Remember the last time that something a friend did caught you off guard? Probably—and that's because the human brain is specially tuned to remember things that are out of the ordinary.

But just how the brain treats those instances has remained uncertain. Some scientists had hypothesized that an unexpected stimulus would trigger a loop that involved both the hippocampus (responsible in part for long-term memory) and nucleus accumbens (involved in reward and pleasure) to make those memories super sticky. But without peering into these centers, researchers could not know for sure.

One research team, however, got just such an opportunity. They were able to implant electrodes in the hippocampus of eight willing patients who were undergoing treatment for epilepsy as well as in the nucleus accumbens of six patients who had volunteered for experimental deep brain stimulation for depression. These electroencephalogram (EEG) readings allow researchers to detect changes in the brain in "milliseconds versus seconds" compared to fMRI (functional magnetic resonance imaging), Nikolai Axmacher, of the University of Bonn in Germany and lead author of the study, wrote in an email to

When the subjects were shown a series of faces against a red background for several seconds and then a picture of a house on a green background, the EEG captured the time at which each of these spots activated when the brain reacted, down to the millisecond. Researchers logged these times and were able to construct the overall pattern of activity between the two groups of subjects.

Axmacher and his team found that in the instance of the unexpected stimulus (the house on the green background), the hippocampus activates twice and that the latter activation—which is closely preceded by activation of the rewarding nucleus accumbens—predicts memory formation. The findings are reported in a study published online February 24 in the journal Neuron.

Why is pinpointing these processes of such keen interest to scientists? "Learning is very selective," Axmacher wrote in the email. Though humans have huge brains, we certainly don't have the capacity to log every aspect of every experience. "Only relevant information receives a 'memory boost' by the reward system, which includes the nucleus accumbens," he noted, so people are more inclined to remember incidents from which they might learn something new.

But sometimes when we encounter something unexpected, we brush it aside because it will not jibe with our understanding of the world. Axmacher explained that in such a case, "suppression of unwanted memories appears to depend on inhibitory top-down control of hippocampal activity by the prefrontal cortex," rather than the rapid response of the hippocampus-nucleus accumbens loop he and his colleagues confirmed.

Electrode implantation has yielded new insights into the brain, including this finding on memory formation as well as previous work on language processing, but it has its drawbacks as well, Axmacher conceded. These sorts of intra-brain readings "can only be obtained from patient populations and only allow one to record from specific regions of the brain," so they provide small snapshots of the brain at work—and in brains that already have a dysfunction. But he and his colleagues note that at least in this study, it is unlikely that the patients' illnesses would change the pattern of these particular neural activations. And "these results could only be obtained in patient populations with implanted electrodes in these regions," Axmacher wrote.

Nevertheless, he and his colleagues noted in their study, their findings probably don't fully explain how and why these surprising instances get preferential memory treatment than humdrum happenings. "The occurrence of an unexpected event likely recruits a network of brain regions that extend well beyond" these two, they wrote. Just remember that next time your friend makes a scene—if it's unexpected, that is.

Image courtesy of iStockphoto/Fitzer