A physiotherapy student tested two-point discrimination on different regions of a person's body. The student touched the skin with the two rounded ends of a...

(a)

\[35\text{ mm}-3\text{ mm}=32\text{ mm}\]

The difference is 32 mm [2].

(b) The lip and fingertip detect the closest points, with thresholds of 2 mm and 3 mm [1]. The palm, and especially the forearm, need a greater separation before two points are detected [1]. Thus two-point discrimination is most sensitive where the threshold separation is smallest.

(c) Fingertips have more sensory receptors or sensory neurones per unit area [1]. Each receptor has a smaller receptive area [1]. Two close points are therefore more likely to stimulate separate receptors [1]. Separate impulses travel to the brain and are interpreted as two points [1].

(d) There is an inconsistency in the supplied mark scheme. It names the palm, but a palm threshold of 12 mm means that points 8 mm apart would be felt as one point, not two. To detect two points 8 mm apart, choose the lip or fingertip [1], because their thresholds, 2 mm and 3 mm, are both below 8 mm [2]. The palm would only be appropriate if the question asked for a region that would not distinguish the two points.

(e) Control any three variables: pressure applied, contact time, person tested, tool or end size, skin temperature, order of tests, and use of an eye cover each time [3]. Controlling these makes body region the main variable affecting the result.