Vision Science 748 Central Visual Mechanisms II

1. 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

2. Class (mostly) – Mon – Fri. 1:00 - 2:50 No class May 7 -11 (ARVO) Exam #1 May 14 (Monday), 9:30 a.m. Worrell Conference Room

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

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

5. Three main purposes of this course: • Learn how visual function is measured (in single cells & whole animals/humans) • Learn basic facts about visual function (what is normal?) • 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!

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

7. Textbook: Norton’s part The Psychophysical Measurement of Visual Function Norton*, Corliss*, Bailey Richmond Products, Inc. 2006 You have what is needed.

8. 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

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

10. 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”

11. 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 • Answer your questions • Ask you questions if you don’t ask me • Both require that you read the material before class

12. 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

13. Equations – must tell what the variables mean 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.

14. 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

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

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

17. 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? Usual arrangement: Response on Y-axis (Dependent Variable) Physical Stimulus on X-axis (Independent Variable)

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

19. 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)

20. 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.

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

22. Two basic types • of psychophysical measures • 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”)

23. 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.

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

25. Neural Example – threshold for detecting a flashed light.

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

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

28. 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 LD50 is also a threshold measurement (the concentration of a drug needed to kill half of a group of animals or people)

29. 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

30. 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?

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

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

40. (again)

43. Action potentials recorded from a single LGN neuron

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

46. 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.

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

49. 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.