A Little Bit More
Placement of the eyes in the head is directly linked to an animal’s “lifestyle”. Eye position varies from an owl’s or monkey’s frontal view to that of a cow or sheep, which is to the side. At one extreme, an American Woodcock (a kind of bird) has eyes that are placed so far to the side that they probably can see a bit behind their head too. The binocular vision (overlapping of the visual fields of both eyes) of owls and hawks is especially important for depth perception in capturing prey. Birds and mammals that capture other animals for food tend to have greater binocular vision. Many birds and mammals however, have laterally placed (side placed) eyes on their head that provide limited binocular vision, but much greater wide-angled vision. Wide-angled vision, means less depth perception, but does give them a much greater field of vision for finding food and seeing enemies; it’s a tradeoff.
Although the eyes of most birds have very limited movement, birds make up for their lack of eye movement with much greater neck mobility. The owls, capable of no eye movement at all, can rotate their heads through an arc of about 270°! But no, they can’t turn their heads all the way round! So, that robin that turns its head as it hops along your yard isn’t listening but by turning its head sideways, is simply making it easier to see the worms with vision that is not very good at looking straight ahead.
Activities
How Well Can I See?
Objectives: Better Understanding of Binocular Vision
Materials: Chalk and blackboard
How does binocular vision work for you? Each of your eyes sees the world from a slightly different position (one is on the left side of your face the other on the right side). Your brain combines these two pictures in a single image that provides information about their color, distance from you and their shape. If you’ve ever seen a 3-D viewer you may know that these pictures are taken with two cameras sitting side by side (like your eyes). You look at the two pictures, one with the left and one with the right eye, and “blend” them just as though you were looking at the real thing, thus providing binocular vision with a photograph!
Explore your own visual abilities!
You may want to try doing this with yourself first, so you know what to do, but students will want to try this exercise for themselves.
Visual Field
- Have a demonstrator face a blackboard at more than an arm’s length away. Draw a circle on the blackboard that represents their head from a bird’s-eye view. (you may want to draw a “nose” on the circle to help people orient – see illustration of overhead view on “A Little Bit More”)
- Looking straight ahead at a point on the blackboard (no fair moving your head or eyes), with arms straight out (maybe even a little back) from their sides, have them wiggle their fingers. Now, while still wiggling their fingers, have them move both arms slowly in an arc toward the front – toward the blackboard.
- Instruct the demonstrator to STOP moving his/her arms forward the moment they can see movement (their fingers) on both the left side and right side. The demonstrator may need to move their arms back and forth a wee bit to establish the exact point where they can just begin to see the wiggling fingers. Demonstrators may find that they see movement on one side sooner than on the other – that’s fine.
- As soon as they see a wiggling finger on the left side and right side, have them hold each arm still. Now have someone with chalk draw on the blackboard the angle formed by both their left and right arms using their head as the center (you’ll need to use a bit of judgement on this).
- Once they’ve found at what angle both the right and left side fingers can be seen, they’ve delineated their visual field (entire area of vision without head movement).
- Binocular Vision:
- To find out their binocular vision, have the demonstrator stand facing the blackboard again. (keep the bird’s-eye view illustration on the board for further use)
- Now have each demonstrator close the left eye (you may need an eye patch to make this work). Extending both the right and left arms, again, keep head and eyes straight ahead, and determine where their field of view is for the right eye only by watching for finger movement on each hand. Have a helper draw on the blackboard a line from the demonstrator’s head at the same angle as both the left and right arm.
- Now have the demonstrator close their right eye and determine their field of view for their left eye. Again, have a helper draw the line on the blackboard.
- Where these two fields overlap is the area in which you have binocular vision.
Now that you’ve determined your visual overlap or where you have binocular vision, try these.
- Try eating your lunch with a patch taped over one eye – you’ll probably find it more awkward than difficult.
- Or, with one eye covered, have a partner stand in front and move two objects toward and/or away from you while you try to decide when they are both the exact same distance from your face.
- Also, try threading a needle with one eye closed – now that’s really difficult! You’ll begin to see just how important having two eyes, with binocular vision, can be.
If you’d like to have a discussion with the class, ask them what happens if someone has sight in only one eye? What kinds of everyday things might be more difficult or impossible?
Driving a car
Parking a car
Throwing a ball
Catching a ball
Estimating distances
Key Concepts
Behavior and Regulation, Predator/Prey Relationships, Structures and Functions
Questions
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