Can mirror neurons tell us if something is alive?

Take a look at this movie (QuickTime Required):

The moving object is exactly the same in each picture, but the background is different. If you're like most people, you'll see one object as an ice skater, and the other as a spinning top.

This puts the objects in two different classes -- animate (something that can move by itself: a human, animal, robot, and so on) and inanimate (something that requires an external force to move). Do we perceive the two objects differently?

Arguably, it's important that we do: if an object can move by itself, it's much more likely to be a threat to us than if it requires some external force, just like it's important to be able to tell the difference between a sleeping dog and a dog-shaped rock.

Perhaps this could be a job for mirror neurons: the mirror system, after all, is involved in both perceiving the motions of others and producing similar motions ourselves. But other areas of the brain are also involved in these types of actions. The social network, for example, is a set of widely distributed brain regions that are activated during social activities ranging from perceiving biological motion to judging the intentions of others.

Some studies have attempted to differentiate between these two systems by providing non-biological examples during fMRI imaging sessions. However, the research until now hasn't eliminated alternative explanations. A team led by Thalia Wheatley has devised a clever set of experiments using displays such as the one depicted above, in order to narrow down whether the mirror system or the social network is responsible for the decision about whether an object is animate or inanimate.

The team created a set of 12 ambiguous moving shapes like the ice skater / spinning top. In each case, two backgrounds were created, one suggesting that the shape was animate, and one suggesting it was inanimate. Then volunteers submitted to fMRI while watching the videos. First they saw the blank background, then later the moving video, and later still they were asked to imagine the motion. This image shows some of the most dramatic results:

You're looking at a single vertical slice of the results, showing areas of this cross-section of the brain that were significantly more active when an apparently animate object was shown than when an inanimate object moved in the same way. The red areas correspond to increased activity when animate motion was inferred (e.g. when the "ice skating" background was shown); the orange areas are when motion was imagined (e.g. the "ice skating" background was shown and participants imagined motion), and the yellow areas show increased activity in both cases.

These more activated areas of the brain when seeing or imagining animate objects correspond to the widely distributed social network of the brain, and not the relatively smaller and less distributed mirror system. While the mirror system was indeed active during both imagined and actual motion, the level of activation was not different for animate versus inanimate objects.

Wheatley et al. conclude that the mirror system is not responsible for determining animacy, whereas clearly the social network is heavily involved in the process. But why is the entire network activated, rather than some specific region devoted to the animacy determination? The researchers suggest that perhaps the entire social network is put on alert when animate motion is detected, in order to be ready for whatever social decision needs to be made next, whether it be fight, flight, or applause for a well-executed triple toe-loop.

Wheatley, T., Milleville, S.C., & Martin, A. (2007). Understanding animate agents: Different roles for the social network and mirror system. Psychological Science, 18(6), 469-474.