What did we leam about the aesthesiometer used by Prof. Allen?

Prof. Todd Allen at the University of Northern Colorado, in Greeley, US, recruited 54 people for his experiment, including 30 who had tattoos. Skin sensitivity was revealed using a common scientific device called an aesthesiometer. The device consists of two adjacent plastic points, each of them capable of readjusting.
    At a certain distance, when the points have moved far enough apart, a person will sense them as two distinct points, instead of just one. If the distance between the points is small when this happens, the skin area being tested is highly sensitive. On the other hand, the two points will not feel discrete on less sensitive areas until they have moved very far apart.
    Prof. Allen tested participants’ reaction to the aesthesiometer on five body parts: the lower back, the back of the calf muscle, the inner forearm, the tip of the index finger and a cheek.
    He found no difference between the sensitivity of the unmarked body parts of tattooed participants and those of their "uninked" counterparts.
    But he did notice that the corresponding marked regions of the tattooed subjects did perform worse on the aesthesiometer test. For example, the tattooed region on the right leg of one participant was less sensitive to touch than the corresponding tattoo-free area on the left leg.
    "There is a relatively small but significant decline in sensitivity, "concludes Prof. Allen. On tattooed regions the points had to be at least 32 millimetres apart, on average, before participants felt two distinct points. The average distance for their corresponding unmarked body parts was 28 mm.
    Prof. Allen says there are several possible explanations as to why tattooed skin has lost this sensitivity. The repeated stimulation of nerves in the skin during the tattooing process could cause them to become inhibited and less easily triggered. Another possibility is that the ink injected into the skin may be interfering with the pressure of touch.