Vision

Question 1

1. GY has an injury to the occipital cortex. He can’t see light, but he accurately points to it 99% of the time. Why?

Answer 1

1. In BLINDSIGHT, V1 IS BYPASSED, but other information can still reach V2 and V5 because of the many minor pathways into extrastriate cortex. GY cannot make mental images, but he has some kind of unconscious vision that helps him “see” his surroundings.

Question 2

2. What is the difference between hyperopia and myopia?

Answer 2

2. HYPEROPIA is farsightedness when your eyeball is short. MYOPIA is nearsightedness when your eyeball is long.

Question 3

3. In the retina, what is the order of cell layers in which light hits first

Answer 3

1. PHOTORECEPTOR LAYER, BIPOLAR CELL LAYER, GANGLION CELL LAYER

Question 4

4. In the retina, what is the order of cells in which the light hits first?

Answer 4

1. PHOTORECEPTORS, (HORIZONTAL CELLS), BIPOLAR CELLS, (AMACRINE CELLS), GANGLION CELLS

Question 5

5. Your retina has two types of photoreceptor cells. What are they?

Answer 5

1. RODS and CONES.

Question 6

6. What pigment, what receptive field size, and what is the best setting for each of the photoreceptors?

Answer 6

1. Rods v. Cones
   a. RODS- contain RHODOPSIN, have LARGE RECEPTIVE FIELDS, and work best in DIM LIGHT. They are in the PERIPHERY of the retina and do not distinguish color.
   b. CONES- contain IODOPSIN, have SMALL RECEPTIVE FIELDS, and work best in BRIGHT LIGHT. They are in the central retina (FOVEA) and distinguish color.

Question 7

7. Where do you find the densest collection of cones?

Answer 7

1. The FOVEA- there are no ganglion or bipolar layers in order to allow light to reach cones and rods at the back of the eye. This gives better visual acuity!

Question 8

8. Where do you find the densest collection of rods?

Answer 8

1. The PERIPHERY of the fovea.

Question 9

9. How does each photoreceptor’s connection to ganglion cells reflect the purpose of each photoreceptor?

Answer 9

1. Rods v. Cones
   a. MANY RODS SHARE EACH GANGLION CELL. This makes them even more sensitive to light, because light signals are readily detected by many rods, but reduces their acuity for other things, because they just send information to one ganglion. They need to be the photoreceptors that, in dark settings, can detect motion.
   b. FEWER CONES ATTACH TO EACH GANGLION CELL. Because each cone has its own ganglion cell (in the fovea), the acuity of each cone matters, and visual acuity overall is high. Cones are important for well-lit settings where color identification is necessary.

Question 10

10. When light strikes rhodopsin, what is activated?

Answer 10

1. RETINAL, a vitamin on the protein Opsin. There is a RPE65 conformation change from bent to straight. This is TRANSDUCTION from light to chemical energy.

Question 11

11. If the RPE65 is defective, what disorder do you have? How can this be cured?

Answer 11

1. You have LEBER’S CONGENITAL OPTIC DEGENERATION, in which photoreceptors degenerate and the patient goes blind. STEM CELLS have cured this disease.

Question 12

12. Explain how transduction happens in the rods.

Answer 12

1. LIGHT CLOSES NA+ CHANNELS, HYPERPOLARIZES, and TURNS RODS OFF (Rods are generally firing in the dark!!). When rods (photoreceptors) are turned off, BIPOLAR CELLS can turn on and send a signal to the GANGLION CELLS to depolarize and conduct an action potential.
   a. At high illumination in sunlight, all sodium channels are closed, because the rods are saturated. This shows just how sensitive rods are to light!

Question 13

13. Photons striking a photoreceptor…

Answer 13

1. HYPERPOLARIZE THE CELL

Question 14

14. Neurons in the retina and LGN have two types of receptive fields. What are the differences between them?

Answer 14

1. Receptive Fields are what make the cell fire. The bipolar cell responds with changes in polarization. The ganglion cell responds with frequency of action potentials.
   a. ON-SURROUND/OFF-CENTER: If you stimulate a little in the surround, you are a little excited. If you stimulate a lot in the surround, you are maximally excited. If you stimulate a little in the center, you are a little inhibited. If you stimulate a lot in the center, you are maximally inhibited. If you stimulate both in its entirety, the signals cancel out. Excitation sends signals to the brain.
   b. ON-CENTER/OFF-SURROUND: If you stimulate a little in the center, you are a little excited. If you stimulate a lot in the center, you are maximally excited. If you stimulate a little in the surround, you are a little inhibited. If you stimulate a lot in the surround, you are maximally inhibited. If you stimulate both in its entirety, the signals cancel out. Excitation sends signals to the brain.

Question 15

15. Bipolar cells are connected laterally by what cells? What do these cells do?

Answer 15

15. HORIZONTAL CELLS connect bipolar cells and are responsible for LATERAL INHIBITION, in which neighboring cells are inhibited to produce CONTRAST. This is how we tell things apart from one another. The first ganglion cell on slide 4 fires more than the second ganglion, as if when the first ganglion is active, it shuts off its neighbor. These two cells are in the dark. The third ganglion cell fires like crazy, trying to put its neighbors down. This third ganglion has a harder time inhibiting the first ganglion, which makes the first ganglion in the dark stronger than the second ganglion in the dark (which is being inhibited more strongly). The second ganglion is being inhibited by both the first and third ganglion very directly. You don’t need to know the exact details, but understand that cells LATERALLY INHIBIT THEIR NEIGHBORS, and this produces CONTRAST and the MACH EFFECT. By contrasting light colors with dark colors, the picture becomes more defined.

Question 16

16. The process that acts to enhance the boundaries of visual objects is called…

Answer 16

16. Lateral inhibition

Question 17

17. Now that bipolar cells have sent the signal to ganglion cells, what happens?

Answer 17

17. The GANGLION CELLS fire the FIRST ACTION POTENTIAL IN THE VISUAL SYSTEM (And they are the ONLY cells that can do this. Other cells generate graded potentials)

Question 18

18. Action potentials in the retina come from…

Answer 18

18. Ganglion cells

Question 19

19. Most axons of the retinal ganglion cells terminate in the…

Answer 19

19. Lateral geniculate

Question 20

20. Ganglion cells together form what?

Answer 20

20. The optic nerve!

Question 21

21. The LGN of the thalamus has three cell types. What are they?

Answer 21

21. PARVOCELLULAR, MAGNOCELLULAR, and KONIOCELLULAR

Question 22

22. Explain the M CHANNEL- the cells involved and purpose.

Answer 22

22. The M-CHANNEL or MAGNOCELLULAR PATHWAY from the LGN to V1 (Primary Visual Cortex) uses MAGNOCELLULAR CELLS, found in the two VENTRAL layers, which are large and have large receptive fields.
    a. Magnocellular cells detect MOTION, LOCATION, and brightness contrast. This is the WHERE PATHWAY. They can easily tell if something is moving toward or away from us. Again, these cells are big and you don’t need a lot of them!
    b. Thus, this channel and pathway is important for ANALYSIS OF OBJECT MOTION. This channel is orientation selective, directional sensitive for movement, and there is no color sensitivity, as expected!
    c. M cells  Magnocellular Layers  DORSOLATERAL PARIETOTEMPORAL CORTEX

Question 23

23. Explain the P-IB CHANNEL- the cells involved and purpose.

Answer 23

1. The PARVOCELLULAR-INTERBLOB CHANNEL from the LGN to V1 uses PARVOCELLULAR CELLS, found in the four DORSAL layers, which are small and have small receptive fields
   a. Parvocellular cells are important for distinguishing SMALL FINE DETAIL. The small receptive field acts similarly to a pixel. The smaller, the more detail. This is the WHAT PATHWAY. You need a lot of pixels.
   b. Thus, this channel and pathway is important for ANALYSIS OF OBJECT SHAPE. This channel has high orientation sensitivity, no color sensitivity, and small receptive fields, as expected.
   P cells  Parvocellular Layers  INFERIOR OCCIPTOTEMPORAL CORTEX (V4)

Question 24

24. Explain the BLOB CHANNEL- the cells involved and purpose.

Answer 24

24. The BLOB CHANNEL from the LGN to V1 uses both PARVOCELLULAR and KONIOCELLULAR CELLS. Koniocellular (from Konio which means “dust”) cells are found all throughout the main layers, and they are very small cells.
    a. Koniocellular cells are important for COLOR ANALYSIS
    b. Thus, this channel and pathway is important for ANALYSIS OF OBJECT COLOR
    c. P Cells  Parvocellular and Intralaminar Regions  INFERIOR OCCIPITOTEMPORAL CORTEX (V4)
    d. K Cells: bring low-acuity info to blobs of V1; also go to superior colliculus and extrastriate cortex in the absence of V1

Question 25

25. What cells receive input from ganglion in the LGN?

Answer 25

25. SIMPLE CORTICAL NEURONS (CELLS).

Question 26

26. When a BAR or EDGE of a particular width, orientation, and location fires, what kind of cells fire?

Answer 26

26. SIMPLE CORTICAL CELLS. They are line detectors. They are part of the recognition/location system. They are more responsible to NEW FLASHES OF LIGHT, than to static, steady illumination or diffuse illumination. One orientation and location specific. If it is not that orientation, it will not fire. This is the WHERE PATHWAY. You have thousands of these that feed into another cell.

Question 27

27. Simple cortical visual cells are most likely to be activated by…

Answer 27

27. Bars of a particular width, orientation, and location in the visual field.

Question 28

28. What do thousands of these cells feed into?

Answer 28

28. One COMPLEX CORTICAL CELL. These receive input from simple cells. This is an example of convergence. Cortical cells act just like simple cells except they don’t care about WHERE in the visual field something is coming from. They are more concerned with recognizing external objects. They take input from all. Anytime simple cells fire, complex cells automatically fire.

Question 29

29. What does the Form Vision Spatial Frequency Theory tell us?

Answer 29

29. That visual cortical cells do a FREQUENCY ANALYSIS OF THE LUMINOSITY VARIATIONS in a scene (just like auditory cells!).
    a. Different visual cortical cells have different sensitivities, not just those required to detect edges
    b. As a result, visual cortical cells can detect not just edges but grades of luminosity change
    c. High frequency transitions are not as meaningful or necessary as low frequencies. If you just filter out high frequencies, you can still recognize the image. If you just filter out the low frequencies, you can’t see most of the basic shapes.
    d. Cortical cells respond to light-dark “gratings” containing a specific combination of frequencies.

Question 30

30. Which cortex would you find these cells, where most visual information first arrives?

Answer 30

30. PRIMARY VISUAL CORTEX (V1) in the occipital cortex. This is also called the striate or extrastriate cortex.

Question 31

31. What is the general function of this cortex?

Answer 31

31. Break down image into individual bars of light for shape, color, location, and where it is coming from. It PERCEIVES OBJECTS AND IS NECESSARY IN FORMING MENTAL IMAGES. This is the biggest bulk of work.
    a. This is the FIRST LEVEL OF ANALYSIS
    b. This detects lines of light, is organized by orientation of the lines, and has ocular dominance columns of left versus right eyes.
    c. V2, V4, V5 work on reassembling the whole image.

Question 32

32. How is this cortex organized?

Answer 32

32. In BLOBS, COLUMNS, and OCULAR DOMINANCE COLUMNS. Ocular dominance columns are a region of the cortex with greater synaptic input from one eye. Remember, both eyes are needed for depth perception, and if you have a problem like amblyopia, you don’t have depth perception! There a many colors representing columns in the diagram. If one color dies, the other overlaps. Cool, huh?

Question 33

33. If you lose this cortex, what happens?

Answer 33

33. You can’t see PATTERNS. If someone hits you so hard in the back of your head, you may get occipital blindness. Additionally, the man with blind sight had a damaged V1 and could not make mental images. V2 and V5 were still intact, so information bypassed V1 to get to the other cortices.

Question 34

34. Brain maps of visual space are mostly devoted to what?

Answer 34

34. The fovea

Question 35

35. V1 can project to V2. What is the function of V2?

Answer 35

35. V2 is the second visual cortex. This perceives complex form and is important for “FILLING IN THE GAPS”. Much of vision is extrapolating (predicting) from what is actually “seen”. You saw the triangle without the triangle actually being drawn. You have learned this by experience and can’t stop seeing the triangle. It is hardwired in you to predict what is there. Object permanence can actually be a problem, because it can cause paranoia (you imagining the leaf is a scorpion).

Question 36

36. Illusory contours are responded to by cells located in…

Answer 36

36. V2

Question 37

37. What is the function of V4?

Answer 37

37. V4 also perceives complex form. It responds to CONCENTRIC AND RADIAL STIMULI, ANYTHING ROUND. It is also involved in COLOR perception along with the koniocellular cells, which are spread throughout the thalamus.

Question 38

38. Cortical area V4 has many cells that respond preferentially to….

Answer 38

38. Concentric and radial stimuli

Question 39

39. What is the function of V5?

Answer 39

39. MOTION PERCEPTION. While in the retinal periphery, you have rods that are sensitive to motion (remember?), in the cortex, V5 is important!! Also in V5, as a whole, it comes together and you see what you are seeing.

Question 40

40. What is akinetopsia?

Answer 40

40. Motion blindness. Perception of motion needs to have accurate tagging. It’s a timing thing! The tagging isn’t working. The video showed you can’t see movement. You see things at individual times, but there are a lot of breaks in between.

Question 41

41. After V5, where is information processed? How?

Answer 41

41. In the INFEROTEMPORAL CORTEX. Cells here respond to complex forms including forms we have LEARNED to recognize. Receptive fields here develop THROUGH LEARNING AND EXPERIENCE. This is the HIGHEST AREA OF PROCESSING. This is very plastic!!! This is the WHAT PATHWAY and HOW YOU RECOGNIZE PEOPLE.

Question 42

42. If you have damage to this area, what problem do you have?

Answer 42

42. AGNOSIA and some reading disorders

Question 43

43. What is the trichromatic hypothesis?

Answer 43

43. The TRICHROMATIC HYPOTHESIS says there are three types of cones (small, medium, and large), each responding to a part of the spectrum and its own separate pathway to the brain. In the RETINA, your photopigments have different iodopsin that respond to different light. You actually see small cones absorb blue, medium cones absorb green, and large cones absorb yellow/green to red. Rods are light blue.

Question 44

44. After images are not supported by the trichromatic hypothesis. What hypothesis does it support?

Answer 44

44. The OPPONENT-PROCESS HYPOTHESIS- three color axes have opposed pairs of colors. Cells in the retinal ganglion and thalamic parvocellular layers fire to some wavelengths, and are inhibited by others.
    a. When one member of the color pair is “fatigued”, inhibition of its corresponding pair member is reduced. This increases the relative activity level of the unfatigued pair member, and so its color is perceived.
    b. Two color opponents fight each other. Cells that respond to, say, red, inhibit responses to cells that respond to green. You can only look at red for so long before it gets weaker and weaker. Once you take your eyes away, green cells are able to easily inhibit red cells.

Question 45

45. If you look at the sun, when you look away, what color would you see?

Answer 45

45. Blue (because you looked at yellow for so long) or green (because you looked at red for so long)

Question 46

46. Which theory easily gives an explanation for color blindness?

Answer 46

46. Trichromatic theory. One of the cones is missing. The most common color blindness is red-green color blindness!

Question 47

47. What do you call it when you experience one sensory modality as another?

Answer 47

47. SYNESTHESIA – everyone has it, but 4% of the population has strong cases of it. The visual and frontal cortex may be activated by spoken words, for example, seeing the number 5 as blue.

Question 48

48. What is Alice in Wonderland Syndrome?

Answer 48

48. When MICROPSIA (small vision) and MACROPSIA (big vision) alternates. Occurs in migraine, EBV (“mono”) infection, and commonly in children. There is a problem with V5!!!!

Question 49

49. If you cannot perceive a visual scene as a whole, like a dollar, dinner table, or “forest for the trees”, what do you have?

Answer 49

49. SIMULTAGNOSIA- an inability to attend to more than a very limited area of the visual field despite normal vision. There is a problem with the INFEROTEMPORAL CORTEX, and this is the end of visual analysis.

Question 50

50. When patients see AFTERIMAGES, what do they have? Why?

Answer 50

50. PALINOPSIA- objects in motion have a continual tracer normally, but in this case, V5 is slow. It is failing to tag multiple images correctly. They may be out of order, even! There is bilateral occipital cortex and middle cortex damage.

Question 51

51. If you have normal sensation and speech, but cannot name objects, what do you have?

Answer 51

51. VISUAL AGNOSIA/OPTIC ATAXIA- you can’t name the object based on seeing it. You can’t use visual cues to guide movement to it, either. However, if the object was in your hands, you could tell what it is. In this case, V5 and the angular and inferotemporal cortex are not functioning properly.

Question 52

52. How could you restore plasticity in the adult visual system in amblyopia?

Answer 52

52. Fluoxetine (Prozac) can rewire the brain and re-establish ocular plasticity.

Question 53

53. In a case study (11.4), after treatment for a temporal lobe tumor, hemianopia developed. The person cannot see to his left. His left temporal lobe had to be cut. But then he lost his left visual field instead of his right. What does this mean?

Answer 53

53. The cancer tumor has come back and is taking over the right visual field. He is going to die.

Question 54

54. In a case study (11.1) a man notices he has a dark spot in his vision that disappeared when he covered his right eye. Where is the lesion?

Answer 54

54. In the retina! It had to occur before the optic chiasm. If there was a problem in the cortex, both of the eyes would have been affected.