The Two-Point Threshold

Although humans rely heavily on the sense of vision to detect and recognize objects, the sense of touch is also very important.
Touch can provide information about an object, such as surface texture, that is not easily detectable by vision. Touch
experiences are triggered by mechanical disturbance of the skin produced by physical contact with an object. The human skin
contains mechanoreceptors, or receptors that are sensitive to mechanical pressure or deformation of the skin. 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 somatosensory
cortex than less sensitive areas. Thus, the area of the brain which receives touch sensations (for example, from the fingertip) is
proportional to the actual sensitivity of the skin area.

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 three separate areas of the skin: the back of the hand, the forearm, and the back of the neck. We shall try various distances until we find the "threshold," sometimes stimulating the subject with two points and sometimes with one point, and asking the participant to report whether one or two were used. If his/her reports are 100% correct the points are too far apart, and if they are 50% correct, he/she is guessing. It is customary to seek the distance which will provide 75% correct reports. However, we need a bit of allowable error, so let us accept results which vary from 70% to 80% correct.

Equipment: An adjustable 2-point caliper and a small steel ruler.

Procedure:  1. Adjust your caliper to 5mm.
                    2. Give ten stimulations, divided at random between stimulations using one point, and stimulations using
                         two points for all three areas.  NOTE: Use the single and the double stimulations in random order, making up
                         a new order for each trial. Click here for Random Numbers Table.
                    3. Record responses. Be sure to record the amount of separation and whether the participant can perceive one
                        or two separate points.
                    4. Repeat procedure for 10mm, 15mm, 20mm, 25mm, and 30mm increments.

**The participant should NOT look at the area to be stimulated, nor at the data sheet.

Lab Report:

1. What is the independent variable?
2. What is the dependent variable?
3. Plot your % correct at each distance for each body part.
4. What is the relationship between the two-point threshold and the tested area? More specifically, what do the different sizes
     of threshold suggest about the distribution of sensitive points in the skin?