Perform the following experiments on your sensory system. For some of the experiments you will have to work in pairs. Answer all of the questions in italics.

1. Vision: The retina is a thin layer of neural tissue lining the back of the eyeball; this tissue contains the receptors for vision that are called "rods" and "cones".

A. The Blind Spot: Part of the inner back surface of the eye is not entirely covered by the retina. There is no retina where the optic nerve connects with the eye; this area is called the papilla. The part of your field of vision projected onto the papilla is called the blind spot because you receive no visual information from this spot.

Blind spot exercises: Hold this paper about 25 cm (about 1 ft.) away from your face. Cover your left eye and focus on the "X" in the top panel with your right eye.

What happens to the black dot in the top panel and why?

Now repeat this procedure while looking at the "X" in the bottom panel.

What do you see to the right of the "X" in the bottom panel?

How can you see something in what you know is your blind spot?

B. Afterimages Exercise: A change in sensitivity limited to a restricted region of the retina is called local adaptation. This can result in the production of afterimages.

Fixate with one eye on the white dot in the center of the geometric pattern on the right for about 30 seconds. Then fixate on the dot in the middle of the circle to the left.

What do you see in the white circle?

How has the sensitivity of your retina changed after staring at the dark geometric shape?

C. More Optical Illusions: In many cases, perception (which occurs in the brain) can be fooled by what the eye sees accurately. Have fun with the following examples.

 

D. Vision Test: Run the Macintosh computer program, "Freiberg Acuity Test"

Describe the visual acuity test and indicate your score:

Describe the contrast test and indicate your score:

E. Color Blindness Test: Due to a problem on the X chromosome, males are more likely to exhibit than females since females have a "backup" X and males don't. About 5% of males have some degree of red-green Color Blindness. On the following test chart, everyone can easily distinguish the orange circle but people with red-green colorblindness will have difficulty identifying the red star.

2. Taste (Gustation) and Smell (Olfaction): There are four quite distinct basic taste sensations: sweet, sour, salty, and bitter. Different regions of the human tongue vary in sensitivity to the four basic qualities. The diagram at left illustrates the areas of the tongue devoted to the four basic test sensations. However, the sense of taste is strongly influenced by the sense of smell. The mouth cavity is in open communication with the nasal cavity, so odor substances can diffuse up to the odor receptors in the nose and give rise to other sensations.

A. Effect of smell on taste: Perform the following experiment to determine if you sense of taste is affected by smell:

1. Unwrap the two hard candies (butterscotch and mint) provided and place them in front of you. 2. Pinch your nose with the clothespin provided. 3. Close your eyes. Have your helper hand you one of the candies without telling you which one it is, and place the candy on your tongue. Let it remain there for a few seconds. 4. Take that candy off your tongue and have your helper give you the other one. 5. Finally, try them both at the same time. 6. Take the clothespin off your nose and taste both candies again.

What were the results of your experiment?

Could you tell the two candies apart with the clothespin on your nose? Explain.

What was different when you took the clothespin off?

 

B. Satiation Test:

Time 1 - Soak a tissue with cologne and have the subject take a good sniff and remain near the soaked tissue.

Time 2 - After 5-10 minutes of exposure to the smell, have the subject describe the smell as and then go get some fresh air for a few minutes.

Time 3 - When they come back, ask them to describe the smell again and compare it to their first exposure.

Although no more cologne has been added to the tissue, the subject usually indicates that the smell is stronger once again at time 3. Explain.

3. Touch (Sensitivity to heat, cold, pain, pressure and touch)

You have multiple sensors in your skin which respond to pressure, touch (tactile sense), pain, and temperature. However, these receptors are at different densities in different locations on your skin. The distance between two tactile stimuli at which the two are just discerned to be separate is known as the "simultaneous spatial threshold".

A. Simultaneous Spatial Threshold:

1. Unbend a paper clip and re-bend it so the tips are close together. Measure the distance. 2. While your partner closes his or her eyes, place the two tips of the paper clips simultaneously on the back of his/her arm. Ask your partner if he or she feels two tips or one. 3. Change the distance between the two tips. Every now and then, touch them with one tip to be sure they aren't just guessing "two" all the time. 4. By doing this procedure repeatedly, you should be able to get a good estimate of the simultaneous spatial threshold of the back of the arm. Measure this distance with a ruler. 5. Repeat using the back of the hand and the palm.

How wide (mm) is the spatial threshold on the back of the arm? The back of the hand? The palm?

Why do you think that these distances vary so much?

What parts of the body might have the smallest simultaneous spatial threshold ?

What parts might be the largest simultaneous spatial threshold?

B. Temperature Distinction: The sensation of temperature is not absolute - it depends upon perceptions and recent experiences. To demonstrate, we will use three basins of water; one with hot water, one with lukewarm water, and one with ice water.

Have the subject place one hand in hot water and the other hand in ice water. After at least 1 minute of immersion, have your partner place one hand in the lukewarm water and describe the temperature. Then have them place the other hand in the same basin and again describe the temperature.

Why is it that although both hands experience the same temperature, the sensation differs?

4. Hearing and Balance:

The sense of hearing depends upon the vibration of sound waves on the thin membrane called the tympanum, which in turn vibrates in turn each of the 3 bones of the middle ear. The last bone activates tiny hairs which send a nervous impulse to the brain.

A. Orientating to sound: First, have a blindfolded subject try to point to a person making a sound without turning their head. Then allow them to turn their head while pointing.

Which is more effective - turning the head or staying still? Why?

B. Orienting to Pitch: Have a blindfolded subject attempt to point to a person who is whistling at a high pitch and one who is whistling at a low pitch.

Which person is easier to locate? Why?

C. Balance: Balance or equilibrium is controlled by the semicircular canals of the inner ear. Each of the three canals is oriented along one of the 3 dimensions of the body (the x, y and z axes), so motion in any direction is felt when the fluid inside the canals triggers tiny hairs on the inside. The brain integrates this information into a sense of balance.

i. Balance and Vision: Have a person stand on both feet without moving while their eyes are closed. Then have them stand on one foot with eyes closed. Finally have them stand on one foot with eyes open and then two feet with eyes open.

Which is easiest? Which is hardest? Why? How does vision work with the sense of balance?

ii. Motion Sickness: Motion sickness is generally caused when the inner ear and the eye disagree about the status of the body. For example, when you are in a smooth riding car and you look out the window, your eye tells you that you are moving rapidly while your inner ear says you are sitting still. In a way, both are true, but the brain can't rationalize these two different experiences and it results in nausea for some people.

Why does staring at a fixed point on the horizon rather than the fast moving terrain sometimes prevent motion sickness?

This eye chart should be read from 20 feet away. 20:20 vision means that at a distance of 20 feet, you can distinguish the letters on this chart as well as the average person can at 20 feet. A person with 20:100 vision is very nearsighted because what they can only see at 20 feet, an average person can see at 100 feet. A person with 30:20 vision has better than average "visual acuity" because they can see at 30 feet what the average person can only see at 20. Sometimes these "acuity ratios" are divided through to give a single number. In that case, a number greater than 1 means above average visual acuity while a 1 is average and a number less than 1 indicates below average visual acuity.