Our Sense of Sight: Part 2.

How We Perceive Movement, Depth and Illusions

Experiment: Depth Perception and Illusions

Developed by Marjorie A. Murray, Ph.D.; Neuroscience for Kids Staff Writer

TEACHER RESOURCE

FEATURING: A "CLASS EXPERIMENT"
PLUS: "TRY YOUR OWN EXPERIMENT"

[Teacher Guide] | [Student Guide]

To view the Teacher Guide and Student Guide, you must have the free Adobe Acrobat Reader.

Index
[Summary] | [Background Concepts] |
[Planning and Teaching Lab Activities] | [References and Suggested Reading] | [Science Education Standards]

SUMMARY

Students learn some ways to investigate visual perception, and find out how to plan and conduct their own experiments.

In the "CLASS EXPERIMENT," students discover how depth perception works by testing their ability to perform motor tasks with one eye or two eyes.

In "TRY YOUR OWN EXPERIMENT," students design experiments investigating visual illusions, such as shadow vs. figure, length of lines, or apparent size. They can investigate visual attention with "minimum difference tests."

SUGGESTED TIMES for these activities: 45 minutes for introducing and discussing the activity, 45 minutes for the "Class Experiment;" and 45 minutes for Explore Time and "Try Your Own Experiment."

BACKGROUND CONCEPTS

Our visual systems allow us to resolve fine detail, track a moving object, perceive depth, and see colors. Somehow, all these components of a visual scene merge so that we have one visual experience; for example, when we see a cat playing with a string, we interpret the scene-paws striking the string, details of claws and whiskers, the cat's paw in front of or behind the string, the colors of the cat-as a unitary visual event, even though we can attend to one or the other of these individually. How do our brains deal with such complex visual information? First, we will consider theories on the processing of motion, form, and color; then we will discuss binocular vision and perceiving depth. For a brief review of how visual information travels to the brain, see Figure 1. (For information on the cells and connections of the visual system, see Part 1 of this unit)
FIGURE 1. The visual pathway, shown in a midsaggital section (cut in the middle, front to back) of the eye and brain. Axons from ganglion cells in the retina travel in the optic nerve to the lateral geniculate nucleus of the thalamus. Here they synapse with other neurons, whose axons go to neurons in the visual cortex in the occipital lobe of the brain. (Figure courtesy of the Society for Neuroscience, copyright by Lydia Kibiuk, 1994.

One possibility for how we perceive a visual scene is that a defined series of neurons and their axons handles all information-motion, shape, and color-from a part of the visual field in a hierarchical manner. That is, a defined set of photoreceptors, other retinal cells, lateral geniculate cells, and cortical cells acts as a serial pathway for information from a block of the visual scene. This information arrives at a "master" cell or group of cells, in a visual association area of the cortex, which combines the current information with memories of previous experiences and make sense of it. In this model, pieces of the visual scene are transmitted like sections of a photograph to the brain.

However, evidence does not support this "photographic transmission" or serial pathway theory, but rather a parallel pathways model. This model states that three primary types of information-color, shape, and motion-are individually "pulled out" of the visual scene and sent through three parallel pathways, beginning right in the retina. In this view, one cone receptor receives light from a small area, for example, on the face of a moving calico cat, and signals from this cone go to several intermediate retinal cells and from these to ganglion cells. Each of the ganglion cells responds to only one of the attributes-color, form, or motion-of the cat's face. The ganglion cells forward their signals to lateral geniculate nucleus (LGN) cells in the thalamus that respond uniquely to that one type of information (color, form, or motion), and so on to the primary visual cortex and further cortical areas. At higher areas, the scene is put back together again, as discussed below.

Several types of evidence, both experimental and clinical, have led to the parallel processing theory. In one important type of experiment, researchers recorded electrical responses from cells in specific areas of "higher" visual cortical areas (where signals go from the primary visual cortex). Cells in an area called V5 responded only to something moving in part of the visual field, while cells in V4 responded to color. Second, clinical evidence came from patients who had selectively lost the ability to perceive one type of visual information. For example, with damage to a particular area of the cortex, a person loses all perception of motion, while still being able to identify objects and colors. (For a vivid description of this syndrome, see http://www.hhmi.org/senses ; look under The Strange Symptoms of Blindness to Motion.) Such selective defects are called agnosias: some of these are listed in Table 1.

Table 1 - The Visual Agnosias
TYPE OF AGNOSIASYMPTOMS
Object agnosiaCannot name, use, or recongize real objects
Agnosia for drawingsDoes not recognize drawn objects
ProsopagnosiaCannot recognize faces
Color agnosiaDoes not associate a color with an object
Color anomiaCannot name colors
AchromatopsiaCannot distinguish hues
Visual spatial agnosiaLoss of stereoscopic vision
Movement agnosiaCannot see objects moving
Table modified from Kandel et al. - see References

Putting all the evidence together, researchers now describe a system that processes motion, the where system, and a system that handles shape or form, the what system. The motion or where system information seems to have its final processing station (after leaving the occipital lobe) in the parietal lobe, while the shape or what system input ends up in the inferior temporal lobe. Color information is processed in a pathway that will be considered in Part 3 of this unit. Color information also appears to come to its final sensory analysis in the inferior temporal lobe, but in different cells from those for shape. From these places, messages go out to other cortical areas-to motor systems, for example. When the brain detects something moving, it coordinates bodily movements through the environment or maintains eye pursuit of a moving object.

Finally, note that the parallel processing theory is still that, a theory, and researchers disagree on how much overlap may exist in the system.

Higher cortical areas reassemble the visual puzzle

Although parallel visual processing seems to fragment what we see, at higher levels the puzzle is reassembled. Further, the immediate visual scene is then interpreted in light of what we know from past visual experiences and what the wiring of our brains allows. While our visual experiences usually make sense to us, we are generally unaware of the cues we are using to interpret scenes until we are challenged with unconventional pictures such as illusions. For example, when we see a friend at some distance, we recognize the person and know that this is a normal adult (or child) even though the image on the retina is much smaller than that of a person standing right next to us. We are using cues from the rest of the image, noting, for example, that the trees and buildings also appear small, and using past knowledge to realize that these cues mean distance, not a miniature world. When cues are not present or confused, we can be fooled, as shown in Figure 2.

Figure 2. The perceived size of an object depends on other objects and cues in the scene. The lines on the floor, walls, and ceiling in these drawings tell us that we are looking into the distance, and the square at the back appears far away. The figure of the man is the same in both drawings, but the man in the drawing on the right appears larger. This happens because our brains have learned to interpret the distance cues and to assume that, in real life, the two people would be about the same size. To convince yourself, you can measure the two figures with a ruler.

Illusions are a window on how the brain puts together the different aspects of visual information and integrates it with previous experiences. The illusions presented in the Teacher Guide of this unit illustrate higher visual processing. However, scientists do not yet know all the brain regions and computations involved in interpreting size, shape, and motion, and thus cannot fully explain how illusions fool us.

Perceiving depth depends on both monocular and binocular cues

Along with information on motion, shape, and color, our brains receive input that indicates both depth, the perception that different objects are different distances from us, and the related concept of stereopsis, the solidity of objects. Studies show that people have two ways of judging depth or distance: using monocular (one-eyed) information, and using binocular (two-eyed) data. Monocular cues operate at distances of around 100 feet or greater, where the retinal images seen by both eyes are almost identical. These cues include:

  1. Previous familiarity: If we know the range of sizes of people, cats, or trees, we can judge how far away they are.
  2. Occlusion: If one object partly hides another, we know that the object in front is closer.
  3. Perspective: Parallel lines such as the edges of a road, the intersections of walls and ceilings, and railroad tracks, appear to converge at a distance. The relative distances between objects in a scene with parallel lines are estimated by their positions along the converging lines.
  4. Motion parallax: As we move our heads or bodies, nearby objects appear to move more quickly than distant objects; for example, telephone poles beside the road appear to pass by much more quickly when viewed from a moving car than do buildings or trees hundreds of feet back from the road.
  5. Shadows and light: Patterns of light and dark can give an impression of depth, and bright colors tend to seem closer than dull colors.

Even though these monocular cues provide some depth vision so that the world does not look "flat" to us when we use just one eye, viewing a scene with two eyes-binocular vision-gives most people a more vivid sense of depth and of stereopsis. This is important when viewing objects closer than about 100 feet. Stereoscopic (three-dimensional) vision depends on the fact that the eyes are separated, on average, by about six centimeters, and thus get slightly different views of the same object. This means that when we fixate an object (place its image on the fovea), we can tell if another object is in front of or behind it by the difference in location of the second object's image on the two retinas. This difference in retinal position is called retinal disparity (Figure 3.) and is essential for stereoscopic vision. Experiments have shown that depth perception occurs at the level of the primary visual cortex or perhaps higher (association cortex), where individual neurons receiving input from the two retinas fire specifically when retinal disparity exists. Although scientists have located such neurons and know they are important for merging or fusing the images from the two eyes, they cannot yet fully explain how the brain accomplishes this.

Figure 3. Schematic drawing looking down onto the head of someone viewing two objects, one indicated by the letter P and the other by the large asterisk. The person is looking directly at the object at P, so its image falls onto the fovea (f in the diagram). When the person notices the other object but does not shift the direction of gaze, the image of the other object falls on different parts of the two retinas as indicated by the small asterisks on the retinas. This retinal disparity information is used by the brain to interpret depth and help produce stereoscopic vision.

We first scan a scene, and then we attend to individual features

Finally, what we see depends on what we pay attention to. Sometimes the nature of the visual scene is such that one object or feature "pops out" at us because of distinctive boundaries. If the elements of a scene are not different enough from each other, no one part gets our attention over another. Vision scientists describe two sequential processes that direct our attention: the first is a rapid scanning system that tells us if a simple property is different in one or more parts of a scene. After the initial scan, we direct our attention to individual features of the scene-color, shape, orientation, size-and compare each part with others to detect subtle differences, or we compare each with a previously known version of the scene or object. Our ability to recognize any differences in what appear initially to be two identical objects, or sets of objects, depends on several factors, such as the degree of differences and previous familiarity with the objects.

One illustration of how we can miss differences in scenes when we first rapidly scan them is to make a copy of a photograph of a well-known person and to carefully alter one or two aspects of the picture: make the eyebrows heavier, change the mouth, add bags under the eyes. When the original and the altered copies are viewed side by side and upside down, the changes can be very difficult to identify. If these two pictures are viewed upright, the differences are immediately apparent. Another way to illustrate visual attention is to make a pattern of identical marks or simple objects, such as a sheet of paper with row upon row of X's, with one row containing one or two Z's. See how long it takes to identify the odd letters or objects. Students can devise tests like these in the experimental part of this unit.

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PLANNING AND TEACHING LAB ACTIVITIES

Provide background information

First, prepare students for lab activities by giving background information according to your teaching practices (e.g., lecture, discussion, handouts, models). Because students have no way of discovering sensory receptors or nerve pathways for themselves, they need some basic anatomical and physiological information. Teachers may choose the degree of detail and the methods of presenting the visual system, based on grade level and time available.

Offer students the chance to create their own experiments

Although students need direction and practice in order to become good (and responsible) laboratory scientists, they also need to learn how to ask and investigate questions they generate themselves. Science classrooms that offer only guided activities with a single "right" answer do not help students learn to formulate questions, think critically, and solve problems. Because students are naturally curious, incorporating student investigations into the classroom is a logical step after they have some

experience with a system. The "Try Your Own Experiment" section of this unit (see the accompanying Teacher and Student Guides) offers students an opportunity to direct some of their own learning after a control system has been established in the "Class Experiment." Because students are personally involved in this type of experience, they tend to remember both the science processes and concepts from these laboratories.

Use "Explore Time" before experimenting

To encourage student participation in planning and conducting experiments, first provide Explore Time or Brainstorming Time. Because of their curiosity, students usually "play" with lab materials first even in a more traditional lab, so taking advantage of this natural behavior is usually successful. Explore Time can occur either before the Class Experiment or before the "Try Your Own Experiment" activity, depending on the nature of the concepts under study.

Explore before the Class Experiment

To use Explore Time before the Class Experiment, set the lab supplies out on a bench before giving instructions for the experiment. Ask the students how these materials, along with the information they have from the lecture and discussion, could be used to investigate how we perceive depth. Give some basic safety precautions, then offer about 10 minutes for investigating the materials. Circulate among students to answer questions and encourage questions. After students gain an interest in the materials and subject, lead the class into the Class Experiment with the Teacher Demonstration and help them to formulate the Lab Question. Wait until this point to hand out the Student Guide and worksheets, so students have a chance to think creatively. (See the accompanying Guides.)

Explore before "Try Your Own Experiment"

To use Explore Time before Try Your Own Experiment, follow the procedure above, adding the new materials for student-generated experiments. Let the students suggest a variety of ideas, then channel their energies to make the lab manageable. For example, when a number of groups come up with similar ideas, help them formulate one lab question so that the groups can compare data. The goal is to encourage students to think and plan independently while providing sufficient limits to keep the classroom focused. The Teacher and Student Guides contain detailed suggestions for conducting good student-generated experiments.

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REFERENCES and SUGGESTED READING

  1. Kandel, E.R., Schwartz, J.H., and Jessell, T.M. (Eds.) (2000). Principles of Neural Science, Fourth ed., New York: McGraw-Hill

  2. McIlwain, J.T. (1996). An Introduction to the Biology of Vision. Cambridge, UK: Cambridge University Press.

  3. Shepherd, G.M. (1994). Neurobiology, Third ed. Oxford: Oxford University Press.

  4. Shepherd, G.M. (1994). Neurobiology, Third ed. Oxford: Oxford University Press. )

Here are some good Web sites to visit for visual perception information:

  1. Optometrists' Network: this site shows you the Magic Eye book type of illusion. Go to Magic Eye 3D.
  2. World of Escher
  3. Sandlot Science (Illusions)
  4. Neuroscience for Kids (Simple Experiments using Vision)
  5. Howard Hughes Medical Institute - see: The Strange Symptoms of Blindness to Motion
  6. Vision Science - click on Motion Integration, then on Stereograms

MEETING SCIENCE EDUCATION STANDARDS

By reaching Project 2061 Benchmarks for Science Literacy, students will also fulfill many of the National Science Education Standards and individual state standards for understanding the content and applying the methods of science. Because the Benchmarks most clearly state what is expected of students, they are used here. The Benchmarks are now on-line at: http://www.project2061.org/tools/benchol/bolframe.htm

The Benchmarks are listed by chapter, grade level, and item number; for instance, 1A, 6-8, #1 indicates Chapter 1, section A, grades 6-8, benchmark 1.

The PROCESS OF INQUIRY used in the Eye and its Connections activities will help students reach the following summarized Benchmarks:

1A, 6-8, #1
When similar investigations give different results, the scientific challenge is to judge whether the differences are trivial or significant, and it often takes further studies to decide.

1B, 6-8, #1
Scientific investigations usually involve the collection of relevant evidence, the use of logical reasoning, and the application of imagination in devising hypotheses and explanations to make sense of the collected evidence.

1B, 6-8, #2
If more than one variable changes at the same time in an experiment, the outcome of the experiment may not be clearly attributable to any one of the variables.

12A, 6-8, #2
Know that hypotheses are valuable, even if they turn out not to be true.

12A, 6-8, #3
Know that often, different explanations can be given for the same evidence, and it is not always possible to tell which one is correct.

12C, 3-5, #3
Keep a notebook that describes observations made, carefully distinguishes actual observations from ideas and speculations about what was observed, and is understandable weeks or months later.

The NEUROSCIENCE CONTENT in the Eye and its Connections activities and Background material will help to meet the following Benchmarks:

5C, 6-8, #1
All living things are composed of cells. Different body tissues and organs are made up of different kinds of cells. The cells in similar tissues and organs in other animals are similar to those in human beings.

6A, 6-8, #1
Like other animals, human beings have body systems for the coordination of body functions.

6C, 6-8, #1
Organs and organ systems are composed of cells and help to provide all cells with basic needs.

6C, 6-8 #6
Interactions among the senses, nerves, and brain make possible the learning that enables human beings to cope with changes in their environment.

6C, 9-12, #2
The nervous system works by electrochemical signals in the nerves and from one nerve to the next.

6D, 3-5, #2
Human beings can use the memory of their past experiences to make judgments about new situations.

6D, 6-8, #3
Human beings can detect a tremendous range of visual and olfactory stimuli.

6D, 9-12, #3
Communication between cells is required to coordinate their diverse activities.

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