HAP Senses Lab Explanations

Peripheral Vision

Your retina - the light-sensitive lining at the back of your eye - is packed with light-receiving cells called rods and cones. Only the cones are sensitive to color. These cells are clustered mainly in the central region of the retina.

When you see something out of the corner of your eye, its image focuses on the periphery of your retina, where there are few cones. Thus, it isn't surprising that you can't distinguish the color of something you see out of the corner of your eye.

The rods are more evenly spread across the retina, but they also become less densely packed toward the outer regions of the retina. Because there are fewer rods, you have a limited ability to resolve the shapes of objects at the periphery of your vision.

In the center of your field of view is a region in which the cones are packed tightly together. This region is called the fovea. This region, which is surprisingly small, gives you the sharpest view of an object. The fraction of your eye covered by the fovea is about the same as the fraction of the night sky covered by the moon.

You can demonstrate this effect more simply by focusing on one of the words on this page while at the same time trying to make out other words to the right or left. You may be able to make out a word or two, depending on how far the page is from your eyes. But the area that you can see clearly is the area imaged on the fovea of your eye.

Generally, you are not aware of the limitations of your peripheral vision. You think that you have a clear view of the world because your eyes are always in motion. Wherever you look, you see a sharp, clear image.

Interestingly, your peripheral vision is very sensitive to motion - a characteristic that probably had strong adaptive value during the earlier stages of human evolution.

Persistence of Vision

Your eye and brain retain a visual impression for about 1/30th of a second. (The exact time depends on the brightness of the image.) This ability to retain an image is known as persistence of vision. As you swing the tube from side to side, the eye is presented with a succession of narrow, slit-shaped images. When you move the tube fast enough, your brain retains the images long enough to build up a complete image of your surroundings.

Persistence of vision accounts for our failure to notice that a motion picture screen is dark about half the time, and that a television image is just one bright, fast, little dot sweeping the screen. Motion pictures show one new frame every 1/24th of a second. Each frame is shown three times during this period. The eye retains the image of each frame long enough to give us the illusion of smooth motion.

Cheshire Cat

Normally, your two eyes see very slightly different pictures of the world around you. Your brain analyzes these two pictures and then combines them to create a single, three-dimensional image.

In this Snack, the mirror lets your eyes see two very different views. One eye looks straight ahead at another person, while the other eye looks at the white wall or screen and your moving hand. Your brain tries to put together a picture that makes sense by selecting bits and pieces from both views.

Your brain is very sensitive to changes and motion. Since the other person is sitting very still, your brain emphasizes the information coming from the moving hand, and parts of the person's face disappear. No one knows how or why parts of the face sometimes remain, but the eyes and the mouth seem to be the last features to disappear.

Seeing Invisible Shadows

The network is the pattern of arteries and veins that supplies blood to your retina. It spreads out from the dark blob of your blind spot.

In human eyes, the blood supply of the retina is in front of the retina. That is, light passes through the blood supply on its way to the retinal detectors. You do not see the retinal blood supply because it never changes, and your eye ignores unchanging images.

The point source of light casts a shadow of the retinal blood supply on your retina. When you move the point of light from side to side, the shadow moves. You can then see the changing shadow.

Depth Perception

One of the clues that your brain uses to judge distance and depth is the very slight difference between what your left eye sees and what your right eye sees. Your brain combines these two views to make a three-dimensional picture of the world.

Taste and Smell

There are only four different types of true tastes - sour, sweet, salt and bitter. Each of these types of receptors bind to a specific structure of a "taste" molecule. Sweet receptors recognize hydroxyl groups (OH) in sugars, sour receptors respond to acids (H+), the metal ions in salts (such as the Na+ in table salt. Alkaloids trigger the bitter receptors - alkaloids are nitrogen containing bases with complex ring structures which have significant physiological activity. Some examples of alkaloids are nicotine, quinine, morphine, strychnine, and reserpine. Many poisons are alkaloids, and the presence of receptors for the bitter taste at the back of the tongue may help to trigger the vomiting response.

Approximately 80-90% of what we perceive as "taste" actually is due to the sense of smell. Just think about how dull food tastes when you have a head cold or a stuffed up nose. At first students may not be able to tell the specific flavor of the candy, just perhaps a sensation of sweetness or sourness. If students are patient, some may notice that as the candy dissolves they can identify the specific taste. This is because some scent molecules volatilize and travel up to the olfactory organ through a "back door" - that is up a passage at the back of the throat and to the nose. Since we can only taste four different true "tastes", it is actually smell that lets us experience the complex, mouth watering flavors we associate with our favorite foods.

Benham's Disk

Different people see different intensities of colors on this spinning disk. Just why people see color here is not fully understood, but the illusion involves color vision cells in your eyes called cones.

There are three types of cones. One is most sensitive to red light, one to green light, and one to blue light. Each type of cone has a different latency time, the time it takes to respond to a color, and a different persistence of response time, the time it keeps responding after the stimulus has been removed. Blue cones, for example, are the slowest to respond (have the longest latency time), and keep responding the longest (have the longest persistence time).

When you gaze at one place on the spinning disk, you are looking at alternating flashes of black and white. When a white flash goes by, all three types of cones respond. But your eyes and brain see the color white only when all three types of cones are responding equally. The fact that some types of cones respond more quickly than others -- and that some types of cones keep responding longer than others -- leads to an imbalance that partly explains why you see colors.

The colors vary across the disk because at different radial positions on the disk the black arcs have different lengths, so that the flashing rate they produce on the retina is also different.

The explanation of the colors produced by Benham's disk is more complicated than the simple explanation outlined above. For example, the short black arcs that are on all Benham's disks must also be thin, or no colors will appear.