EXPERIMENTAL PSYCHOLOGY

EXPERIMENT 2

The Two-Point Threshold

Introduction

The two-point threshold experiment is a fundamental method in psychophysics, used to

determine the minimum distance at which a person can distinguish between two separate
points of contact on their skin. Introduced by Ernst Weber in the 19th century, this experiment
is crucial for understanding tactile spatial resolution and the functioning of the somatosensory
system.

In the experiment, a device called a two-point discriminator or callipers is used to apply two
simultaneous points of contact to the skin. The subject, often blindfolded to eliminate visual
input, is asked to report whether they feel one or two points. By varying the distance between
the points, the experimenter identifies the minimum separation at which the subject
consistently perceives two distinct points. This distance is known as the two-point threshold.

The two-point threshold varies significantly across different body areas. For instance,
fingertips and lips, which have a high density of mechanoreceptors, have a much lower
threshold compared to areas like the back or thighs. These variations provide insights into the
distribution and density of touch receptors in the skin, enhancing our understanding of
sensory perception and its physiological underpinnings. The experiment is widely used in
neurological examinations and research on tactile perception.

Uses of the test

1. Clinical Diagnostics:

- Diagnoses and monitors sensory perception disorders like neuropathies and multiple
sclerosis. Evaluates recovery of tactile function after surgeries or injuries.

2. Research:
 - Maps the distribution and density of mechanoreceptors. Studies changes in tactile
perception with ageing. Conducts comparative studies across species.

3. Ergonomics and Design:

- Informs design of tactile interfaces, such as touchscreens and braille devices. Enhances
workplace safety through better tools and environment design.

4. Rehabilitation:

- Develops tactile therapies and interventions for sensory deficits. Monitors progress in
sensory rehabilitation programs.

5. Educational Tools:

-Teaches sensory systems, neural pathways, and psychophysics principles in educational

settings.

Review of Literature

The two-point threshold test, introduced by Ernst Weber in the 19th century, has significantly
influenced various fields of study. Historically, Weber's work established the test as a measure
of tactile spatial resolution, showing how sensitivity varies across different body areas.

In clinical settings, the test is crucial for diagnosing and monitoring sensory disorders.
Studies, such as those by Moberg (1958) and Dellon and Mackinnon (1989), highlight its role
in evaluating nerve injuries and recovery from surgical interventions. Neurologically, it
provides insights into the somatosensory system’s organisation, with research by Johansson
and Vallbo (1979) mapping mechanoreceptor distribution and Stevens and Choo (1996)
exploring age-related declines in tactile perception.

Comparative studies, like those by Darian-Smith et al. (1982), use the test to understand
evolutionary adaptations in tactile sensitivity across species, revealing that species with more
dexterous hands exhibit lower two-point thresholds. In ergonomics and product design,
research by Klatzky and Lederman (1999) emphasises the test’s importance in designing
tactile interfaces and feedback systems. Lastly, in rehabilitation, the test aids in developing
and assessing tactile therapies, tracking progress in sensory recovery.

Aim

To determine the subject’s two-point thresholds on the left ventral forearm by using

the method of limits.

Plan

A series of stimuli, ranging between 10 mm and 30 mm and increasing by 2mm is

presented in random order, 10 trials of each distance presented, and subject’s responses are

recorded. The absolute threshold is obtained by drawing a Series-Parallel graph.

Subject details

Name: YG

Age: 19

Gender: F

Education: Undergraduate Student

Hypothesis

Subjects vary in their cutaneous sensitivity, as represented by their two-point thresholds.

Materials

1. Two-point Aesthesiometer (touch compass) consists of two points with vernier callipers.

2. Blind fold.

3. Writing materials.

Variables

Independent variable:

Distance between the 2 points of the Aesthesiometer.

Dependent variable:

Subject’s perception of two points.

Experimental controls

1. Subject should be blind folded.

2. The two points of the Aesthesiometer should be presented with equal and steady pressure.
3. Check trials, with one point should be presented occasionally, which should not be

recorded

4. Rub the area periodically to avoid after sensation.

Procedure

Seat the subject comfortably and instruct the subject that his/her ventral forearm will

be touched with either one point or two points. Each time the subject has to report whether

he/she felt one point or two points. Draw a 4cm, line on the subject’s ventral forearm and

blindfold her. After instructing the subject, present the stimuli in random order and record the

subject’s responses. Each trial should be preceded by a ‘ready signal’

Instructions

“I will touch your ventral forearm with either one point or two points. Report to me

each time whether you feel the touch of one point or two points on your skin. I will give you

‘ready’ signal before each trial, so be very attentive”.

Analysis of result

1. Note down the frequency (f) of ‘two’ responses for each distance.

2. Compute cumulative frequency of two-point sensation for each distance.

3. Compute the cumulative percentage (CP) using the value of the last ‘cf’ as the base

number.

cf

CP = --------------------- X 100

base number
1. Plot the subject’s data on a C-P (cumulative-percentage) graph to determine Reiz Limen

(absolute threshold) as follows:

i. The stimulus values are plotted on the abscissa (X-axis) and cumulative percentage

values are plotted on the ordinate (y-axis), and the graph for the subject’s data is drawn.

ii. A horizontal line drawn from the 50 percent point on the ordinate to the graph.

iii. From this point, a vertical line is dropped to the abscissa. The value on the abscissa gives

use the value of the absolute threshold for two points.

2. Compare the two-point thresholds of the subjects and verify the hypothesis.

Calliper Cumulative Cumulative

Measure Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Frequency frequency Percentage
0.5 1 1 1 1 1 0 0 0
1 2 1 1 2 1 2 2 16.66
1.5 1 1 1 1 1 3 25
2 1 2 1 2 3 6 50
2.5 1 1 2 2 2 8 66.6
3 1 1 2 - 0 8 66.6
3.5 2 1 - - 1 9 75
4 1 2 - - 1 10 83.3
4.5 1 2 - - 1 11 91.6
5 2 - - 1 12 100
Graph 1

The graph represents the data collected in ascending series. The Y axis depicts the cumulative
percentage and the X axis depicts the distance at which the stimulus was given. The absolute
threshold of the subject in ascending series is__2.0__ which was received by drawing a
horizontal line at 50% of that determined the Reiz Limen

Calliper Frequen Cumulative Cumulative

Measure Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 cy frequency Percentage
5 2 2 2 2 2 0 0 0
4.5 2 2 2 1 2 1 1 7.69
4 1 2 1 2 1 3 4 30.7
3.5 1 1 2 1 1 4 8 61.5
3 1 2 2 - 1 9 69.2
2.5 - 1 1 - 2 11 84.6
2 1 1 - 2 13 100
1.5 - - - - - 0 0 0
1 - - - - - 0 0 0
0.5 - - - - - 0 0 0
Graph 2

The graph represents the data collected in descending series. The Y axis depicts the
cumulative percentage and the X axis depicts the distance at which the stimulus was given.
The absolute threshold of the subject in descending series is___3.6___ which was received by
drawing a horizontal line at 50% of that determined the Reiz Limen.

Conclusion

The two-point threshold experiment shows different absolute thresholds depending on the
order of stimulus presentation:

Descending Series: The absolute threshold is 3.6 units, where the distance between points
was gradually decreased. The subject may have anticipated the points merging sooner,
leading to a higher threshold.

Ascending Series: The absolute threshold is 2.0 units, where the distance was gradually
increased. The subject may have taken longer to perceive the points as separate, resulting in a
lower threshold.

Conclusion: The order of stimulus presentation significantly impacts the perception of the
two-point threshold, indicating that it is influenced by psychological factors and is not a fixed
value.