Two-Point Threshold Experiment
Neuroscience Class Activity
Handout
An informative experiment adapted by Don Hood, Dave Krantz, Jen Blanck, and
Elizabeth Cottrell
This activity provides a review of the real estate and magnification principles as well as random
and systematic errors, standard deviation, and Gaussian distributions (Habits of Mind, Ch. 5).
Introduction
In hearing, sounds are converted into neural signals and acoustic information is processed within
the brain. How do the real estate and magnification principles apply to the organization of
auditory information within the ear and in the brain? Specific regions of the basilar membrane
and auditory cortex are responsive to specific sound frequencies, illustrating the real estate
principle. The magnification principle is demonstrated by the fact that a disproportionate amount
of cortical space is devoted to the frequencies that characterize human speech (500-4000 Hz; see
Figure 1).
Figure 1. The mechanical tuning of the basilar membrane creates a spatial map of sound frequency carried all the way up to
auditory cortex. Frontiers of Science Online. © 2008 The Trustees of Columbia University.
The real estate and magnification principles also apply to the organization of other regions of the
brain, including the sensory cortex that receives information on touch (a.k.a. the post central
gyrus). Within the sensory cortex, different parts of the body are mapped onto the surface of the
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brain (see Figure 2). When neurons in specific regions of the sensory cortex fire action
potentials, the sensation of touch in different parts of the body is created.
Figure 2. Schematic of the sensory cortex. Adapted from Penfield W and Rasmussen T, 1950. Frontiers of Science
Online. © 2008 The Trustees of Columbia University.
The two-point threshold demonstration will be used to show how the real estate and
magnification principles apply to the organization of the sensory cortex.
Two-point threshold demonstration
The two-point threshold: The two-point threshold is the minimum distance at which two
distinct tactile (touch) stimuli feel like two distinct points. In other words, at any separation
smaller than the two-point threshold, the subject will feel only a single stimulus.
The procedure below allows for a fairly quick and rough estimate of the two-point threshold on
the forearms and thumbs.
1. Each pair of students should obtain a divider and ruler.
2. One student should choose to go first. One student (the experimenter) should apply the
points of the divider to the arm of the other (the subject) while the subject watches. This
allows the subject to see how the stimuli will feel, and the experimenter to get used to
applying the stimuli properly (pressed quickly and evenly). The two points of the divider
should be applied simultaneously and firmly but not so vigorously as to break the skin.
3. Now measure the two-point threshold for the thumb. The subject should close his/her
eyes. When the stimuli are applied, the subject should say “one” or “two” to indicate the
number of stimuli that he/she feels.
4. The experimenter should separate the divider to at least 1 cm and apply the stimuli to the
middle of the thumb, perpendicular to the long axis. The subject should say “two.” If not,
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then open the divider wider until the subject feels two stimuli.
5. The experimenter should decrease the separation of the two stimuli and reapply.
Continue until the subject feels only one stimulus. Record the distance of this threshold.
6. Repeat steps 4 and 5 for the forearm, applying the stimuli roughly 1 inch below the
crease in the arm and perpendicular to the long axis. Start with the dividers at least 4 cm
apart.
Analysis/discussion of results
Share your results in class discussion. You will probably notice that the two-point threshold is
smaller on the thumb than the forearm. In order to address possible reasons, it may be useful to
review the anatomical and physiological steps between the stimulus (pressure on the skin) and
activity in the cortex. If your left thumb or forearm is tapped, nerve receptors in the skin will be
stimulated. The endings of nerve cells will generate action potentials, which then travel in
peripheral nerves and enter the central nervous system. After several synapses, information on
touch crosses to the other side of the brain (via crossing axons). Action potentials originating on
the left side of the body end up activating the right thalamus, and then the right sensory cortex,
specifically a region devoted to the area of the body that was touched (the thumb or forearm). If
you examine Figure 2 carefully, you will notice that the area of the sensory cortex devoted to the
thumb is practically the same size as the region devoted to the entire forearm, leading to finer
detailed information for the fingers. The magnification principle is clearly evident. More cortical
space is devoted to the thumb area because fine touch information from the hands is important.
Discussion questions
Two-point threshold experiments for the thumb and forearm (described above) were carried out
during Frontiers of Science seminars at Columbia University in the Fall of 2004 and Spring of
2005. Two-point threshold data for the thumb and forearm from four of these seminar sections
(two from Fall 2004 and two from Spring 2005) were collected and analyzed. Below is a series
of questions based on these data, which involve random/systematic error, Gaussian distributions,
and standard deviations—all of which are reviewed in Chapter 5 of Habits of Mind.
1. Below are two histograms of data from a student, Phil, collected during the twopoint threshold experiment. Calculate the mean of these data.
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Figure 3: Histograms representing two-point threshold data from a Frontiers of Science student. Elizabeth Cottrell,
Frontiers of Science. ©2008 The Trustees of Columbia University
2. State two reasons why Phil’s data do not perfectly approximate a Gaussian
distribution. (Hint: See Habits of Mind Chapter 5, sections 24-32 for a discussion of
Gaussian distributions.)
3. Below are two histograms of data collected from several seminar sections. Estimate
the mean and standard deviation of these data by fitting a Gaussian curve to each
distribution. (Hint: Use Habits of Mind Chapter 5, sections 24-32 for a guide on what
a Gaussian curve should look like.)
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Figure 4: Histograms representing two-point threshold data from several seminar sections. Elizabeth Cottrell,
Frontiers of Science. ©2008 The Trustees of Columbia University
4. Calculate the standard error for the forearm and thumb data above. Based on the
standard error of the means, does it appear that the two-point threshold of the
forearm is truly greater than the two-point threshold of the thumb? Explain. (Hint:
See Habits of Mind Chapter 5, section 29 for a discussion of standard error.)
5. How do random errors affect the mean and standard deviation of normallydistributed data? How do systematic errors affect the mean and the standard
deviation of normally-distributed data? (You may wish to refer to Habits of Mind,
Chapter 5, sections 20-25 for a discussion of error.)
6. Suppose a few groups mistakenly reported their data in cm instead of mm (i.e., they
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reported a 2-point threshold of 2.5 cm on the thumb instead of 2.5 mm) Describe
where on the figures in part B these data would appear. Is this a systematic or
random error? Explain.
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