I'm looking for research studying:

  1. How small and compressed together do particles of a surface need to be to be perceived as solid surface to human touch? E.g. sand still feels grainy. Has there been research on how much it varies between different areas of the body?
  2. Same question, but for where the line of pain lies - walking on gravel can be painful, while walking on sand isn't.

Any pointers to relevant material much appreciated!

  • $\begingroup$ Both sensations are dependent on pressure (i.e. area over which a force is distributed). Remember that stepping on one nail is incredibly painful but lying on a bed of nails may not be. $\endgroup$
    – Jam
    Jul 19, 2019 at 11:22

1 Answer 1


1) Discerning between solid and granular

It is the distribution of the touch receptors in your skin that enables the distinction between solid and granular objects.

The ability to recognize two separate points applied simultaneously to the skin from a single point is called two point discrimination. Two point discrimination varies with location on body surface. It is about 2 mm on the finger tip and increases to over 40 mm on the back. (vcu.edu)

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Image source: Annals of Neurology, CC licence

2) Touch vs pain

The sense of touch is enabled by touch receptors and pain by pain receptors (nociceptors). In general, pain receptors are activated by greater pressure than touch receptors. Walking on gravel (sharp-edged stones) likely applies greater force on a certain area of your skin than walking on rounded pieces of sand, so it will more likely cause pain.

Mechanoreceptive nociceptors have a high threshold for activation—they respond to mechanical stimulation that is so intense it might damage the tissue. (nobaproject.com)

More info:

  • $\begingroup$ This is exactly what I was looking for - thank you for the summary and references! $\endgroup$
    – Athere
    Sep 4, 2019 at 13:21

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