Vision Science 748 Central Visual Mechanisms II Norton’s Part: Measuring vision; intensity discrimination; acuity; spatial vision; temporal factors Loop: Color; Suprathreshold Liu: Binocular Vision; Depth Perception; Binocular rivalry

Class (mostly) – Mon – Fri. 1:00 - 2:50 No class May (ARVO) Exam #1 May 14 (Monday), 9:30 a.m. Worrell Conference Room

Exam #1 (80 pts) on Norton’s material (quizzes included in the 80 points total) Exam #2 Fri. May 25th (80 pts)

Lab May 4 1 – 3? Or 4? p.m. You will measure your thresholds and plot them

Three main purposes of this course: 1)Learn how visual function is measured (in single cells & whole animals/humans) 2)Learn basic facts about visual function (what is normal?) 3)Relate what you have learned about the neural basis of visual function to measures of vision (Why does the visual system respond as it does?) You’ve been learning neural function - YOU STILL ARE!! What causes visual behavior? NEURONS! Apply what you know about CNS function to what we study now. The answers on exams often should include a description of what neurons are doing to cause the visually-guided behavior!

The neurons in the visual pathway respond to physical stimuli (light) and produce visual function which produces visually-guided behavior

Textbook: Norton’s part The Psychophysical Measurement of Visual Function Norton*, Corliss*, Bailey Richmond Products, Inc You have what is needed.

In Norton’s part: Some or all of 5 Chapters + appendix Ch 1. Principles of Psychophysical Measurement Ch. 9, (5 pages) Chapter 2 – absolute threshold of vision Appendix – Measuring light Ch 3. Intensity Discrimination Ch 5. Spatial Acuity Ch 6. Spatial Vision Ch 7. Temporal Factors in Vision Specific chapter assignments were given earlier

Overview At the beginning of each chapter. Contains a summary of the content of the chapter.

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”

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 (on purpose!) That is why I would prefer to not lecture, but to 1)Answer your questions 2)Ask you questions if you don’t ask me Both require that you read the material before class

Glossary – intended to help you know what terms mean for exam Definitions given in the text – it helps to learn them verbatim Equations – to be a complete answer, must tell what the variables mean

Equations – must tell what the variables mean 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.

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

Graphs – can be confusing What is on the X-axis? (& approx. scale) Physical Stimulus on X-axis (Independent Variable) Usual arrangement:

Graphs – can be confusing What is on the X-axis? (& approx. scale) What is on the Y-axis? (& approx. scale) Response on Y-axis (Dependent Variable) Usual arrangement: Physical Stimulus on X-axis (Independent Variable)

Graphs – can be confusing What 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) Response on Y-axis (Dependent Variable) Usual arrangement:

Graphs What is different in each graph in a “family” of curves?

Chapter 1 Principles of Psychophysical Measurement Measuring visual function in humans occurs in clinical settings & in laboratory settings. Measuring visual function in neurons uses the same tools Applies to neurons as well as whole creatures (animals; humans)

The “Natural Science of the Soul”  Psychophysics (from the Greek psyche [soul] and the Latin physica [natural science]) has been developed as a way to measure the internal sensory and perceptual responses to external stimuli.

We study here visual psychophysics, but there also is auditory psychophysics, somatosensory psychophysics, etc.

Two basic types of psychophysical measures 1)Threshold measures (Do you see it”) Determine the boundary between values that are seen (above threshold) and values that are too small to be seen (below threshold) 2) Sensory Magnitude measures (“What does it look like”)

Relationship between a Stimulus and a Response  The stimulus is always a physical entity that can be measured directly with instruments.  The response can be the number of action potentials produced per second by a neuron (“firing rate”), or it can be a criterion behavior, such as an animal pressing a lever. With humans, it can be a verbal response (“I see it.”)  In all cases, the perception that occurs between the stimulus and response is inferred. –We are not interested in “perception” in this course but in the relationship between the physical stimuli and the response.

Example: Do you see the light? Physical stimulus – light intensity Perceptual response – Seeing the light

Neural Example – threshold for detecting a flashed light.

Example #2: How far down an eye chart can you read? Physical stimulus – Letter size Perceptual response – Identifying letters

 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? How you measure vision changes the results you get!! So, learn the rules for measuring vision.

Could also determine the threshold concentration of an antibody needed to produce a visible reaction on tissue A dose-response curve is a threshold measurement The LD 50 is also a threshold measurement (the concentration of a drug needed to kill half of a group of animals or people)

Psychophysical measurements are fundamental in many laboratory settings Need to know the scientific basis for measuring vision The results you get depend on the way you measure vision – a single neuron or in a whole visual system

Visual thresholds are the most common psychophysical measurement Why are we interested in knowing threshold? It gives useful information. Does the threshold of neurons in V1 match the threshold of the monkey or bird or person? Is the neuronal threshold the reason for the “owner’s” threshold? In a patient with retinal degeneration, which neurons are responsible for loss of vision?

Key in measuring thresholds: Try to keep all dimensions unchanged except the one being measured

There are many possible values of ΔL, But only 1 value (theoretically) for threshold ΔL

(again)

Action potentials recorded from a single LGN neuron

Graded potentials (in the retina, before ganglion cells) Action potentials (“spikes”) – from ganglion cells and from LGN and cortex (and superior colliculus, etc.

Action 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.

Because of variability, threshold isn’t always easy to determine

Assignment for Monday In your own area of research, think of a threshold measurement you have to make. Write a brief description (1 or 2 paragraphs) of how that threshold is measured and which of the three main Threshold Determination Methods is used.

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.

In Class Demo

Rule: Plot straight lines between data points

“Silliest Plotting Error” Plot data points from left to right

“Silliest Plotting Error” Plot data points from left to right

“Most Interesting Curves”

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.

Note that the steeper the slope of the psychometric function, the more accurately defined the threshold is (assuming the x-axis remains the same.)

Other newer ones, like “QUEST”

In Class Demo

Developed during WWII to test bomb detonators

When you correctly identify the side the stimulus is on, the contrast decreases. The first time you are incorrect is a “reversal”. The contrast then is increased until you are “correct.” That is a second “reversal.” Contrast then decreases until you are wrong again, the third reversal, and then increases until you are correct again (4 th reversal). Threshold contrast is the average of the four reversal values. error correct error correct The number of reversals can be small, yet give a good estimate of threshold if the step sizes are adjusted to an optimal size

Frequency with which L T is seen as equal to L

Stimulus value 0, 0% = (0.3 – 0.3)/0.7 *100 Stimulus value 4, 29% = (0.5 – 0.3)/0.7 *100 Stimulus value 10, 100% = (1.0 – 0.3)/0.7 *100 This equation corrects less at higher stimulus values

In Class Demo

Threshold comes out the same either way & it is simpler to use uncorrected with 75% as threshold

5432

5432

With TAFC, usually, don’t apply the correction; just make threshold be at 75%, chance at 50%

For thousands of years, people thought infants couldn’t see more than light and dark. “just a bundle of organs and nerves during the first month” Information from Chapter 9 Sometimes a new or modified method is needed: the Forced-choice Preferential Looking technique (Davida Teller)

The “blooming, buzzing confusion of infancy”

In the 1960’s people began to realize infants could do more than had been thought, like this newborn imitating his father. To learn what infants can see required devising psychophysical techniques that would work with infants.

Two-alternative Forced-choice Preferential Looking (FPL) Children prefer to look at something, over nothing (Fantz) Stimuli of greater complexity are preferred over very simple stimuli In FPL the child is presented with two stimuli. An observer watches the child and must report which side the child looked toward. (Davida Teller and students)

Two-alternative Forced-choice Preferential Looking (FPL) The observer must (is “forced” to) decide that the child looked to one side or the other. The observer’s judgment is recorded and the observer is given feedback (“the side you chose was/was not the side the stimulus was presented on”). When the observer is 100% correct, the child must have looked at the stimulus 100% of the time.

The Acuity Card Procedure

Forced-choice Preferential Looking

Infant’s WILL look! low frequency gratingobserver’s view of infant looking

What do YOU think? Where is the stimulus?Take a guess?

Can the infant see the stripes? YES! NO?

Advantages of Acuity Cards simple apparatusobserver-infant interaction

Two-alternative Forced-choice Preferential Looking (FPL) The observer must (is “forced” to) decide that the child looked to one side or the other. The observer’s judgment is recorded and the observer is given feedback (the side you chose was/was not the side the stimulus was presented on). When the observer is 100% correct, the child must have looked at the stimulus 100% of the time.

Two-alternative Forced-choice Preferential Looking (FPL) As the stimulus is changed so it is closer to threshold, the child (and, therefore, the observer) will make mistakes. When the observer’s responses are 50% correct, the child must not see the stimulus well enough to look at it. Creates a psychometric function with threshold at 75% correct.

New Topic Detecting the response to a visual stimulus against the “noise” of ongoing neural activity

0 occurs 1 time 1 occurs 0 time 2 occurs 1 times 3 occurs 0 times 4 occurs 0 times 5 occurs 1 time 6 occurs 0 times 7 occurs 0 times 8 occurs 0 times 9 occurs 0 times 0 occurs 0 times 1 occurs 0 times 2 occurs 0 times 3 occurs 0 times 4 occurs 0 times 5 occurs 1 time 6 occurs 0 times 7 occurs 0 times 8 occurs 2 times 9 occurs 0 times Number of APs during 50 msec noise “bin” Number of APs during 50 msec signal “bin” This is for 3 rows. Now expand to 30 rows (30 stimulus and noise pairings) 50 spikes/s means 2.5 spikes/50 msec, average over 30 rows (30 stimulus and noise pairings); 200 on the y-axis means 10/50 msec Average, 2.33 spikes per bin Average, 7 spikes per bin

This is for 30 presentation of stimulus and noise

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 grass”

Decide that 6 or more action potentials means “snake”

10 Low threshold: No misses; will always avoid snake, but false alarms will restrict food access

Decide that 10 or more action potentials means “snake”

10 High threshold: No false alarms, so food access is high, but misses mean that the mouse may be eaten

Can calculate hit rate and false alarm rate for ANY criterion

An ROC curve summarizes the Hits and False Alarms for all possible thresholds

False Alarm Rate Hit Rate ROC Curve

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)

In the LGN, changed the detectability of a stimulus by increasing the transfer ratio using bicuculline to block GABAa inhibition

You will hear clinicians talk about the “sensitivity” and “specificity” of diagnostic techniques. Sensitivity is the hit rate Specificity is the absence of false alarms So plot (1 – specificity) on an ROC curve Want a diagnostic tool that has high sensitivity and high specificity

“Do you see it?” Visual thresholds are the most common psychophysical measurement