding it is not present when it is (a Miss).
-
Many false alarmsIf set a low criterion (threshold), hit rate is perfect, but
-
10
-
So, try changing the criterion mouse gets hungrier, willing to take a chance If set a high criterion (threshold) have no false alarms but also fewer hits (more misses)
-
10
-
Can calculate hit rate and false alarm rate for ANY criterion
-
Receiver Operating Characteristic (ROC) curve
Receiver Operating Characteristic (ROC) curve for the responses shown in the previous figure. If the threshold is set at 15 action potentials, there are 0 Hits and 0 False Alarms. If it is set at 14, there will be a few Hits, but 0 False Alarms. As the threshold is decreased further, the P(Hit) increases but the P(False Alarm) remains at 0 until the threshold reaches 9, at which point False Alarms begin to increase. As the threshold is further lowered, through the overlap region in the previous figure, the probability of both Hits and False Alarms increase. For thresholds below 6, there is no further increase in hit rate, but the false alarm rate climbs toward 1.0.
-
Signal Detection Theory also applies to human perceptual responses
Distribution of hypothetical perceptual response in a human subject over many trials when the stimulus was absent (top) and when the stimulus was present (bottom). The criterion value (vertical line) indicates the criterion a subject would adopt if Hits, Misses, False Alarms and Correct Rejections had the rewards and costs listed in another figure.
-
False Alarm Rate0.00.20.40.60.81.0Hit Rate0.00.20.40.60.81.0Receiver Operating Characteristic(ROC) CurveThe distributions on the previous slide would produce this
-
d (d prime) is a measure of the separation of two normal distributions. d = the difference between the means of the noise and signal plus noise distributions divided by the common standard deviation of the two distributions.d quantifies the detectability of the signal (small d = signal is hard to detect)
-
Big point: Where a neuron, or an entire creature (human or animal) sets its criterion depends on circumstances (fear vs. hunger which causes a change in bias). This contributes to threshold variability.
Can Use Signal Detection Theory to control bias when measuring threshold (This is the minor point)
-
Use the Payoff Game
To control a subjects criterion, the examiner provides the subject, in advance, with:
the payoff amounts
and
information on the frequency of stimulus presentation
-
Use the Payoff Game
To control a subjects criterion, the examiner provides the subject, in advance, with:
the payoff amounts
and
information on the frequency of stimulus presentation
-
Skip the text on pages 24, 26, 27 and top of 28
Signal detection theory can be used to control bias when measuring threshold
-
Screening for refractive error: Hits: Correct detection of refractive errorCorrect rejection: pass the screening because child is emmetropicFalse alarm (false positive): incorrectly refer for full exam based on screening (cost, concern, inconvenience)Misses (false negative): fail to detect refractive error
Minimize false positives even though some refractive error is missed
The concepts of Signal Detection Theory form the basis of rational clinical decision making
-
Detecting ocular melanoma: Hits: Correct detection of melanoma (refer for possible surgery)Correct rejection: pass because no melanomaFalse positive incorrectly refer based on screening (alarm, cost, inconvenience)Misses (false negatives): fail to detect melanoma (possible death)
Minimize false negatives even though some false positives occur
The concepts of Signal Detection Theory form the basis of rational clinical decision making
-
You will hear in clinic about the sensitivity and specificity of diagnostic techniques.
Sensitivity is the hit rateSpecificity is the absence of false alarmsSo plot (1 specificity) on an ROC curveWant a diagnostic tool that has high sensitivity and high specificity
-
Do you see it?As was said the first day of classVisual thresholds are the most common psychophysical measurement
-
What does it look like?
The other major type of
psychophysical measurement:
Measuring the magnitude of sensations
-
Increased light intensity is brighter but how much brighter? Increased spot size is larger, but how much larger?
Perceptual responses (sensations) have magnitude but no obvious scale or units
-
To measure sensory magnitude above threshold, use scales that do not rely on any particular units of measurement
-
1) Ratio Production - the subject is presented with a reference stimulus and is asked to adjust the intensity of a test stimulus so that it appears to be some fraction or multiple of the reference stimulus.
2) Ratio Estimation - the examiner sets the physical intensities of a reference and test stimulus and asks the subject to estimate the ratio of the test to the reference stimulus.
-
3) Magnitude Estimation (two variants)
a) an observer is presented with a reference stimulus and told that it has a certain value (10, 100 etc.) A series of test stimuli are then presented and the observer assigns a number to these stimuli to indicate their perceived magnitude relative to the reference stimulus
b) (dont bother with this)
4) Magnitude Production a subject is presented with a reference stimulus and is asked to adjust a test stimulus so that is appears to be some fraction or multiple of the reference stimulus.
-
Stevens' Power Law relates sensory magnitude to the magnitude of the stimulus:
where (psi) is the sensory magnitude, (kappa) is an arbitrary constant determining
the scale unit, (phi) is the stimulus magnitude, and (alpha) is an exponent that
is characteristic of the stimulus used.
Sensory magnitude is proportional to the stimulus magnitude raised to some power
-
When plotted on a log-log scale.
_927461079.unknown
-
Value of Some Vision-related Exponents in the Stevens Power Law Equation
Sensation
Exponent
Stimulus Condition
Brightness
0.33
5SYMBOL 176 \f "Symbol" target in the dark
Brightness
0.50
Point source
Brightness
0.50
Brief flash
Visual Area
0.70
Projected square
Brightness
1.00
Point source briefly flashed
Visual Length
1.00
Projected line
-
Clinicians rely on sensory magnitudes
to make judgments about pathologies
-
What does it look like?
Measuring the magnitude of sensations
-
Chapter 1
Principles of Psychophysical MeasurementObjectives:Psychophysical MethodsThresholdConstant StimuliLimitsAdjustmentSignal detection theorySensory Magnitude
*