Well, afaik we can't easily record individual neurons inside the brain, although there are some studies towards that goal. Most brain studies work on areas of activity. Even MRI is not that precise, to be frank. But for studying human perception, a technique often used is called microneurography, which involves sticking a probe into the nerve fibers along a known neural pathway and recording the electrical stimulations.

From this we can often find things like:

- frequency of impulses increases as stimulus increases

- different numbers of neurons in a similar area fire simultaneously

Things like that. The wikipedia articles linked by wickedagain contain a good summary. I won't link specific sources, since there are thousands, but here's one for instance:

http://jn.physiology.org/content/85/4/1561

There in fig 6 for example you can see that the frequency of nerve firing increases in the tibial nerve as heat is applied to the paw. (Yes, a lot of information comes from horrible animal studies. It's part of the reason I didn't continue in this domain of work -- although rest assured, this technique is possible with humans, without harm.)

So, do impulses in the perceptual nervous system reflect how brains work, how we think? It's hard to say for sure, but it's clear that information is coded according to certain possibilities related to how neurons work, and this information must be combined and processed somehow to create what we call perceptions. Is it such a stretch to imagine that these perceptions become their own codes, abstract reductions of correlations of perceptual information, that are evaluated, compared, and combined in the much, much more nerve-dense central cortex to produce what we call "thought"?

Not necessarily established, but you'll find some hypotheses here:

https://en.wikipedia.org/wiki/Neural_coding

https://en.wikipedia.org/wiki/Neural_decoding