In sandpaper sorting, what neural proccesses in the brain occur during the task?

Question

the sandpaper sorting is when you close your eyes and you try to sort out a variety of sandpaper from roughest to not roughest.

Answer 1

I can only offer what I think happens in my brain. Two characteristics can be felt: frictional force and tactile resolution of individual grains. (Bending stiffness would also be a clue, but I'm assuming you refer only to feeling the surface.)
You note a rise in friction force as grain gets coarser. This occurs because fewer, coarser grains deform your skin more deeply causing more friction force per unit area (similarly to the improved traction your car gets on rougher-grained pavement). Simultaneously, perhaps adjusting the contact force of your fingers, you note how clearly you can feel the individual grains. For this, sensitivity depends on the softness and thickness of your skin. When grain size falls below some minimum value, you lose any perception of individual grains, but up to that point you can distinguish low from high resolution.
These two effects -- friction on the macroscale and grain resolvability on the microscale -- are integrated in your brain to form the final decision. That's the most mysterious, and probably complex, part of the test. If the sandpaper samples are all the same type and condition (e.g., dry/wet, uniform/nonuniform grain size, old/new) conflicts are unlikely to arise. However, should they differ, the brain must use additional decision criteria such as, which do I trust more, friction or resolution? Perhaps weighting factors are used in such cases.