Our Sense of Touch
Experiment: Two-Point DiscriminationDeveloped by Marjorie A. Murray, Ph.D.; Neuroscience for Kids Staff Writer
FEATURING: A "CLASS EXPERIMENT"
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[Summary] | [Background Concepts] |
[Planning and Teaching the Lab] | [References] | [Science Education Standards]
|Students learn how to investigate the touch
sensory system and discover how to plan and carry out their own
In the "CLASS EXPERIMENT," students find that the ability to tell that two points rather than just one are pressing on the skin depends on two things: the density of skin sensory receptors and the connections that the sensory nerve cells make in the brain. They learn basic facts about sensory receptors and nerve connections, and use their estimates of receptor density to predict the size of the brain areas devoted to input from different skin regions.
In "TRY YOUR OWN EXPERIMENT," students design their own experiments, investigating, for example, how touch information is important in motor tasks, or whether additional sensory input can interfere with two-point discrimination.
|SUGGESTED TIMES for these activities:
30-45 minutes for introducing and discussing the activity, 45 minutes for
"Explore Time" the "Class Experiment;" and 45 minutes for the "Try Your
Own Experiment. (Total time may be reduced if the number of skin areas
tested are limited and if materials for "Try Your Own Experiment!" are
1. Skin can detect several types of
Information from our skin allows us to identify several distinct types of sensations, such as tapping, vibration, pressure, pain, heat, and cold. What is it that allows us to make these distinctions? First, human skin contains different kinds of sensory receptors (cells) that respond preferentially to various mechanical, thermal, or chemical stimuli. (The word "receptor" can mean a receptor cell or a membrane receptor in a cell. Here, it refers to a cell.) Next, these receptors convey this information to the brain and spinal cord, also known as the central nervous system (CNS), to areas where we perceive the stimuli. To accomplish this, the nerve endings of the sensory receptors transduce, or convert, mechanical, thermal, or chemical energy into electrical signals. These electrical signals then travel along neuronal extensions called axons, to the CNS. Finally, the way we interpret or understand sensations is shaped not only by the properties of receptors and neurons, but also by previous experiences that are stored in our brains.
In this lab, activities involve the tactile or touch sense of the skin, which allows us to distinguish different kinds of stimuli upon the surface of the body. By using our tactile sense, we detect superficial and deep pressure and sensations we describe as brushing, vibration, flutter, and indentation. As mentioned above, our skin is also sensitive to temperature and pain, which we sense with different sets of receptors. These skin senses, along with muscle/joint position awareness or proprioception, make up the somatic senses.
2. Sensory information forms the basis for our connection to the outside world
How do we use somatic sensory information? Brainstorm with students for ideas and see if they include the following: exploring, evaluating, and enjoying our environment; making decisions about what to wear or where to set the thermostat; keeping ourselves awake and alert; using as feedback for controlling our movements; avoiding harm from hot, cold, or damaging substances. (Note that some of these involve the tactile sense while others involve the pain, temperature, and proprioceptive senses.) The somatic senses and the sense of taste put us in direct contact with our environment, while vision, hearing, and smell gather information from a distance. Other special internal senses include balance, detecting blood pressure, and sensing blood oxygen levels.
3. Different kinds of tactile receptors respond to distinct types of information
The tactile system, which is activated in the two-point discrimination test, employs several types of receptors. A tactile sensory receptor can be defined as the peripheral ending of a sensory neuron and its accessory structures, which may be part of the nerve cell or may come from epithelial or connective tissue. Different kinds of receptors respond to different kinds of stimulation, such as vibration, pressure, or tapping, and convert these into electrical signals. Table 1 below shows a few types of skin receptors, the kinds of input they detect, and their adaptation rate when stimulated. Slowly adapting receptors continue sending impulses to the brain for a relatively long time when a constant stimulus is applied. Rapidly adapting receptors fire at the time a stimulus begins and sometimes again when it is removed, but they do not continue firing to a constant stimulus. Having receptors with different preferences and different "reporting" capabilities allows us to tune in more acutely to our environment and to distinguish a wide variety of sensations.
4. Sensory input is "mapped" onto specific brain areas
Information from each skin receptor is carried along a pathway formed by several neuronal axons to a strip on the top of the brain surface called the somatosensory cortex. The cortex or "rind" is the cell body-containing outer layer of the brain and is about six millimeters, or one-quarter inch, thick. The somatosensory cortex is packed with the cell bodies of CNS neurons, which receive "skin input" from all parts of the body via the "touch-neuron pathway."
Sensory input pours into the CNS neurons in a topographically faithful manner. This means, for instance, that the CNS neurons receiving input from sensory receptors in the right thumb will have neighbor cells that receive input from the right index finger. These, in turn, will have neighbors receiving input from the next finger, and so on. In this way, a sensory "map" of the body surface is created on a section of the brain surface. Neurologists discovered this years ago when they found that they could produce the illusion of sensation in, say, a finger, by electrically stimulating the appropriate spot on the somatosensory cortex: the CNS neurons interpreted the artificial electrical stimulus as input coming from the finger that usually sent it information.
From the somatosensory cortex, messages about sensory input are sent to other brain areas; for example, to motor areas for use in performing actions, and to higher processing areas, for making decisions or enjoying sensations or reflecting on them.
5. Sensory maps in the cortex are "distorted"
Although tactile sensory maps in the cerebral cortex are faithful to the locations of the sensory receptors, they do not reflect the correct proportions of the skin areas. Rather, the cortical area devoted to receiving information from a spot on the skin reflects the density of sensory receptors there, and this number in turn reflects the importance of that body area for gathering information. The fingertips, for example, contain about 100 times more receptors per square centimeter than the skin on the back. Because of this, more CNS neurons must be devoted to receiving fingertip sensations, and consequently the cortical area that receives input from the fingertips is huge compared to the area that receives input from skin on the back.
If pictures of the parts of the body are drawn next to their corresponding brain areas, the fingers are very large and the arms and back are small. This type of picture is called a homunculus, literally, "little man" or person.
All sensory systems feed information into the cerebral cortex in orderly maps, even though the other peripheral sensory receptors, unlike those of the touch or tactile system, are concentrated in small organs: eyes, ears, nose, and tongue. Information from each of these senses is mapped onto a different brain area.
6. Receptor density and the sizes of receptive fields of central neurons determine two-point discrimination ability
What properties of the touch sensory system allow us to discriminate two points pushing on our skin even when they are only 2 or 3 mm apart? One of the necessary properties is high receptor density, and the class should discuss this after students find that the two-point threshold distance on the fingertips is two to three millimeters (mm). In other words, the receptors must be packed closely enough so that a probe stimulates one or more of them. High receptor density alone, however, cannot explain why the fingertip can distinguish points so close together while the arm senses two points only when they are 35 to 40 mm apart. The second property necessary for fine two-point discrimination is that neighboring receptors must connect to different CNS neurons, which in turn means that these CNS neurons must have small receptive fields, as explained below.
Each sensory receptor connects through a series of relay neurons with a CNS neuron. A given central neuron responds to all information from its input area (the skin area that is the gathering field for only that CNS cell) as if it were coming from one point. This skin area is called the receptive field of the central neuron. On the arm, each sensory receptor gathers information from a much larger skin area than a receptor on the fingertip, and this receptor is also connected to a defined central neuron. This central neuron, like the central "finger neuron", interprets all input as coming from one point, even though the skin area in this case is much larger. In order for a person to feel two points, two separate central neuronal populations must be activated by stimulation of their respective receptive fields. When this happens, two points are reported.
To summarize, two-point discrimination depends on activating two separate populations of neurons, and in order to discriminate two closely placed points, the receptive fields of the neurons must be small. This in turn means that the receptors must be densely packed in a sensitive area, so that two points very close together activate different receptors.
7. Sensory information from different receptors is combined at higher brain levels
Although individual receptors respond to only one type of stimulus, such as pressure or vibration, a stimulus in the real world almost always activates several kinds of receptors simultaneously. To form a representative picture of this in our minds, the different sensations must all "get together" somewhere in the brain, and one place this happens is in cortical neurons called feature-detecting neurons. These neurons each receive several different types of information from neurons in the primary somatosensory cortex (which received their information from receptors). This integration of sensations allows us to experience an ice cube as both smooth and cold, or to feel that sand at the beach contains different sized grains and may be hot or cool. As this information is sent to higher brain centers, sensations also take on meaning because of past experiences.
8. Neurologists use two-point discrimination tests to check for nerve damage
Neurologists, doctors who specialize in diseases of the central (brain and spinal cord) and peripheral (nerves to all the organs and muscles) nervous systems, sometimes test patients for two-point discrimination. They may do this if they suspect a problem with sensory information entry to the skin, the pathways to the brain, or the interpretation of sensory information. For example, if a patient has cut a finger badly, a neurologist may test for two-point discrimination at the time of injury to see if the nerve was cut. After the original injury has healed for a number of weeks, the neurologist will again test two-point discrimination and compare it with the normal fingers to see if the nerve has regenerated.
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First, prepare students for lab activities by giving background information according to your teaching practices (e.g., lecture, discussion, handouts, models). Because students have no way of discovering sensory receptors or nerve pathways for themselves, they need some basic anatomical and physiological information. Teachers may choose the degree of detail and the methods of presenting the auditory system based on grade level and time available.
Offer students the chance to create their own experiments
While students do need direction and practice to become good laboratory scientists, they also need to learn how to ask and investigate questions that they generate themselves. Science classrooms that offer only guided activities with a single "right" answer do not help students learn to formulate questions, think critically, and solve problems. Because students are naturally curious, incorporating student investigations into the classroom is a logical step after they have some experience with a system.
The "Try Your Own Experiment" section of this unit (see the accompanying Teacher and Student Guides) offers students an opportunity to direct some of their own learning after a control system has been established in the "Class Experiment." Because students are personally vested in this type of experience, they tend to remember both the science processes and concepts from these laboratories.
Use "Explore Time" before experimenting
To encourage student participation in planning and conducting experiments, first provide Explore Time or Brainstorming Time. Because of their curiosity, students usually "play" with lab materials first even in a more traditional lab, so taking advantage of this natural behavior is usually successful. Explore Time can occur either before the Class Experiment or before the "Try Your Own Experiment" activity, depending on the nature of the concepts under study.
Explore before the Class Experiment
To use Explore Time before the Class Experiment, set the lab supplies out on a bench before giving instructions for the experiment. Ask the students how these materials could be used to investigate the sense of touch in light of the previous lecture and discussion, then offer about 10 minutes for investigating the materials. Give some basic safety precautions, then circulate among students to answer questions and encourage questions. After students gain an interest in the materials and subject, lead the class into the Class Experiment with the Teacher Demonstration and help them to formulate the Lab Question. Wait until this point to hand out the Student Guide, so students have a chance to think creatively. (See the accompanying Guides.)
Explore before "Try Your Own Experiment"
To use Explore Time before Try Your Own Experiment, follow the procedure above, adding the new materials for student-generated experiments. Let the students suggest a variety of ideas, then channel their energies to make the lab manageable. For example, when a number of groups come up with similar ideas, help them formulate one lab question so that the groups can compare data. The goal is to encourage students to think and plan independently while providing sufficient limits to keep the classroom focused. The Teacher and Student Guides contain detailed suggestions for conducting good student-generated experiments.
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By reaching Project 2061 Benchmarks for Science Literacy, students will
also fulfill many of the National Science Education Standards and
individual state standards for understanding the content and applying the
methods of science. Because the Benchmarks most clearly state what is
expected of students, they are used here. Below is a list of Benchmarks
that can be met while teaching the Two-Point Discrimination activities.
The Benchmarks are now on-line at:
The Benchmarks are listed by chapter, grade level, and item number; for instance, 1A, 6-8, #1 indicates Chapter 1, section A, grades 6-8, benchmark 1.
The process of inquiry used in the Two-Point Discrimination activities will help students reach the following summarized Benchmarks:
The neuroscience content in the Two-Point Discrimination activities and Background material will help to meet the following Benchmarks:
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