Two points touching the skin at the same moment feel, below a certain distance, like one. On a fingertip that distance is tiny. On the back it is far larger than you would guess. You can measure it tonight with a paper clip and someone you trust.
Scroll: the skin moves from fingertip to back.
Fingertip
On the pad of the index finger, two points 2 millimetres apart usually feel like two. The dotted circle marks the gap between them. The ridges are the fingerprint's.
Palm
In the palm the same sensation needs about 10 millimetres. The fingertip has about 240 nerve fibres sensitive to light touch in each square centimetre of skin; the palm, about 58.
Forearm
On the inside of the forearm the threshold climbs to almost 4 centimetres. Closer than that, most two-point touches feel like a single press.
Lower back
On the back, again about 4 centimetres: 40 millimetres. This is the most-quoted number, measured by Weinstein in 1968. Check it on your own back.
The test, for two
One of you is touched, eyes closed. The other holds the paper clip and the phone. The phone tells the toucher, in secret, what to do: one point or two. The person being touched says only “one” or “two”.
A paper clip bent into a U, with both ends level.
A ruler. If you have none, the phone draws one: match it to a bank card first.
Five minutes per area. The back is the most interesting.
How to touch
Touch lightly, for about a second, equally hard every time.
With two points, both must meet the skin at the same moment.
With one point, tilt the clip so only one end touches.
Move the spot a little between touches, within the same area.
The test needs a browser with scripts turned on. You can also run it on paper: at each distance, four touches with two points and two with one, in shuffled order.
Choose the area:
Bend the clip to mm
Does the on-screen ruler look wrong? Match it to a card
Lay a bank card over the outline below and move the slider until the outline is exactly the size of the card.
Show the screen only to the toucher.
What your skin felt
Weinstein, 1968
Mancini, 2014, simultaneous touches
Mancini, 2014, one after the other
You
The coloured marks are the same as in the chart below: Weinstein in 1968 and Flavia Mancini's team in 2014. The black dot is you.
Same back, different number
In 2014, Flavia Mancini and her colleagues at University College London mapped the whole body again. They used thin one-gram filaments mounted on an electronic calliper, and mixed single-point touches in among the two-point ones, a quarter of the total. The test above does the same, with a third.
On the lower back they found about 23 millimetres, roughly half of Weinstein's number. When the two touches came one after the other, three seconds apart, and people only had to say which way the second lay from the first, the threshold fell to about 13 millimetres. On the fingertip, every method gives almost the same answer: 2–3 millimetres.
Weinstein, 1968
Mancini, 2014, simultaneous touches
Mancini, 2014, one after the other
The distance below which two points feel like one, on nine areas of the body. The scale is logarithmic: from fingertip to calf, the threshold grows more than tenfold.Area · mm
Values read off Figure 5 of Mancini et al. 2014, to within about a millimetre.
Area
Weinstein 1968
Mancini, simultaneous
Mancini, one after the other
Fingertip
~2 mm
~3 mm
~3 mm
Palm
~10 mm
~7 mm
~5 mm
Forehead
~17 mm
~11 mm
~7 mm
Sole
~21 mm
~14 mm
~7 mm
Forearm
~39 mm
~21 mm
~17 mm
Lower back
~40 mm
~23 mm
~13 mm
Shoulder
~41 mm
~29 mm
~22 mm
Thigh
~44 mm
~24 mm
~23 mm
Calf
~46 mm
~37 mm
~28 mm
One point or two, at zero millimetres
In 2013, at McMaster University in Canada, Jonathan Tong, Oliver Mao and Daniel Goldreich checked the classic test in a simple way: they brought the points together until the gap was zero. People kept telling one point from two clearly above chance.
The explanation is intensity. Even pressed equally deep, two points close together make the nerve fibres beneath fire fewer impulses than a single point: about 30% fewer at a one-millimetre gap. The person being touched does not feel two points; they feel a weaker touch, and can use the difference as a clue. Of 19 young adults tested at four sites on the hand and forearm, only 5 had a clear threshold at every site, against 15 with the test described below.
If you answered “two” to a single point during your test, you ran into exactly this trap. That is why the single-point touches are mixed in: without them, such an answer would look like very fine skin.
Traditional two-point performance remained significantly above chance levels even at zero point separation.Tong, Mao and Goldreich, Frontiers in Human Neuroscience, 2013
A better test: which way the points lie
Tong and his colleagues recommend a different question: there are always two points, and the person says whether they lie along the finger or across it. Intensity gives no help with that answer, and as the gap shrinks the answers fall to chance, as they should.
Measured with fine grooved gratings turned two ways, the fingertip's limit of resolution, measured this way, is close to one millimetre: 0.94 mm in 15 adults, and 0.51 mm on the lip (Van Boven and Johnson, 1994). The back stays far less sensitive than a fingertip, but not as insensitive as the 40-millimetre number says.
The threshold does not stay put for life. In a study of 122 people aged 8 to 87, the loss from youth to old age averaged about 130% at the fingertip and about 400% at the big toe (Stevens and Choo, 1996).
Tonight
Open the paper clip to three centimetres, close your eyes and draw it slowly, without pressing, from your palm towards your elbow. The points seem to come together, though they have not moved. Ernst Heinrich Weber described the effect almost two centuries ago, according to his translators; it now bears his name. Then ask someone to write a letter on your back with a fingertip, large, then smaller and smaller, and find where you can no longer read it.
Sources
The values per body area are read off Figure 5 of Mancini et al. 2014, which plots their own measurements next to Weinstein's (1968) and Weber's (1834). The readout is accurate to about a millimetre. Your test threshold is the smallest distance at which, there and at every larger distance, at least three of four two-point touches were felt as two. It is a home test with a paper clip, not a laboratory measurement.
Weinstein S (1968). Intensive and extensive aspects of tactile sensitivity as a function of body part, sex, and laterality. In Kenshalo DR (ed.), The Skin Senses, pp. 195–222. Values as plotted in Mancini et al. 2014.