Our Sense of Sight: Part 2.
How We Perceive Movement, Depth and Illusions
Experiment: Depth Perception and IllusionsDeveloped by Marjorie A. Murray, Ph.D.; Neuroscience for Kids Staff Writer
TEACHER RESOURCEFEATURING: A "CLASS EXPERIMENT"
PLUS: "TRY YOUR OWN EXPERIMENT"
To view the Teacher Guide and Student Guide, you must have the free Adobe Acrobat Reader.
[Summary] | [Background Concepts] |
[Planning and Teaching Lab Activities] | [References and Suggested Reading] | [Science Education Standards]
| 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."
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
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.
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.
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:
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.
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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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
Here are some good Web sites to visit for visual perception information:
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:
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
1B, 6-8, #1
1B, 6-8, #2
12A, 6-8, #2
12A, 6-8, #3
12C, 3-5, #3
The NEUROSCIENCE CONTENT in the Eye and its Connections activities and Background material will help to meet the following Benchmarks:
5C, 6-8, #1
6A, 6-8, #1
6C, 6-8, #1
6C, 6-8 #6
6C, 9-12, #2
6D, 3-5, #2
6D, 6-8, #3
6D, 9-12, #3
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