Researchers have been trying to teach absolute pitch ability in laboratory settings for more than a century, and various commercial absolute-pitch training courses have been offered to the public since the early 1900s. In 2013, experimenters reported that adult men who took the antiseizure drug valproate (VPA) "learned to identify pitch significantly better than those taking placebo—evidence that VPA facilitated critical-period learning in the adult human brain". However, no adult has ever been documented to have acquired absolute listening ability, because all adults who have been formally tested after AP training have failed to demonstrate "an unqualified level of accuracy... comparable to that of AP possessors".