IB Biology
International School of Toulouse : Centre Number 1203

The Two-Point Threshold Touch Acuity Experiment

Aim: To test the hypothesis that some areas of the skin contain more mechanoreceptors than other areas.

The human skin contains mechanoreceptors, or receptors that are sensitive to mechanical pressure or deformation of the skin.  The sense of touch is dependent on pressure receptors that are sensitive to either strong or slight pressures.

However, the concentration of mechanoreceptors within the skin is not uniform. Rather, the highly sensitive areas of skin, such as the lips and fingertips, contain densely packed mechanoreceptors, while insensitive areas, such as the stomach and back, contain lower concentrations of mechanoreceptors.

More sensitive areas of the skin also project to a larger proportion of the the brain which receives touch sensations.

Experiment

Touch acuity is conventionally measured using the two-point threshold test.
The basic question is this: How far apart do two separate points need to be before they are perceived as two points rather than one?

In this experiment we will test the sensitivity of two separate areas of the skin.

Begin the experiment by forming a 2 person team.  Next, decide who will do the testing and who will be the participant.

The person chosen to be the experimenter will begin by calibrating the test apparatus (an adjustable 2-point caliper or a hair grip) to a inter-point gap size of 1 mm (using a ruler).

Adjustable 2-point caliper

Start with the first area and then test the other testing areas using a gap size of 1mm.

Touch the skin with one or two points randomly (in a way that the participant can't see)

Repeat this until you have done 10 tests (5 with 2 points and 5 with 1 point - randomly)

After testing each area record whether the participant perceives one or two points on the skin surface and if they are correct.

Once you have tested both areas at 1mm of separation (and record the responses) proceed to test both areas in the same sequence at the following increments: 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, up to 20mm.

Once the participant has perceived two points correctly on two consecutive trials feel free to stop testing and use the 1^st two-point increment as the threshold value. For example, if on the back of the hand the participant feels two points correctly at 10mm and 15mm, use 10mm as the threshold.

Proceed through the remaining increments until the participant can feel two separate points on both testing areas.

Lab Assessment of Data Collection, Data Processing and Conclusion

Record your data and present it appropriately (including uncertainties).

Process the data mathematically in a way that helps the interpretation of the data.  (including uncertainties)

Present the processing and the processed data in a good way.

Explain what you have found out, explain why and if you can find a book that says something about your findings.

Evaluate the procedure for weaknesses and errors. Suggest improvements   to the procedure.(with explanations why they will improve it.)

Questions to help us plan experiments in the near future

1. What is the independent variable?

2. What is the dependent variable?