Why two nearby touches can merge on your back
Touch begins in sensory nerve endings in the skin. Each ending responds to pressure in a small area, but those areas overlap, and the nervous system has to infer where a touch happened from the pattern of activity they send. If two pokes are close together, their signals can produce a pattern that is hard to distinguish from one touch spread across a wider area. Your brain may therefore perceive one point instead of two.
The brain gets a less precise map
The back is generally less sensitive to fine spatial differences than the fingertips. Fingertips have many touch receptors packed into a small space, and the brain devotes substantial processing capacity to interpreting their signals. On the back, touch information is less finely resolved, so nearby points are more likely to fall within the same effective patch of sensation.
This ability to tell two simultaneous touches apart is called two-point discrimination. The distance needed to feel two points separately varies across the body and from person to person; it is not a fixed boundary. A wider gap makes the two sources easier to distinguish because they are more likely to create clearly different patterns of neural activity.
Why the same test feels different elsewhere
Try the same gentle, simultaneous touches on a fingertip and then on your back. On the fingertip, you may feel two distinct points even when they are close. On the back, they may seem like one broad contact. The difference is not that the back cannot detect touch: it is that its sensory map is less precise for locating neighboring touches. That is why a pair of pokes can become one sensation before you ever consciously identify where either one landed.
