Touching on touch
I spent part of this afternoon at a fascinating talk on the sense of touch - one of those things that seem so simple yet have a tremendous degree of complexity when you get into them. (And much of what we know, it seems, comes from when these systems, through illness or injury, go wrong.)
So what contributes to your sense of touch?
(a) Meissner's endings (which sense dynamic pressure, i.e. movement)
(b) Merkel's cells (which are biased towards static pressure, allowing the modulation of your force so you don't squeeze say a glass to smithereens)
The above are close to the surface, while deeper down are:
(c) Pacinian endings (which sense vibration)
(d) Ruffini endings (which sense skin stretch)
Then there's proprioception, which gives you the sense of where your limbs are. (Why even with your eyes closed if someone moves one limb to a position, you can put the matching one in the same position.)
Contributors to this are:
(a)Muscle spindles (the nerve fibres in which sense position and movements)
(b) Golgi tendon organs (muscle tension)
(c) Extra-muscular receptors
(d) Joint capsule receptors
(e) Those Ruffini endings again
All of these separate pieces of information apparently travel independently to the brain, where they are processed in different areas of the brain (as seen in EM imaging) , so there are "dimensions of touch", each perceived in a different part of the brain.
Then there are varying actions for touch, which seems to follow from above
1. Lateral motion e.g. rubbing a finger across the surface, which gives texture
2. Static contact - fingers resting on the surface - for temperature
3. Enclosure, holding grasping the object - for shape
4. Unsupported , holding the object in the hand - for weight
5. Contour - tracing around the edges - for global and exact shape
If you try to use an action to make the "wrong" judgement, it will be highly ineffective.
Interestingly, the touch part of your brain is very close to the movement part, while sight is much further away.
However, vision is a holistic sense, while processing of touch happens serially - each one being acknowledged in turn,
No wonder the robot-makers have so many problems when they try to make their inventions carry a glass of water across a room. (Although looking around this I found one way they are trying to deal with this is to create artificial skin.)
And here's an artist's take on the issue.
(I got into the outer edges of this area of knowledge in my master's thesis, when looking at proprioceptive coherence", the way in which an practiced tool user can experience the tool as an extension of their body. )