FECTEAU, S., PASCUALLEONE, A., & THEORET, H. (2008). Psychopathy and the mirror neuron system: Preliminary findings from a non-psychiatric sample Psychiatry Research, 160 (2), 137-144 DOI: 10.1016/j.psychres.2007.08.022
AGNEW, Z., BHAKOO, K., & PURI, B. (2007). The human mirror system: A motor resonance theory of mind-reading Brain Research Reviews, 54 (2), 286-293 DOI: 10.1016/j.brainresrev.2007.04.003
One of the main hallmarks of psychopathy is a lack of empathy. A psychopath is able to look at a person who's suffering and not feel the unease that you or I (hopefully) would. The idea, then, is that this empathic understanding that other people have the same feelings as you or I is a major deterrent to harming others; we feel what we cause them to feel, so ultimately we're looking out for our own self-interest in not feeling bad.
Well, within the past few years it looks as though researchers have identified one of the major components of how empathy is generated in the brain. Agnew, Bhakroo, and Puri do a good job of summing up the current state of knowledge, but a few highlights: Electrode studies in monkeys have revealed that particular neurons are activated whenever the monkey performs a certain action, but also when observing another monkey perform that action. Hence the name, "mirror neurons." Importantly, mirror neurons provide a link between perception and action. Beyond simple movements, mirror neurons also provide an explanation for how people can decipher others' intentions; we can differentiate whether a person is reaching for a cup or just moving our hands towards it, for example. In addition, mirror neurons have been linked with the ability to interpret and feel the emotions of others. This is what is usually referred to by the term "empathy," or emotional empathy, which can be contrasted with the motor empathy observed when viewing others' actions.
So the idea is that psychopaths are literally unable to imagine and feel the pain that they inflict on others. Fecteau and his colleagues set out to test this idea using TMS - Transcranial Magnetic Stimulation. (Warning: excessive abbreviations in this article. I'll do my best to navigate through it, but it can still get confusing. Damn psychiatrists.) The TMS apparatus is a high-powered (but hand-held) electromagnetic coil that can pick up patterns of neural activity. It can work in reverse, too, acting as a signal emitter as well as a receiver. Other researchers have used it to selectively knock out brain regions temporarily. Previous studies have found that witnessing a painful event will result in a temporary dampening in neural firing in response to TM stimulation (a motor-evoked potential, or MEP), but which is specific to the nerves connecting to the region that's receiving the pain. So: stimulate the somatosensory cortex with TMS and you get a MEP you can pick up on an electromyocardiogram; see someone getting hurt in the region associated with the stimulated area and the MEP gets dampened. If this dampening of the MEP signal is not observed, then the mirror neurons aren't responding to this particular stimulus, and aren't interpreting perception of others' pain into empathic pain.
So is this what they saw in psychopaths? Quick answer: no. Long answer: they saw the opposite.
Okay, to be fair, they didn't actually look at psychopaths. Instead they used the Psychopathic Personality Inventory to get a scalar measure of psychopathic traits in a normal population. There was some variation in the scores, however, so they had a range of personality profiles among their subjects. And back to the results, they actually found that people who scored high on one subscale, coldheartedness, actually showed greater dampening of the MEP signal when witnessing pain. What's more, coldheartedness is the subscale that most directly assesses empathy - whether people are sensitive to the suffering of others or not. So what's going on? Well, the authors argue that they're measuring motor empathy, but that in psychopathic populations this becomes divorced from emotional empathy. In fact, psychopathic people don't have any impairment in recognizing emotions or identifying when others are in pain; in fact, if anything they're better at it than non-psychopaths. This is part of the reason why they're so good at manipulating people. If this theory is correct (and to bookend with my title), then a psychopath knows exactly what you're going through; they just don't care.