Visual Perception
Student Laboratory Kit
Materials Included In Kit
Station 1 Clay, 1 stick Craft sticks, 6 Plastic supports, 6 Rulers, 30 cm, 6 String, 6 m Vision blocker and background templates, 8½" x 11", 3 Station 2 Mirrors, glass, 10 cm x 15 cm, 3
Plastic supports, 6 Station 3 Peripheral Vision Disk templates, 11" x 13", 3 Reading Sight Cards template, 8½" x 11" Station 4 Colorful Afterimages Apple cards, 4 Colorful Afterimages Flag cards, 4
Additional Materials Required
Meter stick* Scissors* Tape (optional)* Station 1 Chairs (optional), 3 Desk or table space at least 2" long, 3 Station 2 Books Stopwatches, 3
Tape, transparent Station 3 Plastic bags, zipper-lock, 5 Station 4 Colored pencils Paper, white Timer or clock with second hand *for Prelab Preparation
Prelab Preparation
Station 1 Each setup for this activity requires a desk or table at least 2 feet long for each student group. Students should sit in a chair or kneel on the floor about a foot away from the table.
- Obtain the vision blocker and background templates and cut along the dotted lines. The solid 5½" x 8" piece is the background and the other piece with the 1" x 2" “window” cut out is the vision blocker (see Figure 11).
{10836_Preparation_Figure_11}
- Divide the clay into 24 large pea-size pieces.
- Cut the string into three 2-m lengths.
- Place one background piece at the far end of the table (at least 2 feet from the near end, but not more than 4 feet), with the short side at the top and bottom. Use two pieces of clay at the bottom corners to hold the background upright (see Figure 12).
{10836_Preparation_Figure_12}
- Place two rulers parallel on the table against the center of the background, ½ inch apart. The metric units should be on the same side of each ruler. Use four pieces of clay, one under each end of each ruler, to keep the rulers in place (see Figure 13).
{10836_Preparation_Figure_13}
- Obtain two craft sticks and two plastic supports. Gently squeeze the legs of one support to open the slot, and insert a craft stick. Make sure the stick is aligned vertically in the slot, not leaning left or right.
- Place the stick with its support against the 15-cm mark on the outside of one of the rulers. The flat side of the stick should face the background (see Figure 14).
{10836_Preparation_Figure_14}
- Tie one end of a 2-m length of string around the foot of the other support. Tie the string tightly and position the knot in the front center of the leg. (Optional) Use a piece of tape to hold the string in place.
- Insert the second craft stick into the support with the string, making sure the stick is aligned vertically.
- Place the stick and support with the string between the two rulers (see Figure 15).
{10836_Preparation_Figure_15}
- Place one vision blocker at the near edge of the table (2 to 4 feet from the background), with the “window” at the bottom. Use two pieces of clay to hold the vision blocker upright (see Figure 6).
{10836_Preparation_Figure_16}
- Thread the string from the support through the window, allowing excess string to hang from the edge of the table (see Figure 16). Pulling on the string should move the stick and support forward between the rulers.
- Repeat steps 4–12 for the remaining two setups.
Station 2
- Photocopy “Running the Mirror Maze” Test A, three each for each student.
- Photocopy “Running the Mirror Maze” Test B, one for each student.
Station 3
- Cut the reading sight cards into five sets of eight cards each. A set is one row across. Place each set of eight individual cards into a separate small zipper-lock plastic bag. (Two extra sets of cards are included.)
- Cut out each peripheral vision disk.
Safety Precautions
Handle the mirror carefully by the edges and keep it away from the edge of the testing surface. Please observe all laboratory safety guidelines. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.
Disposal
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Clay may be placed in the trash according to Flinn Suggested Disposal Method #26a. The materials from each lab should be saved and stored in their original containers for future use.
Lab Hints
- Enough materials are provided in this kit for 24 students working in pairs or for 12 groups of students. Three student groups may work at each station at the same time. Students should rotate through the stations every 12–15 minutes. All four stations of this laboratory activity can reasonably be completed in two 45- to 50-minute class periods. If three activities are completed the first day, students may complete the fourth activity and the data compilation and questions the second day.
- Each activity can easily be done in groups of three. In this case, allow more time at each station. Two activities can be done each day and the questions assigned as homework.
- Prepare copies of the student pages for each student and hand out the day before the lab. Encourage students to read through the entire lab before coming to class to facilitate more efficient use of time at each station.
- This lab station activity kit can be used with life science units on the nervous system or the senses, as well as with a physical science unit on optics.
- A smooth desk or tabletop is necessary for the Station 1 setup to allow the support to slide when the string is pulled down toward the floor. Practice pulling the strings a few times to make sure the supports slide forward easily.
- The tester for Station 1 should pull the string so the test subject cannot judge the distance by how far the string has been pulled.
- The first tester in each group at Station 1 may have a slight advantage as a test subject, having seen the results of the test. The subject should not try to “second guess” the position of the moveable stick.
- Tracing a path while looking in a mirror at Station 2 involves visual-motor feedback that is unusual. The eyes send a message to the brain, indicating which direction the pencil needs to go. The test subject knowing the mirror has reversed the image results in the brain trying to send a message to the hand to go in the opposite direction. The result is confusion (and often much laughter)!
- Students at Station 2 can expect their scores to improve with repeated trials in Test A. The brain and the eyes learn to “cooperate” with the perceived discrepancies in direction. However, in Test B, the confusion is increased when the pencil must change two directions at once. The angled corners change the direction to the right and down, for example.
- Be sure students adhere to the 3-minute time limit at Station 2, especially for the Test B Maze.
- During the Station 3 activity, as the sight card approaches the center of vision, it becomes very difficult for the subject to keep his eyes focused on the zero point. Audible reminders from the tester will help keep the subject from averting his gaze.
- Each student may want a “trial run” for the Station 3 activity before actually recording results. Do not allow more than one trial run, since improvements in peripheral vision are possible with practice.
- A normal eye has a near point (closest distance at which the eye can focus) of approximately 20–25 cm. The peripheral vision disk is within this range. If focusing on the zero mark is difficult for a student, he or she may focus on a point beyond the peripheral vision disk (the index finger of the opposite hand with arm outstretched, for example), as long as the point is aligned with the zero.
- Bright white paper (the same size as the cards or larger) will provide the best results for the Station 4 activity.
- Good illumination, preferably natural light, is ideal for the Station 4 activity.
- Students should maintain focus on the appropriate point-of-focus for each Afterimage card. If their eyes wander they will not be able to see the afterimage.
- Students who are colorblind may find the Station 4 activity difficult to complete.
Teacher Tips
- Depth perception depends on the phenomenon of parallax. This involves viewing an object against a background from two different angles (see the next tip), thus creating a triangulation effect. The binocular effect of two eyes in the front of the head gives the object being viewed shape and depth. Closing one eye leaves only cues from the surroundings as aids in judging distance. Because the background and objects in this experiment give very few cues, students will probably experience a decrease in their ability to judge distance when they close one eye.
- To verify that the image received by each retina is slightly different, have students close one eye and align the index finger of one hand with a distant object. They should then simultaneously open the one eye as they close the other. The finger will appear to move and will no longer be aligned with the object.
- Artists use many techniques, called ambiguous depth cues, to create an illusion of depth. One such famous illusion is the Adelbert Ames’ room. Students may enjoy researching the techniques used to fool the eyes in this distorted room.
- An interesting extension to the Station 2 activity would be to average individual scores for certain groups to see if any differences occur—for example, between right- and left-handed people or between genders.
- The testing done in the Station 3 activity should not be considered for diagnostic purposes, but students may be interested to know that normal range for peripheral vision is 180º when using both eyes. When using one eye, the total visual field from left to right is normally 150º. A good extension to this activity would be to repeat the steps with one eye covered, and again with the other eye covered.
- Loss of peripheral vision is called tunnel vision. Students can research causes of tunnel vision such as glaucoma, retinitis pigmentosa, retinopathy or stroke.
- Research has shown that cell phone usage while driving an automobile severely limits use of peripheral vision, even for a period of time after the driver has stopped using the phone. Have students discuss the need for peripheral vision while driving, and debate the pros and cons of restricting cell phone usage in cars.
- Some animals have wide peripheral vision to better detect predators. Have students research peripheral vision in animals and compare to humans.
- Cone cells in our retina are photoreceptors responsible for color vision. Three types of cones are sensitive to different primary colors of light—red, green or blue. Each image printed on the cards for Station 4 uses the primary color pigments—yellow, cyan, and/or magenta. These primary pigments are complements of the primary colors of light. Magenta is a combination of red and blue, yellow is a combination of red and green, and cyan is a combination of green and blue. When the student focuses on one of the images for a long time, the cone cells where the image is formed on the retina are overworked and stop firing (responding). When the retina is then exposed to a white background, the fatigued cones are temporarily unable to fire. The cones of the complementary color which are not fatigued send messages to the brain, and only the complementary color is seen. For example, when the eye focuses on the cyan color of the apple, the green and blue cones where the image is formed become fatigued. When the eye then shifts to a white background, the brain only receives a red signal. The cone cells are less overworked when viewing the darkest part of a figure (such as the black stripes on the flag). Since these cells are less fatigued than the neighboring cones, the brightest part of the afterimage is seen (white stripes) where the darkest part had been. The afterimage effect usually only lasts a few seconds, at which time the overworked cones have rested enough to begin firing again.
Sample Data
{10836_Data_Table_1}
{10836_Data_Table_2}
Observations
Visual Field: Motion and Reading Mark an “M” for motion and an “R” for reading at the appropriate angles on each side of the diagram.
{10836_Data_Figure_17}
{10836_Data_Table_3}
Answers to Questions
Station 1
- Calculate the average distance between sticks when the left eye was covered; add the measurements for the two trials and then divide the total by two. Record the average in Data Table 1. Do the same for the other two tests.
- In which test was the average distance between the sticks the smallest?
The average distance between the sticks was smallest when both eyes were open. The moveable stick was positioned 4 mm away from the stationary stick.
- How did keeping both eyes open affect how well you could judge distance?
Keeping both eyes open greatly increased the accuracy of judging how close the two sticks were. The distances with one eye covered ranged from 17 to 42 mm difference, with an average difference of 30 mm with the right eye open and 29 mm with the left eye open. With both eyes open, the average distance between the sticks was only 4 mm.
Station 2
- Calculate your score for each trial in the following manner. Count every 10 seconds (round to the nearest 10 seconds) as one point. Count each error as one point. A lower score indicates a better performance than a higher score. See Examples 1 and 2. Record your scores in Data Table 2.
Time 0:55 Errors: 3 Score: 6 + 3 = 9 Example 1 Time 2:04 (124 seconds) Errors: 0 Score: 12 + 0 = 12 Example 2
- Where in the maze did you have the most difficulty following the path for Test A? Give an explanation.
For Test A, the horizontal paths (left or right) were more difficult than the vertical. When looking in a mirror, left and right are reversed, so the signal from the eyes indicates the hand needs to go in one direction, while the brain realizes the hand needs to go in the opposite direction.
- Describe how your score changed from Run 1 to Run 3 for Test A. Why do you think it changed in this way?
The scores improved with each trial. With practice, the brain adjusted to the reversed image and the visual-motor feedback became more coordinated.
- How did changing from Test A to Test B affect your score? Why do you think this happened?
The Test B score was much worse than the score for Test A. For Test B, most changes in direction were difficult. Since the corners were not right angles, a change in two directions was necessary (left and down, for example), resulting in greater confusion with the visual-motor feedback.
- Do you think doing more runs with Test B would result in an improved score? Why or why not?
Scores would most likely improve with practice since they improved in Test A; however, reaching the same score as Test A might take many more trials since Test B is more difficult.
Station 3
- Compare your motion and reading fields of vision (Total Visual Field). Which one is greater?
The total visual field for motion is much greater than the reading field of vision.
- On what area of the retina was the image of the sight card focused when it was first detected? On what area of the retina was the image of the letters focused? Which type of nerve cell is more numerous in each area of the retina?
The sight card was first focused on the periphery of the retina, which has more rods than cones. The letters were focused on the macula, where a greater concentration of cones is found.
- Think of activities or occupations where good peripheral vision would be advantageous. List several and explain why peripheral vision is important in each.
Athletes need good peripheral vision. Basketball players can pass without looking directly at the intended receiver. A baseball pitcher uses peripheral vision to help him pick off a base runner. Driving is an activity where peripheral vision is essential in order to see road signs, pedestrians, and other vehicles to the right or left. Firefighters and law enforcement officers use peripheral vision to alert themselves to danger.
Station 4
- Use Figure 10 to determine which color-sensitive cones were over-stimulated with each colored part of the U.S. Flag image. Which ones were over-stimulated with each colored part of the Apple image?
With the U.S. Flag image, the cones that are sensitive to red and green are overworked from viewing the star field and the green and blue cones are overworked from viewing the cyan stripes. With the Apple image, the cones that are sensitive to green and blue are overworked from viewing the body of the apple and the red and blue cones are overworked from viewing the worm and stem.
- What afterimage color did you see in place of the black stars and stripes on the U.S. Flag image? Why do you think this happened?
A white afterimage was seen in place of the black stars and stripes. The cones were not overworked as much when viewing the black stars and stripes, so all color-sensitive cones were firing, producing white.
References
Afterimage. http://www.psychologie.tu-dresden.de/i1/kaw/diverses%20Material/www.illusionworks.com/html/afterimage.html (Accessed May 2007)
Color Vision. http://faculty.washington.edu/chudler/eyecol.html (Accessed May 2007)
“Out of Sight!” Neuroscience for Kids, http://faculty.Washington.edu/chudler/heurok.html (Accessed April 2007).
Peripheral Vision, http://www.exploratorium.edu/snacks/perpheral_vision.html (Accessed April 2007).
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