First the sensory part:
When a friend hits her thumb with a hammer, you don't have to put much effort into imagining how this feels. You know it immediately. You will probably tense up, your "Ouch!" may arise even quicker than your friend's, and chances are that you will feel a little pain yourself. Of course, you will then thoughtfully offer consolation and bandages, but your initial reaction seems just about automatic. Why?
Neuroscience now offers you an answer: A recent line of research has demonstrated that seeing other people being touched activates primary sensory areas of your brain, much like experiencing the same touch yourself would do. What these findings suggest is beautiful in its simplicity—that you literally "feel with" others.
The comparison with the emotions part:
Despite the lack of a universally agreed-upon definition of empathy, the mechanisms of sharing and understanding another’s experience have always been of scientific and public interest—and particularly so since the introduction of “mirror neurons.” This important discovery was made two decades ago by Giacomo Rizzolatti and his co-workers at the University of Parma, who were studying motor neuron properties in macaque monkeys. To compensate for the tedious electrophysiological recordings required, the monkey was occasionally given food rewards. During these incidental actions something unexpected happened: When the monkey, remaining perfectly still, saw the food being grasped by an experimenter in a specific way, some of its motor neurons discharged. Remarkably, these neurons normally fired when the monkey itself grasped the food in this way. It was as if the monkey’s brain was directly mirroring the actions it observed. This “neural resonance,” which was later also demonstrated in humans, suggested the existence of a special type of "mirror" neurons that help us understand other people’s actions.
The interesting part is that they seem to be related in that people who self report high empathy also show stronger mirror neuron activity:
Michael Schaefer and his colleagues also scanned their participants’ brains while they were watching movie clips of touches applied to human hands. Consistent with earlier results, participants’ primary somatosensory cortex (the brain’s representation of the body surface) responded vicariously to the observation of touch. However, participants also completed the Interpersonal Reactivity Index (IRI), a paper-and-pencil test measuring four specific dimensions of our ability to empathize with others. And guess what? The higher participants scored on the “Perspective taking” subscale of the IRI, the stronger their primary somatosensory cortex reacted to observed touch. These data suggest that the brain’s mirroring responses are in fact associated with personal empathic ability. How much you empathize with other people seems to reflect how strongly your brain—your primary somatosensory cortex—“feels with” them when you see them being touched.
It's interesting how little we understand the concept of empathy, including what role our physical sensations have in the process (and perhaps in feeling our own emotions?). The whole thing sort of reminds me of studying music and honing my skill of audiation, which is the process of imagining (or basically hearing) pitches in one's head. You can try it too -- sing a song to yourself without making a sound and you are audiating. What I noticed about myself is that there is a physical connection with my audiating. Specifically, when I audiate, my vocal chords, throat, and some muscles in my mouth and face adjust as if I were about to sing or hum the pitch I'm imagining. When I think of a high pitch, my eyebrows and soft palate go up. For a low note, my throat expands.
I know that I do other small physical manipulations like this to affect my mental state, for instance purposefully yawning to make myself more tired or making my face slack like I am already asleep to fall asleep more quickly. I also do this with emotions, like smiling to be happy. Sometimes I try them in response to a curiosity of other people's emotions. But just like how I can't seem to imagine a pitch without being able to sing it (e.g. if it is out of my singing range), I can't seem to imagine an emotion without having experienced it once myself. Do all forms of empathy have this limitation?