Chapter 3: Introduction to Vision



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CHAPTER 3: INTRODUCTION TO VISION

1. A Day Without Sight

2. The Human Eye

3. Filling In

4. Types of Cones

5. Cross Section of the Retina

6. Visual Path Within the Eyeball

7. Receptor Wiring and Sensitivity

8. Receptor Wiring and Acuity

9. Lateral Inhibition

10. Lateral Inhibition in the Hermann Grid

11. Receptive Fields of Retinal Ganglion Cells

12. Intensity and Brightness in Mach Bands

13. Vasarely Illusion

14. Pyramid Illusion

15. Simultaneous Contrast

16. Simultaneous Contrast: Dynamic

17. Simultaneous Contrast 2

18. White’s Illusion

19. Craik-Obrien-Cornsweet Effect

20. Criss-Cross Illusion

21. Haze Illusion

22. Knill and Kersten’s Illusion

23. Koffka Ring

24. Corrugated Plaid

25. Snake Illusion

26. Hermann Grid, Curving
1. ABC Video: “A Day Without Sight”

In this news feature, Diane Sawyer notes that the number of people in the United States who have lost their sight is projected to double in the next 15 years. She emphasizes that sight loss can strike at any time and at a much earlier age than most would expect. Sawyer interviews people who lost their sight as adults due to diseases such as optic neuropathy, macular degeneration, glaucoma, and cataracts and attempts to put the viewer “in their shoes”. We hear about their personal experiences adapting to life with low vision. We see example images of the world simulated to appear as it would to patients at different stages of macular degeneration and glaucoma and learn about how blind patients adapt their daily living routines.


In macular degeneration, the cells in the central portion of the retina (called the macula) are affected. The macula is critical for central vision, which we rely upon to read, drive, see faces, and perceive fine detail in objects. Macular degeneration results in blurriness and blind spots in central vision. Both macular degeneration and glaucoma can lead to complete vision loss in their advanced stages.
RESULTS & DISCUSSION
1. According to the president of the Lighthouse for the Blind and Visually Impaired organization, what are two major causes of blindness in the baby boomer generation?

2. According to this video, at what age can blindness strike? What percentage of the blind is born blind?

3. Describe how macular degeneration and glaucoma affect perception.

4. What advice is offered by the blind people interviewed with regard to offering help to blind people?



2. The Human Eye

This exercise will let you practice labeling the major structures of the human eye. After studying the related material in your text, drag and drop the name of each structure to its appropriate location. If you are wrong, your answer won’t stick.


RESULTS & DISCUSSION
1. Identify the major parts of the eye included in this exercise. Which one is the rather large, light green structure inside the eye? What is its function?

2. Where is the optic nerve located? What is the relationship between it and the blind spot?

3. Where is the fovea located in relation to the lens? In relation to the line of sight (the line determined by extending a line from an object at which a person is looking to the center of the lens)?

3. Filling in

There is a region in each retina called the blind spot. This region contains no photoreceptive cells, so it sends no visual input to the brain. Even though this region is of significant size, you do not perceive a gap in your vision under normal circumstances. One reason for this is that the information that falls within the blind spot of one eye will not fall within the blind spot of the other eye. However, the blind spot is not normally noticed even when only one eye is open, so something else is involved in preventing awareness of the blind spot.


In this demonstration you will observe a phenomenon usually called “filling in”. Be sure to look steadily at the fixation point, but note what happens to the circular region on the lower right. Then restart the demonstration, but this time look directly at the circular region, rather than at the fixation point. (Note that this demonstration illustrates a general filling-in process that can occur even if stimuli are not located on the blind spot. It has been proposed that a similar process of filling in also occurs when stimuli fall on the blind spot.)
RESULTS & DISCUSSION
1. Describe what you observed. Did the circular area change when you kept your fixation on the small dot? Did you observe the same change when you looked directly at the circular area?

2. How can what you observed be applied to explaining why the blind spot is not usually noticed?

3. Why might this be considered an adaptive, positive attribute of the perceptual system?

4. Types of Cones

This exercise shows the absorption spectra (the amount of light absorbed vs. wavelength) for each type of human cone. All of these curves have been set at the same maximum level, for comparison purposes.


Click on a cone to see that cone’s characteristics.
Clicking on the cone with mixed colors will plot all three absorption spectra in the same graph.
RESULTS & DISCUSSION
1. At which wavelength does each cone absorb the most light?
Short-wavelength cone:

Medium-wavelength cone:

Long-wavelength cone:
2. Do the different cones respond to a narrow or broad range of wavelengths?

3. What is the absorbance of each cone to 500-nm light?


Short-wavelength cone:

Medium-wavelength cone:

Long-wavelength cone:


5. Cross Section of the Retina

The visual receptors and their associated neurons are located in the retina of the eye. Their arrangement and characteristics are important factors that influence visual perception. As you will see in this cross-section of the retina, there are several different types of cells located in the retina. Each one has a particular relationship to the others and its own function in transmitting visual input.


One interesting thing that is not indicated in this diagram is the relationship of the retinal structures to the incoming light. Although common sense might suggest that the receptor cells should be pointing toward the incoming light; in reality, they are deep in the retina and point toward the back of the eye, away from the incoming light. The light has to penetrate through the outer layers of cells before reaching the receptors, and yet we still see clearly.
Drag and drop each label into its appropriate place in the diagram. If you are wrong, your answer won’t stick.
RESULTS & DISCUSSION
1. Identify the five types of cells shown in the cross-section of the retina. Describe their positions.

2. The receptor cells have three different parts. Name the three parts and identify which one contains the photosensitive pigment.

3. Which cells transmit information from the receptors toward the optic nerve, and which transmit information laterally (side to side)?
Receptors to Optic nerve:
Laterally:

6. Visual Path Within the Eyeball

This illustrates the sequence of events triggered when light enters the eye and activates the visual receptors. Note that the light must pass through several layers of cells before it contacts the receptors. Pay special attention to the number of receptors associated with each bipolar cell, and the number of bipolar cells associated with each ganglion cell.


RESULTS & DISCUSSION
1. What is the sequence of neurons activated from receptor to optic nerve?

2. What kind of cells make up the optic nerve?

3. Does each ganglion cell carry information from a single receptor cell? How might the number of receptors associated with a ganglion cell be important in influencing what is seen?

7. Receptor Wiring and Sensitivity

This exercise illustrates the physical relationship between the ganglion cells and the rods and cones and demonstrates the impact this arrangement has on how sensitive the rods and cones are to light. Click on the lights to illuminate one type of receptor. Click on the numbers under INTENSITY to vary the intensity of the illumination. Note whether a flashing green spot appears on the ganglion cell’s axon, signifying a neural response. Note that the rate of flashing relates to the frequency of the neural response. Note also what level of stimulation at the ganglion level is required for a response to occur, and what must occur for this level to be reached. Repeat for the other receptor. An explanation is available in text or audio format.


RESULTS & DISCUSSION
1. What illumination level was required to cause the ganglion cells connected to (a) the rods and (b) the cones to respond?
Rod illumination for response:
Cone illumination for response:

2. How can the wiring of the receptors explain why rod-based vision is more sensitive than cone-based vision?




8. Receptor Wiring and Acuity

The cones result in much better acuity than the rods. For example, you might have noticed that you can easily read a word in very small print when you look directly at it, but that you struggle to see it clearly if it is in the periphery (to the side) of your visual field. This occurs because when you look directly at a word, its image falls in the fovea, a location in the retina that has only cones, but when a word is in the periphery, its image falls on a retinal area with mostly rods. The difference in the rod and cone photopigments cannot explain this, so what can? The answer involves the neural relationship between the receptors and the ganglion cells. This exercise illustrates this relationship and how it affects acuity.


Begin by noting how the rods and cones are linked to the ganglion cells. Click on the far left light above the rods, and then click on the adjacent light. Both lights will remain on, stimulating two different rods. Repeat with the other lights. For each pair of lights, note whether the information at the level of the ganglion cells indicates that two points of stimulation are perceived, rather than just one. Repeat this procedure with the cones. For an explanation, click on either the SCRIPT or AUDIO buttons.
RESULTS & DISCUSSION
1. Did the rod ganglion cell response indicate that two points had been illuminated when the far left receptor and the far right receptor were illuminated? Explain your conclusion.

2. For the cones, how far apart do two stimulated receptors have to be in order to indicate that two separate points of light were illuminated?

3. If your back is lightly touched by the tines of a fork, you probably will feel a continuous, line-like pattern of pressure. If your cheek is similarly touched, you probably will feel each of the four tines individually. Given what you’ve learned in this exercise, what do you predict might account for the difference in perception at different places on the body?

9. Lateral Inhibition

In lateral inhibition, a neuron’s activity is influenced by the level of activity in adjacent neurons. Specifically, the response of the neuron associated with one receptor can be inhibited when stimulation of another receptor sends inhibition along the lateral connections connecting the neurons. Note how the response of the neuron changes when each light is turned on, and pay attention to the influence that Light B’s intensity has on the firing rate of A.


RESULTS & DISCUSSION
1. How does stimulus intensity relate to lateral inhibition?

2. Why does the firing rate of A decrease when the intensity of B is increased?

3. The lateral plexus is responsible for lateral inhibition in the visual system of the horseshoe crab. The human eye does not have a lateral plexus, but lateral inhibition occurs. What structures in the human retina allow lateral influences on neural activity?

10. Lateral Inhibition in the Hermann Grid

Although the white “alleys” in the Hermann grid are the same along their entire length, dark “spots” are perceived at the intersections. Follow the following procedure to see how this effect can be explained by lateral inhibition.


1. Click on the center receptor (A) to get output of A with no inhibition.

2. Click on surrounding receptors, two at a time.

3. Enter outputs for A below.

4. Repeat for receptor B and enter outputs below.

RESULTS & DISCUSSION
1. Indicate the response of receptor A when
Only A is stimulated ______

A and neighbors L and R are stimulated ______


A and all 4 neighbors are stimulated ______

2. Indicate the response of receptor B when


Only B is stimulated ______

B and neighbors L and R are stimulated ______

B and all 4 neighbors are stimulated ______

3. Why does Cell L inhibit Cell A more than Cell L inhibits Cell B?



11. Receptive Fields of Retinal Ganglion Cells (Note: This is the same as #9 in Chapter 2)

This classic 1972 film (titled “Lateral Inhibition in the Retina”) by vision researcher Colin Blakemore, describes the cells of the retina and then illustrates how responses are recorded from ganglion cells in the cat. This is a good introduction to receptive fields. Note that the way the stimuli are presented on the television screen was state-of-the-art technology at the time.


RESULTS & DISCUSSION
1. What happens when the room lights are tuned on and off?

2. What is the configuration (arrangement of excitatory regions) of the receptive field plotted by Blakemore?

3. What is the effect of presenting light to the surround of the receptive field?

4. How are the center and surround of the ganglion cell receptive field wired up to produce the inhibition that is caused by the inhibitory surround?

5. What percentage of ganglion cells in the cat has off-centers and on-surrounds?

6. What kinds of objects cause the best response in on-center and off-center cells?




12. Intensity and Brightness in Mach Bands

It is important to distinguish between intensity and brightness. Intensity is a purely physical characteristic. Brightness is perceptual. Although brightness usually increases as intensity is increased, brightness does not always correspond to stimulus intensity. This is illustrated by Mach bands, in which thin bands next to borders appear lighter or darker than the rest of the physically identical area.


First click on the INTENSITY button and drag the cursor across the stimulus display to map the physical intensity. Repeat for BRIGHTNESS and note how the two functions differ.
RESULTS & DISCUSSION
1. How does intensity change across the stimulus? How is the brightness function different?

2. Where in the pattern do the differences occur?

3. Describe how the brightness plot is related to your perception of the pattern.

13. Vasarely Illusion

The Vasarely illusion, which occurs in some paintings by optical artist Victor Vasarely, is sometimes also referred to as the Vasarely “nested squares” illusion or Vasarely’s Pyramid.

Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. How would you describe the brightness across each of the individual squares that appear in the movie?

2. Describe the new perception that occurs when the squares have been stacked.

3. What mechanism has been proposed to explain this perception?

14. Pyramid Illusion

In this version of the Vasarely illusion (also called the Pyramid Illusion) clicking on RUN causes squares to be stacked on top of another. After viewing this animation, click STOP and 3 PARTS. Then click on the plus (+) buttons in order to find how many nested squares are necessary for you to perceive an illusory image.


Demonstration courtesy of Michael Bach.
RESULTS & DISCUSSION
1. . How many boxes were required for you to begin perceiving the illusory “X” superimposed on the boxes?

2. How many boxes are required to make the “X” stand out so it is easy to see?

3. Why do you think the illusory “X” becomes stronger when there are more squares?

15. Simultaneous Contrast 1

Simultaneous contrast occurs when two physically identical fields appear different in brightness because they are placed on backgrounds that differ in intensity. Begin by clicking on INTENSITY and sliding the cursor to plot the intensity of the stimulus. Note that the intensities of the inner squares are the same. Then, plot the brightness. Note how the two functions compare, paying particular attention to the regions representing the inner square. Next, change the intensity of the background in the right-hand figure by adjusting the vertical control. Note how the brightness of the inner square changes, even though only the background is being manipulated.


RESULTS & DISCUSSION
1. Describe the appearance of the inner squares in the initial stimulus. How did this compare to their physical characteristics?

2 How did your perception of the inner square change as the background was varied? When did the inner square look brighter than the constant (left) inner square? When did it look darker? What is the general principle that describes simultaneous brightness contrast?

3. Lateral inhibition or neural receptive fields have provided explanations for other brightness illusions. Would either of these concepts provide a possible explanation for simultaneous contrast? Explain your answer.

16. Simultaneous Contrast: Dynamic

This demonstration illustrates how the perceived lightness of a circular disc is affected as it moves back and forth on a changing background. Note the change in lightness of the central circle as the background changes. Then click on WHITE (OR BLACK) and view the circle as it moves across a uniform background.


RESULTS & DISCUSSION
1. Report what you perceived as the disc moved back and forth across the changing background.

2. What does changing the background to black or white illustrate about how the physical intensity of the disc changes (or doesn’t change) as it moves back and forth?



17. Simultaneous Contrast 2

Another demonstration of how the perception of a small field can be changed by different backgrounds.


Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. Describe the appearance of the two small squares when they are first presented alone.

2. How does adding (a) solid and (b) articulated backgrounds change the perception of the squares?

3. How is this effect explained at the end of the movie?

18. White’s Illusion

Simultaneous contrast occurs when our perception of the brightness or color of one area is affected by the presence of an adjacent or surrounding area and is often explained by lateral inhibition in the retina. Lateral inhibition predicts that a gray bar surrounded by a lighter area will look darker, and will appear lighter when surrounded by a darker area. White’s illusion produces the opposite effect.


Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. How did the lightness of the two sets of gray bars appear initially?

2. Place checks in correct spaces below:


Left rectangle Right rectangle

Surrounded by more white







Surrounded by more dark







Should appear lighter according to lateral inhibition explanation








Perceived as darker







3. What two mechanisms are proposed in the video to explain this effect?


4. What is the “belongingness” explanation for this illusion?

19. Craik-O’brien-Cornsweet Effect

In the Craik-O’brien-Cornsweet effect, two identical squares that each change gradually from slightly lighter to slightly darker from left to right are placed side by side. The illusion created can not be easily explained by lateral inhibition. Visual system responds strongly to sudden changes in illumination and weakly to gradual changes.


Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. What do you perceive when the two squares are placed next to one another?

2. Describe your perception of changes of light and dark within the final display.



20. Criss-Cross Illusion

The criss-cross illusion is another demonstration of how perception of the brightness of a small field can be influenced by background fields.


Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. Can the perception of the tilted rectangles be explained by lateral inhibition?

2. What additional explanation is proposed at the end of the movie?



21. Haze Illusion

This illusion causes one of two identical diamond-shaped regions to appear hazy by adding circular patterns at the junctions.


Demonstration courtesy of Edward Adelson.
1. Describe the stimulus conditions that cause first the left and then the right regions to look hazy.

2. What does this demonstration tell us about the relationship between stimulation presented to the retina and perception?



22. Knill and Kersten’s Illusion

Knill and Kersten’s illusion demonstrates that lightness perception is affected by information about the spatial layout of surfaces.


Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. Which display is most likely to be interpreted as two shadowed surfaces?

2. Which display is most likely to be interpreted as a painted surface?

3. Which display results in a greater perceived difference between the left and right fields?

4. What do the results of this lab tell us about the relationship between the physical pattern that is imaged on the retina and perception?



23. Koffka Ring

The Gestalt movement in psychology was started by Max Wertheimer in the early 1900s. With regards to perception, Gestalt psychologists emphasized that the object is perceived as a whole configuration and not merely as the sum of its parts. Kurt Koffka was a Gestalt psychologist who was interested in studying how the elements of a percept combine to form the overall perception of patterns and objects. The Koffka Ring display provides a clear illustration of how the perception of an object can be influenced by the spatial configuration of its individual elements.


Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. How would you describe the gray ring that is initially presented? What kind of background is it initially presented on?

2. What happens to your perception of the lightness and darkness of the gray ring when it is separated into two parts?

3. The explanation at the end of the movie presents the idea that spatial configuration can affect our perception of lightness. Based on your observations, what can you conclude about why the effect of the background is larger in the “split” and “slid” versions than in the other version?

24. The Corrugated Plaid

This demonstration shows that when the pattern is changed to create a different 3-D shape perception, it changes the perceived lightness of two of the squares in the plaid.


Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. How would you describe the two squares as they are presented in the first image?

2. How does your perception of the lightness and darkness of the two squares change when they are a part of the second image (a plaid pattern changed to create the perception of a new 3-D shape)?


3. Explain what inferences the visual system might be making about the source (or direction) of light in the second plaid pattern? How could this potentially explain the percept you reported above?

25. The Snake Illusion

The snake illusion provides another demonstration of how the surrounds can affect perception of the lightness of a smaller, surrounded field. The main point of this demonstration is in the comparison of the “snake” and “antisnake” versions. The key to understanding this illusion is to recognize that (a) the small diamonds are all physically the same and (b) In both snake and anti-snake versions the diamonds are surrounded by the same background. The diamonds on the top are surrounded by a dark background and the ones on the bottom by a light background.


Demonstration courtesy of Edward Adelson.
RESULTS & DISCUSSION
1. How does the lightness of the top and bottom diamonds compare in the snake version?

2. How does the lightness compare in the anti-snake version?

3. According to the explanation at the end of the movie, the larger effect in the snake version is caused by the way X-junctions created a perception of transparency or shadow in the snake version. If perception of transparency or shadow can influence perception of lightness, what does this say about the relation between stimulation on the retina and perception? About higher-order processes and perception?


26. Hermann Grid, Curving

Although the white “alleys” in the Hermann grid are the same along their entire length, dark “spots” are perceived at the intersections. Notice how changing the straight lines to curved lines affects perception of the dark spots that appear at the intersections in the straight-line version.


Demonstration courtesy of Michael Bach.
RESULTS & DISCUSSION
1. Describe how your perception of lightness at the intersections changed when the lines became curved.

2. What does this say about the mechanism responsible for creating the spots?





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