Presentation on theme: "Psychophysical Assessment"— Presentation transcript:
1Psychophysical Assessment of Visual FunctionAs an O.D. you will measure (assess) vision.How well does the person see?Results depend on how you make the measurement
2You: Any problems with your vision? Pt: Don’t seem to see so well, Doc.What do you do (how do you learn how well the patient sees)?
3You measure the patient’s vision. This course is about the science that stands behind why you measure vision certain ways in the clinic.
4There are many different eye charts 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?The acuity you get will differ depending these factors
5It 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.That’s what this course is about and why it is clinically relevant
6Dr. Tom Norton606 Worrell BuildingGraduate Student Teaching Assistant:Jason Wilson
7Class – Mon- 9-10:50 Tues, 9-10:50 Wed, 11:00–11:50 Lab on 4 Thursdays Check the schedule for your day and time(Schedule will be distributed tomorrow)This week: Group C 1-3; gp A 3-5
8Exams #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 end of course
9LabsAttend 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
10Student-submitted exam questions A way to control your own future!Procedure:Due several days before exam ( 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
11Three main purposes of course Learn 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?
12Textbook The Psychophysical Measurement of Visual Function Norton*, Corliss, BaileyRichmond Products, Inc(*TTN’s author royalties [$ so far] donated to the UABSO)
13We will cover 9 Chapters Principles of Psychophysical Measurement Absolute Threshold of VisionIntensity DiscriminationAdaptation to Light and DarkSpatial AcuitySpatial VisionTemporal Factors in VisionSkip Chapter 8 (color)8) Postnatal Human Vision DevelopmentThe Aging Visual System
14Overview At the beginning of each chapter. Contains a summary of the content of the chapter.
15Declarative 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”
16Study GuideQuestions 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 don’t cover in class!
17Glossary – intended to help you know what terms mean for exam Definitions given in the text – definite full credit if you know them verbatimEquations – must tell what the variables mean
18Equations – must tell what the variables mean “What 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.
19Graphs – The hardest part of this class (because they tend to all look alike)… but important because they show the relationship between stimuli and responses
20Graphs – how to dissect and learn them What is on the X-axis? (& approx. scale)Usual arrangement:Physical Stimulus on X-axis (Independent Variable)
21Graphs – how to dissect and learn them What is on the X-axis? (& approx. scale)What is on the Y-axis? (& approx. scale)Usual arrangement:Response on Y-axisWhat you are measuring(Dependent Variable)Physical Stimulus on X-axis (Independent Variable)
22Graphs – how to dissect and learn them What is on the X-axis? (& approx. scale)What is on the Y-axis? (& approx. scale)How plot a data point?Usual arrangement:Response on Y-axisWhat you are measuring(Dependent Variable)Physical Stimulus on X-axis(Independent Variable)
23GraphsWhat is different in each graph in a “family” of curves?
24Lots of details to learn. Philosophy: better to have learned and forgotten than to not have learned in the first place.example
25“Joke break”Break the monotony… but remember that the course has a serious purpose, and the exams can be difficult.
26Student Response System Test to see if it worksWill use for feedbackWill not look at who respondsSet to room code (23)
27Principles of Psychophysical Measurement Chapter 1Principles of Psychophysical MeasurementObjectives:Psychophysical MethodsThresholdConstant StimuliLimitsAdjustmentSignal detection theorySensory Magnitude
28We study visual psychophysics, but there also is auditory psychophysics, somatosensory psychophysics, etc.
29Why are there so many graphs in this course? Because graphs show relationshipsUsual arrangement:Response on Y-axisWhat you are measuring(Dependent Variable)Physical Stimulus on X-axis(Independent Variable)
30of psychophysical measures Two basic typesof psychophysical measuresThreshold measures (Do you see it”)2) Sensory Magnitude measures(“What does it look like”)
31Do you see the light?Physical stimulus – light intensityPerceptual response – Seeing the light
32How far down an eye chart can you read? Physical stimulus – Letter sizePerceptual response – Identifying letters
33letter size is the stimulus identifying letters is responseWe use psychophysical tools to find the threshold – the letter size you can see 50% of the time
34Which is better, 1 or 2?Physical stimulus – Lens powerPerceptual response – Clarity of the image
35Why study psychophysics? Psychophysical measurements are fundamental in clinical practiceNeed to know the scientific basis for measuring visionThe 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
51For us to see, neural signals must leave the retina and travel to central brain structures.
52In 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 cell’s axon to the next location (lateral geniculate nucleus, then to visual cortex)
53Graded 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.
54We 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 cell’s responses
55Action potentials recorded from a single LGN neuron Neural 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.
56This 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)
59Figure 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.
65Figure 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.
70The 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
71The 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.
72Another way to mis-estimate threshold We are looking for the 50% point, not the closest data point, so we use linear interpolation
73We want to measure threshold as accurately as possible We want to measure threshold as accurately as possible. Why be satisfied with “6” when 5.8 is more accurate?
101But if there are two alternatives (two-alternative forced-choice) you know the guessing rate is 0.5
102Results from yesterday’s Method of Constant Stimuli Threshold Measurement 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.
115Stimuli that are near threshold always are difficult to see Stimuli that are near threshold always are difficult to see! Did I see that, or didn’t I?The brain (comprised of neurons) must “decide” if a stimulus was present against a background of neural “noise”.
116Your brain causes perception 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.
117We 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 cell’s responses
118The visual system has to decide if a stimulus is present “on the fly” – as events happen In 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”
120Below 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”
121We 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.
122Making a frequency distribution of neural responses during “noise” and “signal + noise” 50 ms “bins”Stimulus + noise“noise”Number of action potentials in each bin3823During “noise”, 0 spikes occur 1 time, 3 spikes occur 1 time, 2 occur 1 time215During “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
124How 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”
125One 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 grass”This changing threshold is partly responsible for fluctuations in threshold.
126Imagine the situation faced by a mouse, needing to forage for food, but worrying that a snake might be hanging aroundand eat the mouse when the mouse goes out to eat
127Set criterion low, to always detect the snake If 6 or more action potentials, decide “snake!!”
128Set criterion low, to always detect the snake Problem: will also “see” snake some times when it is just the noise of the visual systemIf 6 or more action potentials, decide “snake!!”
133So, try changing the criterion – mouse gets hungrier, willing to 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)
139The distributions on the previous slide would produce this Hit RateReceiver Operating Characteristic(ROC) Curve1.00.80.60.40.20.00.00.184.108.40.206.0False Alarm Rate
140d’ (“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)
143Big 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.
146Skip the text on pages 24, 26, 27 and top of 28 “Signal detection theory can be used to control bias when measuring threshold”
147Screening 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 errorMinimize false positives even though some refractive error is missed
148Detecting 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
149You 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
150As was said the first day of class Visual thresholds are the most common psychophysical measurement“Do you see it?”