Scenario 1 – Human Sensitivity to Touch

You are working in a pain clinic investigating different ways to measure a type of neuropathic (nerve) pain called allodynia. Typically, individuals suffering from allodynia are extremely sensitive to touch. You are interested in establishing whether the sensitivity to touch in these patients varies across the body and you are going to compare difference thresholds on the forearm with that on the lower leg. In the clinic, you typically use a two-point discrimination test to measure sensitivity – this utilises a probe that has two points that touch the skin. The distance between the points can be varied and the patient has to report whether they feel one or two points against their skin. When they report that they can only feel a single point, the distance between the probes is regarded as their threshold.

Prior to making your measurements, you've been advised to make some predictions as to what the difference thresholds might be, based on previous literature. You have established that that the Weber fraction for touch sensitivity on the forearm is 0.2, and for the lower leg it is 0.5. You plan to use a two-point discrimination probe that will produce distances between the two points of 10, 15, 20, 25, 30, 35 and 40 millimetres.

Calculate the predicted difference thresholds for the forearm.

Calculate the predicted difference thresholds for the lower leg.

On the same graph, plot the difference thresholds for both forearm and lower leg against stimulus intensity.

Which part of the body is more sensitive to touch?

Provide a brief explanation for your answer.