Get three bowls of water, one hot (not so hot as it will hurt anyone), one cold, and one in between get them to put one hand in the hot, and one in the cold. Wait a few seconds, then get tehm to put them both in the warm one.

You should find they look very confused as one hand will feel the warm water as hot and one as cold.

Try the 2 "opposites" with different hands, then use the intermediate one with both hands, and see if they feel different.

This shows that a lot of our senses are relative rather than absolute, you are much more sensitive to a change in temperature than the absolute temperature. I suppose this is an extreem, where a lobster won't really react if the water temperature is increased slowly enough...

If you go from a dark room into bright sunlight it
feels very bright but if you are in a light room and go outside it doesn't
seem so bright as there is less change in brightness. The same is true of
temperature and vibration. If you put one hand in a bowl of hot water it
feels hot because the nerves which detect higher temperatures become more
active. If you leave your hand there the nerves gradually become less
active even though the temperature is staying the same. The same works for
your other hand - the one in the cold water. The nerves that detect raised
temperature become less active initially so you feel cold but if you leave
your hand there then they become a bit more active even though the
temperature is the same. When you move both hands into the normal
temperature water your brain compares the temperature with the previous
temperature. The hand from the hot water is used to warm temperatures and
so feels cold. The hand from the cold water is used to cooler temperatures
so feels hot.

This makes sense, as the important tings are more normally the changes in a sense rather that the absolute value, eg it is interesting that the temperature ha suddenly changed, as this may mean a change in the weather, your house has fallen down, etc. but if your nervous system kept shouting 'It's quite warm' all day, you would be distracted from the sabre tooth tiger creeping up on you...

Roughness

Another manifestation of this is rubbing your hands on rough and smooth sandpaper, and then both on some intermediate sandpaper...

You get a similar reaction as the temperature - it is very strange, as there is nothing physical changing in your hands this time...

I think you have vibration sensors in your skin which detect the amount of vibration, instead of each movement, and the nervous system must just me more sensitive to changes than absolute values in general??

Additional background for demonstrators

"Thermal receptors are very sensitive to differences between the temperature of the skin and the temperature of objects that are touched. Rapid changes in skin temperature evoke dynamic responses, with increases in temp. signalled by warmth receptors firing and cold receptors silencing and vice versa. If contact with the object is maintained forseveral seconds, the firing rate of the receptor falls to a lower rate. This adaption of the spike discharge corresponds to the phenomenon of sensory adaption."

This is a bit of an aside: Sensory adaption is commonly talked about with touch receptors, and means that the receptor notices changes in pressure, not the actual level of pressure. It is an economical way of saving on neural transmission. Receptors in the skin which do this are called rapidly adapting sensory receptors. Some rapidly adapting sensory receptors, eg. Pacinian corpuscules and meissner's corpuscules, can detect vibration - they give a spike of an action potential for the peak of every bump in a bumpy surface.

Now Pacinian corpuscles work @ high frequencies (like fine sandpaper - I haven't lost the thread of my argument yet!), while meissner's work at middle range frequencies, and Merkel's discs, which are not rapidly adapting (so they just fire all the time to say what's going on), work at the lowest frequencies, which I think might equate to rough sandpaper. This means that the smallest skin indentations from sandpaper, if close to the optimal frequency for a certain receptor, will only activate one sort of receptor, as it is the only receptor good enough to detect such small indentations at the frequency.

The "amount of bumpiness" you perceive (not the frequency), according to my book (well, it calls it the "intensity of vibration signalled"), is based on how many nerves are firing at once - if loads are firing, it means that the indentations made into the skin are deep (ie. you're pressing harder or could this also be from the grains of sand being bigger?) and so loads of the optimal, and a great many sub-optimal receptors are firing.

I think we must be able to perceive the frequency of the bumps by amassing information from which specific fibres are firing action potentials.

I wonder then, why it is that when you go from v. bumpy to q. bumpy, the resultant feeling is "smooth" ( and why from smooth to q. bumpy the feeling is "v. bumpy")? I'd say it might be to do with the sorts of receptors that are firing changing so quickly - perhaps this is a brainstem or a cerebral mechanism for alerting you to changes? You could wire things up in the brainstem so that a decent period of all one frequency/intensity pattern caused all the receptors that weren't involved in that pattern to senstize. Or perhaps the sensitization occurs higher up in the consciousness scale - the brain actively n the lookout for changes.

On a neural/receptor level, it is hard to account for because the changes to the receptors are not gradual (eg. a slow warming of the hand as it enters warmer water) but instant. But I may be entirely wrong...!

As far as the eye is concerned, I think the small size of the iris makes the room look dark when you come indoors, but there's also something the eye does to adjust its way of looking at things to enable it to see changes in light and dark at any level of light on an absolutely enormous scale (10^1- 10^20, I think). This it does chemically, i think, by allowing the light to "bleach out" the activity of some receptors... I think...