Stare at an object straight ahead. Can you see anything else out of the “corner of your eye”? Seeing beyond the center of our visual field (everything seen while looking straight ahead) is known as peripheral vision. Explore the range of your own peripheral vision.
- Peripheral vision
- Rods versus cones
The retina, a thin tissue that lines the back of the eye, contains specialized nerve cells, called photoreceptors, that are sensitive to light. The two different photoreceptors are known as rods and cones, so named because of their shapes. Rods are more numerous (over 120 million) and are more concentrated around the periphery (outside edges) of the retina (see Figure 1). They are more sensitive to light than cones and help us see in dim light. After walking into a darkened room, at first it is difficult to see anything because the cones have been active in the brighter light. After several minutes in dimmer light, the eyes “adjust” and objects can be distinguished again. The adjustment occurs because the cones become less active and the rods are stimulated by the lower-level light. Rods are not sensitive to color, however, which is why it is difficult to distinguish colors in a dark room. Rods are very good at detecting motion. A moving object can usually be detected in the peripheral vision before the object can be clearly identified.
Although cone receptors are found throughout the retina, the center of the retina, the macula
, has a much higher density of cones than the periphery. In the center of the macula is the fovea
—a densely packed area of cones with no rods (see Figure 1). Cones are responsible for color vision. Even though the eye has fewer cone receptors (6–7 million) than rod receptors, cones are vital to our central vision and the ability to see fine details. Cones are used primarily when we read. Try focusing on a letter in the middle of a word in a sentence and see how many other words you can read to the right or left without moving your eyes.
When one looks at something directly, the image is focused on the macula. When something is seen out of the corner of the eye, the image is focused on the periphery of the retina, where more rods and fewer cones are found.
The purpose of this activity is to measure the peripheral field of vision. The angle at which motion and details such as color and shape can be detected will be measured using a peripheral vision disk.
Envelope with 8 color sight cards
Envelope with 8 reading sight cards
Peripheral vision disk
- Which type of photoreceptors in the retina is used mainly for peripheral vision?
- Why is it hard to distinguish colors at night?
- Predict the sequence in which the following will be observed by using peripheral vision, from first to last—identifying color, detecting motion, identifying letters or numbers and identifying shape.
Although the materials in this activity are considered nonhazardous, please observe all laboratory safety guidelines.
Part 1. Visual Field: Motion and Reading
Part 2. Visual Field: Motion, Color, Reading and Shape
- Choose one member of the group to be the subject, one to be the tester, and one to be the recorder. These roles will rotate so everyone will have his or her peripheral vision tested. Read the Procedure section completely before beginning the activity.
- The subject obtains the peripheral vision disk and sits at a desk or table.
- The subject holds the vision disk horizontally to his face, placing his nose in the center curve. The disk should be about halfway between the top and the tip of the nose. Use the thumb and forefinger of either hand to hold the disk at the zero point, with the forefinger on the zero (see Figure 2).
- The tester obtains the reading (black and white) sight cards and stands in front of the subject.
- The tester chooses one of the eight reading sight cards without letting the subject see which one.
- The recorder uses the Peripheral Vision Worksheet and a pencil to mark the angle at which the subject can distinguish the object and details (marking on the subject’s worksheet, not the recorder’s). The recorder should sit or stand where she can read the sight card as it moves, as well as the angle markings on the peripheral vision disk.
- The tester holds the lower half of the sight card with letters facing inward against the disk before the largest angle (110°) on the subject’s left side, out of the subject’s sight. The vision disk should intersect the card, with the letters above the disk and the tester’s hand below the disk (see Figure 3).
- The subject focuses on his finger at the zero point, not moving his eyes to the right or left at any time during the test. The tester must watch the subject’s eyes to make sure they stay focused straight ahead. This task may be somewhat difficult for the subject initially, as the eyes may tend to wander, especially as the card becomes easier to distinguish. The subject may try to peek to see if his guesses are correct. (It may help if the tester audibly reminds the subject periodically by saying, “Look straight ahead,” or “Stay focused on your finger.”)
- The tester slowly and steadily moves the card toward the zero point. As soon as the subject detects the card moving into his field of vision, he should say, “Motion.” The tester pauses the motion of the card at this point, allowing the recorder time to mark the angle on the left side of the diagram of the disk on Part 1 of the subject’s Peripheral Vision Worksheet, using the letter “M” for motion.
- The tester continues to move the sight card slowly toward the zero point, reminding the subject to continue gazing straight ahead, until the subject can recognize the two letters and/or numbers on the card. The subject should say the letters/numbers out loud—for example, if the card reads 3T, the subject should say “Three tee.” If correct, the recorder marks this point on the diagram with the letter “R” for reading. If the subject does not correctly read the card, the recorder does not mark on the diagram and the tester continues to move the card toward the zero point, until the card is read correctly by the subject.
- If at any time during the test, the subject averts his eyes from the center gaze and glances toward the card, the tester chooses a different card and places the new card at the same place the original card had reached. The procedure continues with the new card.
- Repeat the entire procedure on the subject’s right side, choosing a different card and marking the angles on the right side of the disk diagram.
- Repeat steps 1–12, assigning a different role to each group member. Continue until each group member has been tested.
- Place the reading sight cards back in the envelope.
- Follow steps 1–9 of Part 1, substituting the eight color sight cards for the black and white reading cards.
- The tester continues to move the sight card slowly toward the zero point until the subject can recognize the color, the letter or number, or the shape on the card. The subject should announce the detail out loud—for example, “Square,” “Three,” or “Blue.” If correct, the tester pauses and the recorder marks this point on the diagram with the letter “C” for color, the letter “R” for reading, and the letter “S” for shape. If the subject does not correctly identify a particular detail, the tester continues to move the card toward the zero point, until the detail is announced correctly by the subject.
- The tester continues to move the sight card toward the zero point until all details of the card—color, reading and shape (not necessarily in that order)—have been correctly identified.
- If at any time during the test, the subject averts his eyes from the center gaze, and glances toward the card, the tester should choose a different card and place the new card at the same place the original card had reached. The procedure continues with the new card.
- Repeat steps 15–18 on the subject’s right side, marking the angles on the right side of the disk diagram for Part 2. Remember to use a different card.
- Repeat steps 15–19, assigning a different role to each group member. Continue until each group member has been tested.
- Place the color sight cards back in the envelope.
- Fill in the data tables for your own results to the nearest 5 degrees. Add the left and right angles for the Total Field of Vision.