Is perfect pitch really perfect?

BBC : Only one in 10 000 people have perfect pitchâmdash;the ability to identify a given musical note without the help of a reference note. Although long believed to be innate, perfect pitch may not be so perfect after all, say Howard Nusbaum of the University of Chicago and his colleagues. In their paper published in Psychological Science, the researchers discuss their study in which they played a long piece of music to a group of 27 people with perfect pitch. Over a period of 45 minutes, the researchers gradually turned the tones slightly flat. Not only did the change go unnoticed, but when the tones were returned to normal, the listeners claimed they sounded too sharp. The researchers determined that the ability to recognize pitch, rather than being absolute, can change with listening experience. Next they hope to see whether adults without perfect pitch can improve in their ability to recognize notes.