Some things are too good to be true, and are still true. What Eisenhower said about mucking with Social Security qualifies well enough, but that's the current Internet Meme, so it can be left to everybody else. Here's another one.
When you're about to do something, certain nerve cells in the brain region called Broca's area will fire; different ones for different actions. No surprise here. Something obviously has to happen in the brain to start off an action.
About ten years ago, it was found that when you watch someone make that same action, some of these same nerve cells fire. Naturally, they're now called mirror neurons. (Neuron, nerve cell, nerve, I can now use them interchangeably if you don't mind.) This is cool: it's a physical basis for empathy, for experiencing what someone else is experiencing; by extension, for why your skin crawls when the tarantula shows up on the pillow next to James Bond.
It gets better: a monkey, watching the action of picking something up, has the same reaction in the corresponding bit of its brain. In fact, that's where the effect was found first.
Broca's area also generates actions in the production of speech. And listening to speech -- back to human subjects now -- starts activity in mirror neurons that are involved in producing speech.
But that's just the beginning. A person might pick up a cup, for instance, in order to drink out of it, or to clear up the table, or for no apparent reason; video clips can set up context that implies one action or another. When you watch the video, the person's intention at the time of starting to pick up the cup affects the set of mirror neurons that fire in your brain. More neurons fire for drinking than for cleaning up; more for cleaning up than for the action with no context.
This is getting almost spooky. Here you have some nerves doing the primitive function of getting a muscular action going, really unintellectual and mechanical; and the way they fire depends on your instant interpretation of what is intended.
And this is where reality goes berserk: monkeys' brains work this way too. Train a monkey to respond to certain cues either by picking up a piece of food and eating it or by picking it up and putting it in a container. Then show it a video of a human hand starting to pick up the food, with one of those cues present. The nerves' response depends on what the person is about to do with the fruit.
To me, this is no less wonderful than Hieronymus Bosch action figures.
All this and more is in an article in the News Focus section of the 13 May 2005 issue of Science; no point giving a link, since you need a membership to read it, and if you have that, you can surely find the item easily. The original work on the monkeys' reaction to people's intentions is in the 26 April issue.
[I wanted to title this The Physiological Bases of the Conscience, to tickle the Lord Peter Wimsey fans; but write a headline like that, and it will look like Science Stuff, which is as attractive to potential readers as publishing a book with footnotes and equations. It's already bad enough to have a blog with no pictures or conversations. So one makes it less obvsiously Science by using a friendly term like empathy, while inserting a quasi-pun with "sympathetic nervous system" that only biology freaks will perceive. BTW did you know that empathy is not in the original edition of the OED? For the adjective, empathic is a bit older and therefore better, but empathetic is acceptable.]