How You See The lens focuses light onto the retina, the inner layer of the eye. Photoreceptors are arranged in a layer in the retina. The photoreceptors convert light energy into nerve impulses that are carried to the brain through the optic nerve. There are two types of photoreceptors: rods and cones. Rods are extremely sensitive to light, but they do not distinguish different colors. They only allow us to see black and white. Cones are less sensitive than rods, but they do respond to different colors, producing color vision. Cones are concentrated in the fovea, the site of sharpest vision.

The impulses assembled by this complicated layer of interconnected cells leave each eye by way of the optic nerve, which carry the impulses to the appropriate regions of the brain. There are no photoreceptors where the optic nerve passes through the back of the eye, producing a blind spot in part of each image sent to the brain. During the processing of the nerve impulses, the brain fills in the holes of the blind spot with information.

If the eye merely took photographs, the images would be no more detailed than the blurry images taken by an inexpensive camera and would be incomplete. The images we actually see of the world, however, are much more detailed, and the reason is the sophisticated way in which the brain processes and interprets visual information.

In Your Notebook Make a flowchart that shows the sequence of how light and nerve impulses travel from the outside environment to the brain.

31.4 Assessment

Review Key Concepts

1. 1. Review What three types of sensations do receptors in the skin respond to?

   2. Predict Do you think that the soles of your feet or the back of your neck has the greater concentration of sensory receptors? Explain.

2. 1. Review What are the five basic tastes detected by taste buds?

   2. Apply Concepts Why can't you taste food when you have a bad cold?

3. 1. Review Which structures in the ear gather information about the position of your body?

   2. Apply Concepts If you spin around for a time, the fluid in your semicircular canals also moves. When you stop suddenly, why do you think you feel like you are still moving?

4. 1. Review Identify the relationship between the cornea, pupil, lens, retina, and optic nerve and the photoreceptors of the eye.

   2. Infer Some people suffer from night blindness. Which type of photoreceptor is likely not functioning correctly? Explain.

WRITE ABOUT SCIENCE

Creative Writing

5. Imagine that you have lost your sense of taste for one day. Write a 3- to 4-paragraph essay describing how the absence of this sense would affect your day.

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