When you see something unexpected, your brain kicks into high gear--even when you're not consciously aware of the novelty. That surprising conclusion, from a study reported in today's issue of Science,* directly counters the prevailing wisdom that conscious awareness is needed for detecting novelty.
Past studies have shown that subjects change their reactions when they are aware of novel stimuli. But researchers have had a hard time designing an experiment to determine whether subjects register novelty without awareness. To do so, University of Pittsburgh psychiatrist Gregory Berns and his colleagues at Pittsburgh and at Washington University School of Medicine in St. Louis had 10 volunteers spend an hour doing a reaction-time task: The number 1, 2, or 3 appeared on a screen, and the subject had to press the corresponding key on the computer as fast as possible. As the volunteers pressed the keys, the researchers took periodic PET scans, which map the blood flow into regions of the brain.
The trick was that the numbers didn't appear randomly; they followed a subtle set of rules called a grammar. The number displayed on the screen was influenced by the preceding five numbers, but the relationship was fairly complex. Then, in the middle of the hour, the researchers suddenly switched the grammar. Interviews with the volunteers following the experiment showed that the change was so subtle that none of them noticed that the rules had changed. But the PET scans revealed that a particular region deep within in the volunteers' brains--called the ventral striatum--had picked up the switch.
As soon as the Pittsburgh team switched the grammar, not only did the patients' reaction times decrease, but the ventral striatum's blood flow increased immediately, suggesting that this brain region was hard at work trying to make sense of the novelty. The ventral striatum, which has been linked with pattern detection, is believed to "make guesses at likely future stimuli," says P. Read Montague, a neuroscientist at the Baylor College of Medicine in Houston.
"It's a pretty cool paper," says Montague. "Your brain makes thousands of decisions all the time that we thought to be [conscious], but it seems to be working all the time, autonomously." Terrence Sejnowski, a computational neuroscientist at the Salk Institute in San Diego, agrees. Contrary to previous notions of how novelty is detected, he says, this paper shows that "under the conscious experience, there is this iceberg that's always there, monitoring."