Activity 2A: Visual Pathways Topic: Vision Part of: Unit: Challenges and Changes: Sensitivity to Vision & Hearing Compromises Lesson: How We See

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How We See Teacher Handout

How We See Student Handout

 

Keywords

visual pathway, eye, function, sight, brain, optic nerve, occipital lobe, visual cortex

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Activity Description

Use the handout "How We See." This handout has an outline of the visual system. A labeled copy of the handout ("How We See: Teacher Handout") is provided for the teacher to use in the lesson. Give each student a copy of "How We See." Using the labeled copy as a guide, trace the path of vision from the object to the visual cortex of the cerebrum (occipital lobes).

Steps:
1. Begin with the object, here displayed as a stick figure of a person. Two stick figures are provided, one for each eye, to avoid confusion in tracing the visual path. Explain to the students that each eye has a different angle on viewing a single object.

2. Light enters the eye through the cornea and iris to the lens. The eye adjusts the amount of light entering by changing the diameter of the pupil. (N.B. Both pupils constrict even when light is shone into just one eye - this is a normal light reflex that optometrists check for during an eye exam).

3. The image of the object is inverted by the lens ( the same thing happens in the lens of a camera, the lens inverts the image on the film ). The ciliary muscle changes the tension on the lens which adjusts its focal length and "projects" (focuses) the image on the retina.

4. An inverted image is now present at the level of the retina. For example, the head of the stick figure is on the right side of the body but is now on the left side of the retina.

5. In the retina, photoreceptors (the rods and cones) take the light energy and transform it into neurochemical signals.

6. The neurochemical signals are transmitted by nerve cells (ganglion cells) through the optic nerve to visual centers in the brain.

7. Half of the nerve fibers from the optic nerve cross over in the optic chiasm to project to the opposite side of the brain. The nerve fibers that cross over arise from the medial (nasal) side of the retina.

8. In the visual projections, the images from both retinae are carried to the occipital lobes of the brain (these are in the back of the skull).

9. Note that the images from the left halves of both retinae project to the left visual cortex. Also, images from the right halves of both retinae project to the right visual cortex. Thus the left brain sees what is on the right half of the body and the right brain sees what is on the left half of the body. If a defect occurs in the visual projec-tions or cortex, the visual field is cut in half and information is lost from only one side of the body (a condition called a homonymous hemianopsia).

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