Like an invisible brass ring or dangling piece of bacon, subliminal rewards can drive people to work harder without them even knowing it.
Most of us consciously work for the paycheck, along with other material and emotional benefits, but new research points to this type of unconscious motivation, in which a person completes a task but is unaware of the reward or goal at stake.
"As far as we know, this is the first study looking at [subliminal perception] in relation to motivation," said study team member Chris Frith of the Wellcome Trust Center for Neuroimaging at the University College London. The details are published in the April 12 online edition of the journal Science.
The scientists, led by Mathias Pessiglione, also of the Wellcome Trust Center, tested the effects of subliminal rewards in a group of 18 volunteers, 9 women and 9 men, telling them that the harder they squeezed a hand grip, the more money they would receive. Prior to squeezing, participants watched an image of a one-cent coin or one English pound (equivalent to about $2) flash onto a computer screen for 17, 50 or 100 milliseconds. A prior experiment revealed that while 17 and 50 milliseconds produced subliminal cues, 100 milliseconds led to a conscious picture of the coins.
The scientists measured brain activity and so-called skin conductance response, which indicates whether one's level of excitement is "off to the races."
During the unconscious and conscious stimuli, the participants gripped with significantly more force when the visually suggested monetary stakes were highest. Brain recordings showed much higher activity in an area in the basal forebrain called the ventral pallidum for the one-pound images, even during the 50-millisecond subliminal flashes.
The skin conductance measurements followed the same trend with an increase for more money shown, but only with the 100- and 50-millisecond flashes. Frith thinks the lack of response for 17-millisecond flashes is due to the statistical method used.
From the brain maps, the scientists think the ventral pallidum, which has been linked with reward-related processes in other studies, sends a message to the brain's motor regions that is commensurate with the value of the reward. The motor region then orchestrates the appropriate behavior even when a person has no conscious idea of the reward.
"It's further evidence that a great deal of what our brains do happens subliminally," Frith told LiveScience.
Being unaware of your actions has its benefits, he said. "It's an advantage to do lots of things using the subliminal perception because it's much faster," Frith explained. "As soon as you have to start thinking about things everything is slowed down."