The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

The upper part of the IPL, the supramarginal gyrus, is another structure unique to humans. Damage here leads to a disorder called ideomotor apraxia: a failure to perform skilled actions in response to the doctor’s commands. Asked to pretend he is combing his hair, an apraxic will raise his arm, look at it, and flail it around his head. Asked to mime hammering a nail, he will make a fist and bang it on the table. This happens even though his hand isn’t paralyzed (he will spontaneously scratch an itch) and he knows what “combing” means (“It means I am using a comb to tidy up my hair, Doctor”). What he lacks is the ability to conjure up a mental picture of the required action—in this case combing—which must precede and orchestrate the actual execution of the action. These are functions one would normally associate with mirror neurons, and indeed the supramarginal gyrus has mirror neurons. If our speculations are on the right track, then one would expect patients with apraxia to be terrible at understanding and imitating other people’s movements. Although we have seen some hints of this, the matter requires careful investigation.

One also wonders about the evolutionary origin of metaphors. Once the cross-modal abstraction mechanism was set up between vision and touch in the IPL (originally for grasping branches), this mechanism could have paved the way for cross-sensory metaphors (“stinging rebuke,” “loud shirt”) and eventually for metaphors in general. This is supported by our recent observations that patients with angular gyrus lesions not only have difficulty with bouba-kiki, but also with understanding simple proverbs, interpreting them literally rather than metaphorically. Obviously these observations need to be confirmed on a larger sample of patients. It is easy to imagine how cross-modal abstraction might work for bouba-kiki, but how do you explain metaphors that combine very abstract concepts like “it is the east, and Juliet is the sun” given the seemingly infinite number of such concepts in the brain? The surprising answer to this question is that the number of concepts is not infinite, nor is the number of words that represent them. For all practical purposes, most English speakers have a vocabulary of about ten thousand words (although you can get by with far fewer if you are a surfer). There may be only some mappings that make sense. As the eminent cognitive scientist and polymath Jaron Lanier pointed out to me, Juliet can be the sun, but it makes little sense to say she is a stone or an orange juice carton. Bear in mind that the metaphors that get repeated and become immortal are the apt ones, the resonant ones. In doggerel, comically bad metaphors abound.

Mirror neurons play another important role in the uniqueness of the human condition: They allow us to imitate. You already know about tongue protrusion mimicry in infants, but once we reach a certain age, we can mime very complex motor skills, such as your mom’s baseball swing or a thumbs-up gesture. No ape can match our imitative talents. However, I will note as an interesting aside here, the ape that comes closest to us in this regard is not our nearest cousin, the chimpanzee, but the orangutan. Orangutans can even open locks or use an oar to row, once they have seen someone else do it. They are also the most arboreal and prehensile of the great apes, so their brains may be jam-packed with mirror neurons for allowing their babies to watch mom in order to learn how to negotiate trees without the penalties of trial and error. If by some miracle an isolated pocket of orangs in Borneo survives the environmental holocaust that Homo sapiens seems hell-bent on bringing about, these meek apes may well inherit the earth.

Miming may not seem like an important skill—after all, “aping” someone is a derogatory term, which is ironic given that most apes are actually not very good at imitation. But as I have previously argued, miming may have been the key step in hominin evolution, resulting in our ability to transmit knowledge through example. When this step was taken, our species suddenly made the transition from gene-based Darwinian evolution through natural selection—which can take millions of years—to cultural evolution. A complex skill initially acquired through trial and error (or by accident, as when some ancestral hominid first saw a shrub catching fire from lava) could be transmitted rapidly to every member of a tribe, both young and old. Other researchers including Merlin Donald have made the same point, although not in relation to mirror neurons.³