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.
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.
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):
Data Table : Responses of the subject were presented in the following Table 4.1
Results:
The overall mean threshold for the 20 trials was calculated to be 11.65 mm (233/20). The standard deviation (SD) for the total trials was 1.194 mm.
For the ascending series, the mean threshold was 10.8 mm, with an SD of 0.92 mm. The standard error (SE) of the mean for the ascending series was 0.31 mm.
For the descending series, the mean threshold was 12.5 mm, with an SD of 0.77 mm. The SE of the mean for the descending series was 0.26 mm.
The SE of the difference between the means of the two series was 0.40 mm, and the ‘t’ value for the difference between the ascending and descending series was 4.25 (p < .01), which indicates that the difference between the means of the two series was statistically significant.
Additionally, the mean threshold for the first half of the 20 trials was 11.6 mm, and for the second half, it was 10.7 mm. The SD for the first half was 1.14 mm, while the SD for the second half was 1.64 mm. The SE of the mean for the first half was 0.38 mm, and for the second half, it was 0.55 mm.
The SE of the difference between the means of the two halves was 0.67 mm, and the ‘t’ value for the difference between the two halves was 1.34, indicating no significant difference between the two halves of the trials.
Discussion:
The objective of this experiment was to determine the right palm’s two-point threshold (also referred to as the RL) using the method of limits. The experiment was conducted in two series, ascending and descending, and the threshold values were calculated for both series. The data from the trials were analyzed by computing the mean, standard deviation (SD), and standard error (SE) of the mean for each series as well as for each half of the total trials. The differences between the two series and the two halves were tested using the t-test.
The overall mean distance between the two points, which represents the two-point threshold, was found to be 11.65 mm. This is the minimum distance required for the subject to perceive two distinct points at least 50% of the time. The mean for the ascending series was 10.8 mm, and for the descending series, it was 12.5 mm. This showed a difference of 1.7 mm between the two series, which was statistically significant (p < .01).
The threshold for the descending series was larger than that for the ascending series. The difference of 1.8 mm between the two series was statistically significant. This result suggests that the subject’s responses were influenced by expectation error. If the threshold in the ascending series had been larger than in the descending series, it would have indicated the presence of a habituation error.
Overall, the two-point threshold for the subject’s right palm was found to be 11.65 mm, with the ascending series threshold being 10.8 mm, and the descending series threshold being 12.5 mm. The larger threshold in the descending series was most likely caused by the expectation error, where the subject anticipated the two points to be closer together, leading to a higher threshold.
Conclusion:
Based on the results of the experiment, it can be concluded that the overall two-point threshold for the subject’s right palm was 11.65 mm. The threshold for the descending series was significantly larger than the threshold for the ascending series, which indicates that expectation error influenced the subject’s responses.
