Magic trick reveals unconscious knowledge

WASHINGTON — Magic tricks prey on people’s subpar powers of perception, but new work finds that the brain has tricks of its own up its sleeve: People notice more than they think.

In the research, presented November 12 at the annual meeting of the Society for Neuroscience, Luis Martinez of CSIC- Miguel Hernandez University in Spain and colleagues amazingly “read minds” with the Princess Card Trick, invented by magician Henry Hardin in 1905. Volunteers mentally chose a playing card from a panel of six cards, which then disappeared. When a second group of cards appeared, the researchers had miraculously figured out which card a person had in mind and removed it. Few people caught the trick: All the cards in the second set were different, not just the card people had chosen.

A few seconds after viewing the two panels of cards, participants were asked which of two new cards was present in the first panel. None of the volunteers could consciously recall which card was present. Despite these avowals of ignorance, when forced to choose, people got the right answer about 80 percent of the time. “People say they don’t know, but they do,” Martinez said. “The information is still there, and we can use it unconsciously if we are forced to.”

To see whether this unconscious knowledge works for objects other than cards, Martinez and his colleagues performed a similar experiment with pictures of men’s faces. A similar kind of visual short-term memory helped people choose which face they had seen before, even when volunteers didn’t perceive that they knew the correct answer.

These unconscious, short-term memories are finicky, Martinez and his colleagues found. If the researchers talked to the volunteers while performing the trick, the ability to identify the card that had been present worsened. (Magicians may deploy a steady stream of patter for this very reason.) And if the researchers revealed the secret of the trick, participants performed no better than chance at identifying the card.