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

by V.S. Ramachandran

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.