Practical 1 : Determination of Two-Point Threshold or RL by the Method of Limits

1. Problem:

Conduct an experiment to determine two-point threshold or RL or aesthesiometric index of a subject’s right palm by the method of limits.

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2. Introduction:

The two-point threshold is an important measure of tactile sensitivity. It is defined as the smallest distance between two stimuli that can be recognized as distinct points. This experiment uses the method of limits, where the distance between the two points is gradually increased and decreased in a series of trials. The subject is asked to report whether they perceive one or two points of contact. The distances are adjusted according to ascending and descending series to minimize errors due to habituation and anticipation.

The aesthesiometric index, also known as the two-point threshold, is measured using an aesthesiometer. The Vernier type of aesthesiometer, with adjustable sharp-ended needles, is particularly useful because it allows for precise control over the distance between the two points. The experimenter gradually increases and decreases the distance between the points during the trials and records the subject’s responses. The point at which the subject consistently perceives two points in 50% of the trials and one point in the remaining 50% is recorded as the two-point threshold. This is considered the absolute threshold, or RL, of the subject’s tactile sensitivity.

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3. Method:

3.1. Subject:

• A healthy, 20-year-old male undergraduate student with no known sensory impairments.

3.2. Apparatus and Materials:

• Aesthesiometer (Vernier type, with adjustable sharp-ended needles).
• Wooden screen to block the subject’s view of the palm.
• Paper and pencil for recording responses.
• Blindfold or handkerchief to prevent the subject from seeing the instrument.

3.3. Design:

The experiment will be conducted in two alternating series, ascending and descending. There will be a total of 20 trials, with 10 trials in each series. The distance between the two points will vary from 6 mm to 16 mm. In the ascending series, the distance starts at 6 mm and increases by 1 mm after each trial. In the descending series, the distance begins at 16 mm and decreases by 1 mm after each trial. This design ensures that the experiment controls for errors due to anticipation and habituation.

3.4. Instructions:

Before beginning the experiment, the experimenter will explain the procedure to the subject: “I will touch the palm of your right hand with either one or two pencil points. Every time, you need to report whether you feel one point or two. You will be given a ready signal just before I touch your palm, so you can prepare for the sensation.”

3.5. Procedure:

The subject will be seated comfortably with their right hand exposed. The experimenter will first mark an inch line on the subject’s palm to identify the test area. The subject will then either be blindfolded or asked to place their right forearm through a window of the wooden screen, ensuring that they cannot see the aesthesiometer. Preliminary trials will be conducted to roughly identify the transition zone where the two-point threshold may lie.

Once the preliminary trials are completed, the actual experiment will begin. The first series will be conducted in ascending order, starting with a distance of 6 mm between the two points of the aesthesiometer. The subject will be asked whether they perceive one point or two, and their response will be recorded. If they report “one,” the experimenter will increase the distance by 1 mm and repeat the process. The trials will continue in this manner until the subject perceives two distinct points. After the ascending series, the descending series will begin. In this series, the distance will start at 16 mm, and the distance will be decreased by 1 mm after each trial if the subject reports feeling two points. The trials will continue until the subject reports feeling one point. The responses from both series will be recorded in a table.

The alternating use of ascending and descending series helps control for habituation and anticipation errors. In some cases, subjects may develop a tendency to repeatedly respond with the same answer (habituation error), or they may change their responses when no change is warranted (anticipation error). By alternating the series and using equal numbers of trials, these errors are minimized.

After completing all the trials, the mid-points of the ascending and descending series will be averaged to determine the two-point threshold for the subject. The subject will then be asked to share their feelings about the experiment, including any moments of uncertainty, patterns they noticed, or difficulties in distinguishing the points.

3.6. Introspective Report (Subject’s Feedback):

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Data Table : Responses of the subject were presented in the following Table 4.1

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