Human Senses

This lab will introduce you to some of the sense organs and the brain, which processes messages from the sense organs. Specifically, you will examine the senses of vision, hearing, touch, and taste. (Chapter 46 and 47). You will need your text for this laboratory exercise.

Objectives: upon completion of this lab, students should:

1. Be able to identify and state the function of the structures of the eye and ear.
2. Be able to explain where the highest concentrations of skin touch receptors are found.
3. Be able to identify the major taste areas of the tongue.
4. Be able to identify and state the function of selected parts of the mammalian brain.
5. Be able to describe the experiments that make up this laboratory exercise and explain their significance.

PART 1 - THE EYE

Sclera

Cornea

Retina

Lens

Iris

Pupil

Optic Nerve

Use your text to answer the following questions:

1. What is the function of rods?
2. What is the function of cones?
3. Where are rods and cones found?
4. Which of the structures listed aid in refracting and focusing light?

A. Afterimages

Sometimes, when you stare intently at an object and then look away the brain retains an image of what you were viewing. This retained image is called an afterimage and will soon disappear. The experiment described below will demonstrate color relations of afterimages:

1. There are two afterimage cards, each with two colors - one on each side. Make sure that you have all four colors: red, blue, green and yellow. You will also need a sheet of clean white paper.
2. Place the afterimage card with the red square under a bright light. Stare intently at the red square on the afterimage card for 1 minute. Without looking away, place the index card over the red square. Record the color of the square (afterimage) you see on the white index card in the table below. Repeat this for the other three colors.

AFTERIMAGE EXPERIMENT

Card Color                    Afterimage Color

Red

Blue

Green

Yellow

B. Blind spot

Procedure - Blind Spot of Left Eye
 

1. Hold this page on its side so that the circle and cross above are one foot away from your eyes. Close your RIGHT eye. Stare at the CROSS ONLY with your LEFT eye. Even though you are staring at the cross only, you should be able to see the circle in the same field of vision.
2. Move the page toward you until the circle disappears. This must be done very slowly. Measure the distance from your eye to the page at the point where the circle disappears and record it in the following blank.

Left eye blind spot is ___________ inches.

Repeat the above procedure, but this time close only your LEFT eye, and stare at the CIRCLE with your RIGHT eye. Slowly move the page closer to your eyes until the cross disappears. Measure and record this distance for the blind spot of your right eye.

Right eye blind spot is ______________ inches.

If you continue to move this page toward you (closer than the distance for your blind spot), the cross or circle reappears. Why do you think this is so?

C. Focus

The eye focuses on objects at different distances by changing the shape of the lens. Lens shape is controlled by the ciliary muscles (within the ciliary body) attached to the lens. To focus on a distant object, the lens flattens. To focus on a closer object, the lens becomes more rounded. The elasticity of the lens, then, determines how well the eye can focus. The elasticity of the lens declines with age such that a younger lens can more easily focus on closer objects. The following experiment will determine the "age" of your lenses. If you wear contact lenses, just do the experiment once, with the contacts on. If you wear glasses, do the experiment with the glasses on, and again with the glasses off.

Hold a pencil or teasing needle so that the sharp end is pointed up. Close your LEFT eye and move the pencil/needle from arm's length toward your RIGHT eye. Focus on the point of the pencil/needle as you move it toward you. Move the pencil/needle until the point becomes blurred (out of focus). Measure the distance from the point to your right eye IN CENTIMETERS.

Near point focus distance for right eye = cm

Repeat the above procedure, but this time close your RIGHT eye and move the point toward your LEFT eye, as you focus on the point with the left eye. Again, measure the distance from your left eye to the point when the point becomes out of focus.

Near point focus distance for left eye = cm

The nearest distance at which the pencil/needle point can be clearly seen (i.e. where it first blurs) is called the near point. Use the table of age/near point to determine the age of your left and right eyes.

AGE of EYE (years) 10  20  30  40  50  60

NEAR POINT (cm)    9  10  13 18  50  83

How old is your right eye?

How old is your left eye?

How old is your right eye without glasses?

How old is your left eye without glasses?

D. Superimposition

1. Hold the paper tube up to one eye. Keeping both eyes open, focus on the farthest corner of the room through the tube.
2. Hold your free hand up at arm's length right next to the tube. Continue to focus on the far corner through the tube (keep both eyes open) as you bring your free hand slowly towards you, keeping it right next to the tube.

What happens to the image of your free hand?

The optic chiasma is a region in the brain that integrates visual stimuli and sends it to both sides of the brain. Both eyes normally view almost the same field of view, and because of optic chiasma, both eyes send information about their field of view to both sides of the brain.  If each eye views a different field (as in the above experiment), the information is mixed in the brain. In other words, the two fields of view are superimposed over each other in the brain. This is called stereoscopic vision.

What advantage(s) might there be to having stereoscopic vision, as opposed to monoscopic vision (able to interpret information from only one eye at a time)?

E. Determining Eye Dominance

You are all familiar with preferences for using a particular hand for tasks such as writing, batting, and throwing. Our eyes also exhibit right-left dominance in a similar way.

1. Hold your hands at arm's length, straight out from your eyes. Overlap your fingertips and thumbs such that a triangular shaped opening is formed between your hands
2. With BOTH EYES OPEN, focus on an object 3 to 5 meters away.
3. Without moving your hands, close one eye and then the other. With which eye was the object still visible through the opening? With which eye was the object obscured? The eye to which the object was visible is your dominant eye. If the object remained visible for both eyes, you lack eye dominance (central dominance).

Do you have right, left, or central dominance?

If you demonstrate dominance, is your dominant eye the same as your dominant hand? If not, you are crossed dominant (right hand - left eye, or left hand - right eye).

Eye dominance is important for how we see and react to our world. For example, right-handed hitters in baseball have their right hand in the upper control position when they hit. Similarly, 65% of baseball players are right-eye and right-hand dominant. Only about 17% are crossed dominant (right hand-left eye or left hand-right eye), while another 18% have no eye dominance. Players with no eye dominance see the world from a point halfway between both eyes. Interestingly, the best hitters (as judged by batting average) are either crossed dominant or lack dominance.

How could eye dominance affect one's ability to hit a baseball?

F. Determining Visual Acuity

Visual acuity is the sharpness of a visual image, and is usually measured with a Snellen eye chart. The size of letters on the chart are such that you should be able to see the first line (the letter E) of the chart from 200 feet away. Conversely, line 7 of the chart is large enough so that it should be seen at 20 feet away. Thus, if a person can read line 7 from 20 feet away, their visual acuity is designated as 20/20. Snellen charts typically have letter sizes for assessing acuity of 20/15, which is better than normal acuity, to 20/200, which is very poor acuity.

The eyes of farsighted people focus the image behind their retina. They are referred to as farsighted because they see distant (far) objects clearer than close objects (see the figure below). Conversely, the eyes of nearsighted people focus the image in front of the retina. These people see close objects better than they do distant objects. Both conditions can be corrected with glasses or contact lenses.

1. Stand 20 feet from the Snellen eye chart in your lab classroom (tape on floor).
2. Cover one eye and read the letters that your lab partner points to on the chart. Begin at the top of the chart and work your way down.
3. Note the lowest row of letters that you can read accurately. Record the number printed next to that row: That number is the farthest distance (measured in feet) that a person with normal vision can read the letters in that row. For example, if the number is 40 then the person has 20-40 vision, meaning that the person can see at 20 feet what a person with normal vision can see at 40 feet.
4. Test both eyes. If you wear glasses or contacts, test your eyes with and without your lenses.

What is your visual acuity?

right eye

left eye

Part 2 - THE EAR (Hearing)

The human ear is a large, complex sense organ containing many sensory receptors. The cochlea contains hair cells, which allow us to hear, while the semicircular canals have hair cells that help us maintain balance.

The outline below shows the name, (outer ear, middle ear, or inner ear) of the major parts of the ear. Using your textbook, list their functions.

1. Which of the above structures transmit sound waves?
2. Which contains the receptors for hearing?
3. Which transmits a nerve impulse to the brain?

A. Sound Direction

Humans locate the direction of sound by how fast it is detected by each of the ears.

1. Work in pairs, swapping roles as subject and experimenter. Subject sits in a chair and closes eyes.
2. Experimenter strikes two metal objects together and the subject attempts to point out exactly where the sound came from. The experimenter should strike the metal objects together at the places shown below. Record where the subject indicates the sound comes from in the spaces below.

• Directly below and behind the head:

• Directly behind the head:

• Directly above the head:

• Directly in front of the face:

• To the left side of the head:

• To the right side of the head:

• Did the subject get them all correct? Explain:

B. Balance

The tiny "hair-like" receptors of the semicircular canals function in dynamic balance. They help us maintain our equilibrium when our body is in motion. The semicircular canals contain a fluid which stimulates the hair receptors as it moves over them. When the body moves, the fluid in the semicircular canals also moves, providing sensory information for the brain. This process can be demonstrated in the following experiment.

1. Work in teams of four during this experiment, with one student acting as the subject and the other three surrounding the subject to prevent injury.
2. Take one of the revolving stools to a clear corner of the room as far away from sharp edges, etc. as possible. The subject will sit on the stool with eyes closed and head on one shoulder while the stool is spun at a constant moderate speed for 30 seconds to 1 minute.
3. Stop the stool and allow the subject to stand and attempt to walk away. It is important that all three observers watch the subject carefully to prevent him/her from falling or crashing into any objects.

Which way does the subject go? Explain the direction, based on the function of the semicircular canals:

Part 3 - TOUCH (Skin Receptors)

A. Skin Sensitivity

Although the skin has receptors for a number of senses, including heat/cold, pain, and touch, we will only examine the sense of touch. 

1. Work in pairs, reversing roles so both act as experimenter and subject.
2. Subject sits in a chair with eyes closed. Experimenter holds the points of a pair of fine scissors against the skin in the areas indicated below. Experimenter should vary whether or not one point or two points are touching the skin to keep the subject "honest." Start with the points farther apart and then move them closer together each time you touch the skin in each area until the subject can no longer tell that there are two points (The shorter the distance, the more receptors there are.). Record the shortest distance for a two-point discrimination for each area listed below:

• Forearm

• Back of Neck

• Index Finger

• Back of Hand

Part 4 -THE BRAIN

You have spent some time examining the various sense organs. The last organ you will study today is the one that processes the input from all of these sense organs - the brain. The brain is a wonderfully complex organ, much of which is still not clearly understood. We will only examine some major structures and their primary functions.

Using your text, identify the following structures on the preserved sheep brain and write the functions below.

• Cerebral hemispheres -
•  What are the names of the lobes of the Cerebral hemispheres?
• Corpus callosum -

• Thalamus -
• Hypothalamus -

Hindbrain:

• Cerebellum -
• Pons -
• Medulla oblongata -

1. What two senses are provided by the ear?
2. The negative blotch of color that remained in your field of view in the test with the colored cared is called the ________________________.
3. The small region in your retina with no space for rods and cones is occupied ba a bundle called the ___________________.
4. Which sense organ uses photoreceptors?
5. The structure beneath your brain responsible for the ability to see a "hole" in your hand is the _______________________.
6. The organs we studied today are all part of the _________________ system.
7. Variation in the ability to focus in a population is influenced most by ___________.
8. What structures affect our balance and where are they located?
9. What is the structure in the eye that adjusts to near or far objects?
10. In the touch receptor experiment, where did the highest concentration of touch receptors seem to be?