TWO-POINT DISCRIMINATION LAB INTRODUCTION: 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) the respond preferentially to various mechanical, thermal, or chemical stimuli. 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 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. 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 to electrical signals. The table 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. Receptor Merkel’s disk Meissner’s corpuscle Ruffini’s corpuscle Pacinian corpuscle Hair receptor

Stimulus steady indentation low frequency vibration rapid indentation vibration hair deflection

Sensation pressure gentle fluttering

Adaptation slow rapid

stretch vibration brushing

slow rapid rapid or slow

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 before, 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. Two-point discrimination depends on activating two separate populations of neurons, and in order to discriminate two closely spaced points, the receptive fields of the neurons must be small. This means that the receptors must be densely packed in a sensitive area, so that two points very close together activate different receptors. Neurologists 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 finger to see if the nerve has regenerated properly.

PROCEDURE: 1. Choose a subject and a tester in your group. The subject must either close his/her eyes or wear a blindfold. The subject may not watch the procedure – this would give away the answer! 2. The tester should use two toothpicks and start with the toothpicks about 50 millimeters apart. Making sure that the two points touch the skin at the same time, the tester will touch the toothpicks to the subject’s forehead. 3. The subject will tell the tester how many points were felt. If the person feels two, move the points closer together – about 40 millimeters apart, and check again. Continue the procedure until you find the smallest distance the points can be separated for the person to feel two points instead of one. When the person reports “one point” for the first time, move the two points apart only one or two millimeters at a time and try to make a very accurate measurement. 4. When the smallest distance is found, measure the distance in millimeters between the two points and record the distance on your data table. 5. Continue this process for the rest of the skin areas on the data table. 6. Repeat for other subject. 7. Average your data and add to a class chart on the board. DATA TABLE:

SKIN AREA FOR TESTING

MINIMUM DISTANCE FOR TWO POINT DISCRIMINATION (IN MILLIMETERS) Subject 1

Average

Subject 2

FOREHEAD CHEEK FOREARM PALM OF HAND TIP OF THUMB TIP OF INDEX FINGER BACK OF LOWER LEG

ANALYSIS: 1. How do your results compare with those of other groups? 2. Are the two-point distances on different areas of the skin the same – for example, is the measurement on fingertips the same as the measurement on the back of the leg? 3. Which parts of the body are best at telling two points are touching them even when the points are very close together? 4. Which skin areas do you think have more receptors, areas that have small two-point distances or large two-point distances? Why do you think so? 5. How does information from sensory receptors in the skin get to the brain? 6. List three findings you think are important from today’s experiment. Were you surprised by anything you found?