STUDENT INVESTIGATION 7.2

An Examination of How Your Eye Contributes to the Body's Need to Balance: The Vestibulo-Ocular Reflex (VOR) and Nystagmus

Background

If you'll recall, all of the various signals generated by the body about its orientation relative to the world around us, are connected and integrated within the brain and central nervous system. Each of our senses provides a unique signal, and when any one signal is eliminated or changed, the brain must adapt to using only those signals that are available. Think of a person who is blind or deaf or even one who is paralyzed with no feeling in their limbs. For such individuals, the brain has certainly had to program itself to analyze and comprehend sensory information differently from those with all of their senses intact. In fact, research has shown that brain structures actually do differ, in both appearance and function, among those with different sensory capabilities. In this exercise, we are going to examine how our visual system serves our balance system.

Figure 12. (a) The electromagnetic spectrum is shown in units of Angstrom (1 Å = 10-10 meter). (b) The spectrum of visible light comprises a very small range of wavelengths shown in units of nanometers (1nm = 10-9 meter).

Sight has two main functions in its interaction with the nervous system: to receive and transform images into cognitive signals for the brain to analyze, reflect upon, and then act on; and to receive and transform images into spatial orientation signals for the brain to analyze and immediately act on without reflection. In either case, the eyes receive energy from the electromagnetic spectrum (Figure 12) and transform that energy into nerve impulses that lead to the brain. Only a limited part of this spectrum can excite the photoreceptors (rods and cones in the eye that receive light) in the retina of the eye. Electromagnetic energy with wavelengths between 400 and 700 nanometers (nary) comprise visible light. It is in their role of providing spatial information signals that the eyes work so closely with our other senses to complete "the picture" of how our bodies are physically oriented. We will discuss this in a moment, but let's first look at the structure of the eyeball.

Figure 13. The internal anatomy of the eye.

Figure 13 illustrates the major parts of the eyeball. The outermost layer of the eye is a tough coat of connective tissue called the sclera. This can be seen externally as the white of the eyes. The tissue of the sclera is continuous with the transparent cornea. Light passes through the cornea to enter the anterior chamber of the eye. Light then passes through an opening called the pupil, within a colored muscle called the iris. The iris regulates the diameter of the pupil and, therefore, the amount of light entering the eye. Light that passes through the pupil enters the lens. The lens is responsible for refracting (deflecting or bending) light and focusing the image that we receive. The retina, which is the inner layer of the back portion of the eyeball, is considered the neural layer (neural refers to neurons and the nervous system) and it contains photoreceptors that are activated by light. Neurons in the retina contribute fibers that are gathered together in a region called the optic disc to exit the retina as the optic nerve. This is also the site of entry and exit of blood vessels.

The role of the visual signal is very powerful in our balance system. In fact, the visual and vestibular signals function particularly close together to create our vestibulo-ocular (ocular refers to the eye) reflex (VOR). This reflex is an effort of the semicircular canal to compensate for the changing visual field that arises when we move our head. Let's consider how the visual and vestibular systems work together to help maintain balance and how a combination of sensory information provides confirmation of your movement.

Visual cues normally work to confirm the orientation information provided by the vestibular, proprioceptive, tactile, and motor signals. For example, tilt your head toward your right shoulder. The following events will occur simultaneously:

  1. The image of the stationary (not moving) visual field on your retina rotates as though your external visual field were moved counter-clockwise, your semicircular canals signal a clockwise movement of your head, and together they come up with an estimate of the change in head angle.
  2. In addition, your eyes roll opposite to the direction of tilt to stabilize the changing visual field. This is the obvious part of the VOR called ocular counterrolling.
  3. Your otolith signals register a new head orientation with respect to the vertical force of gravity, and this helps you confirm your semicircular canal and visual cues. Of course, in space, this otolith signal is not activated.
  4. And if all of that wasn't enough, your neck joint angle receptors and probably your neck muscle sensors also confirm the new head position.
These events illustrate a clear case of confirming visual and vestibular signals, leading to a strong perception of spatial orientation. As you know, spatial orientation is affected by the absence of gravity. We'll learn more about that subject when we review the results of Dr. Young's space flight investigation. For now, we are about to begin our own investigation. For this Student Investigation, we are going to participate in two very simple activities: Part A is a demonstration that allows you to experience the power of the VOR, and Part B is designed to help clarify your understanding of ocular counterrolling. Each of the two exercises should be carried out independently by each student so that everyone will have a personal experience with these two examples of different vestibular and visual signal interactions. READ ALL OF THE DIRECTIONS AND QUESTIONS BEFORE BEGINNING YOUR ACTIVITIES. This is important so that you can develop your hypotheses before you begin the exercises and so that you will know what questions to consider as you perform each activity. Let's get started!

Materials

Part 1: A watch with a second hand or a stopwatch for each group of two students.

Part 2: A small mirror for each student.

Procedure

PART 1

Your Vestibulo-Ocular Reflex (VOR)

  1. For this demonstration, each student will individually perform two kinds of movements (A and B) that might seem very similar, but that are really very different. Teams of two students should work together so that each can make sure the other is carrying out the movements correctly and can time the movements of his or her partner.
    Figure 14.

    1. For the first movement, you will hold your hand in front of your face and look at it from the side with your thumb about 12 to 18 inches in front of the tip of your nose (Figure 14). Then you will begin moving your hand back and forth, to an angle of about 10° on either side of the nose, in front of your stationary head. Increase the back-and-forth speed of your hand until the image of your hand blurs. The blurring occurs as the image of your hand begins to slip on the retina. (This is a hint that may help you answer some of the questions!)
    2. For the second movement, you will be moving your head back and forth in front of your stationary hand, increasing the speed until your hand blurs again.
  2. For each of the two situations, A and B. you will determine the frequency of movement of either your hand or your head at the point that blurring occurs. The frequency can be determined by counting the number of complete back and forth movements over a 5 second period and multiplying that number by 12 to obtain a frequency per minute.
  3. Develop a hypothesis before you begin your demonstration regarding which movement ends in a blur at a lower frequency than the other, the hand movement or the head movement, and why.
  4. Begin your experiment with the help of your lab partner, who will time the 5 second period necessary to determine movement frequency at the point of blurred vision. During this 5 second period for each movement, count the number of complete back-and-forth movements for both A and B. Then calculate the frequencies and compare them. Answer the following questions in complete sentences.

Questions

  1. How were the two movements different?
  2. Why were you able to focus on your hand longer during one movement than the other?
  3. What were the primary sensory signals that were used in each movement?
  4. Hypothesize about what you would expect if you carried out this experiment in space and why.
  5. Combine the answers to Questions #2 and #3 to state what you have learned about the vestibulo-ocular reflex.

PART 2

Recognizing the Effect of Ocular Counterrolling

  1. For this demonstration, each student will perform two kinds of movements (A and B) that, as in the previous exercise, might seem very similar but that use very different vestibular and visual signals. (That's a hint for you to be able to answer one of the questions at the end.) Teams of two students should work together so that one student can verify that the other student is carrying out the movements correctly and can record the other student's observations. Read the following descriptions before you begin this exercise.
    1. For the first movement, you will hold a mirror in front of your face so that you can see your eyes very clearly. You will then begin to slowly tilt your head toward your right shoulder. While tilting, carefully observe your eyeball. You will be looki ng for any kind of movement. If you're unable to detect eye movement, tilt your head a little bit faster, but not too much for you not to see the eyes. Verbally describe your eye movement to your lab partner so that your observations can be recorded.
    2. For the second movement, you will again hold a mirror in front of your face so that you can see your eyes clearly. This time, however, you will be lying on your back. You will begin to slowly tilt your head toward your right shoulder again while observing your eyes. Verbally describe your eye movement to your lab partner so that your observations can be recorded.
  2. Before you begin any part of this exercise, develop an hypothesis about what you expect the eye to do in each movement situation and why. Then begin your experiment based on your teacher's instructions. Compare any differences in eye movement that you observe. Finally, answer the following questions.

Questions

  1. Why was there a difference in eye movement when you tilted your head while in a vertical position versus when you tilted your head while in a horizontal position? (The hint is in the question!)
  2. What are the rotational axes (pitch, yaw, or roll) that you participate in for each of the two different movements, and which sensory organs are at work to detect movements in each of the two instances?
  3. What is ocular counterrolling and why does it occur?
  4. What does this demonstration indicate about the vestibulo-ocular reflex?
  5. Hypothesize about what you would expect if you carried out this experiment in space and why.
  6. How is this demonstration in Part 2 similar to and different from the demonstration in Part 1 of this Student Investigation?
Let's move on to examine the results of Dr. Young's space flight investigation!

Prev: Student Investigation 1   |   Up: Table of Contents   |   Next: Space Flight Investigation