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14 November 2011. Beyond flash cards and mnemonics, hard-wired differences in brain structure and neural firing rhythms may give some people an edge in their ability to remember things. That’s the gist of a study, published in the November 3 Current Biology online, which used a combination of brain imaging techniques to correlate memory function with structural connectivity between the prefrontal cortex (PFC) and the hippocampus. In another study—described in the November 2 Journal of Neuroscience—researchers mapped electrical oscillations underlying memory formation to the PFC as well. The findings strengthen the idea that oscillatory signals detected during memory formation may predict the success of later recall.

Traditionally, scientists have pegged the prefrontal cortex as the brain’s working memory center, holding information needed for complex reasoning and learning, while they attribute long-term memory to the hippocampus. As it turns out, connections between these two brain regions drive both types of memory, yet studying the structures in concert in living people is challenging.

In the Current Biology paper, Michael Cohen of the University of Amsterdam, The Netherlands, designed a dual-pronged approach to study how the hippocampus and prefrontal cortex contribute to memory. In one set of experiments, Cohen used a type of MRI called diffusion-weighted imaging to trace white matter connections between the prefrontal cortex and hippocampus in 20 healthy, young adults. “This has nothing to do with function; it’s all about structure. It’s measuring a quality of their brain that is physically there all the time,” Cohen told ARF. Then, in a separate analysis done on the same people a week or two later, he used electroencephalography (EEG) to record prefrontal cortex activity during a difficult memory task. Study participants saw a series of line drawings flash momentarily onto a computer screen, then had to remember whether the drawing was rotated slightly clockwise or counterclockwise when it reappeared five seconds later. Long-term memory was tested about an hour later, when the participants viewed some of the original pictures along with deceptively similar “lures," and indicated whether they thought they had previously seen the drawing.

“People with stronger connectivity (measured by MRI) between the hippocampus and prefrontal cortex were better able to encode the pictures and remember them,” Cohen said. “These are fundamental, hard-wired, physical features of your brain.”

Though the study’s main focus was linking memory performance to structural connectivity, Cohen noted, the subjects with stronger hippocampus-PFC connections also had slower task-related brain oscillations, as measured by the EEG. Oscillations are rhythmic fluctuations in brain activity that reflect the excitability of large cell populations. Measuring this electrical activity allows scientists to gauge how well the brain stores and transmits information from one region to another. The present data confirm earlier theoretical studies suggesting that slow oscillations play a key role in memory maintenance, Cohen said.

The present study also represents a methodological advance. By linking a fairly old method (EEG) with a more recent technique (diffusion-tensor MRI), the multimodal approach links brain structure to function and “helps overcome one of the main limitations of EEG: the inability to measure deep brain structures,” Cohen wrote. Ongoing work in his lab is addressing whether age or other factors such as learning might influence the strength of hippocampal-PFC connectivity.

Rather than correlate brain function with underlying structure, the Journal of Neuroscience study linked memory function with EEG oscillations at signature frequencies, namely β (13-30 Hz) and ϑ (4-8 Hz). The analysis began as an attempt to replicate counterintuitive results of a previous study, said first author Simon Hanslmayr of the University of Konstanz, Germany. In that study (Hanslmayr et al., 2009), he and senior author Karl-Heinz Bäuml linked semantic memory with a decrease in β oscillations. This was unexpected because less β power means less synchronous neuronal firing, and “we had very little idea how that could serve memory encoding,” Hanslmayr told ARF. It made more sense that stronger synaptic connections, measured as greater synchrony, or increased β power, drove memory, he noted.

In the current paper, Hanslmayr and colleagues not only confirmed their 2009 findings, but also mapped the decrease in β synchrony to a specific brain area—the left inferior frontal gyrus. “It’s not just background noise,” Hanslmayr said. His team found this by doing simultaneous EEG and functional MRI (fMRI) on 24 healthy volunteers (mean age 23 years) while they memorized word lists. For each word, the researchers noted whether the volunteers successfully recalled it, then looked at their blood oxygen level-dependent (BOLD) fMRI changes to see which part of the brain was active during encoding of words that were later recalled successfully. They found that BOLD signal was up, and β power down, in the left inferior frontal gyrus, but only for words the volunteer remembered. The scientists saw no such correlation for items that were not recalled.

Alongside the decline in β signal, the scientists also found that good memory correlated with increased ϑ oscillations in medial temporal lobe regions, consistent with prior studies (Rutishauser et al., 2010; see also review by Axmacher et al., 2006).

The paper shows that BOLD signal and electrophysiology are correlated only when subjects are attentively processing the items, presumably leading to good memory later, Hanslmayr said. And, reassuringly, Cohen said the “regions showing the oscillatory signatures are in the same part of the PFC that had strong connectivity with the hippocampus and predicted subjects’ memory [in our study].” “I think a very important message of our paper is that researchers should not neglect the middle-range frequencies (like β and α oscillations) and should really look at power decreases,” said Hanslmayr. He added that the mid-range frequencies are important for long-term memory.

On the methodological side, Hanslmayr’s research suggests that β power measured by EEG may be able to serve as a proxy for memory encoding, without need for fMRI—a scanning procedure that can be hard to perform on ailing elderly or those with dementia. Hanslmayr hopes his paper will inspire other scientists to look more closely at power decreases in the middle-range frequencies, e.g., β and α oscillations, and their role in long-term memory.—Esther Landhuis.

References:
Cohen MX. Hippocampal-Prefrontal Connectivity Predicts Midfrontal Oscillations and Long-Term Memory Performance. Curr Biol. 3 Nov 2011. Abstract

Hanslmayr S, Volberg G, Wimber M, Raabe M, Greenlee MW, Bäuml KT. The Relationship between Brain Oscillations and BOLD Signal during Memory Formation: A Combined EEG-fMRI Study. J Neurosci. 2 Nov 2011;31(44):15674-15680. Abstract

This is Part 2 of a three-part series. See also Part 1 and Part 3. Download a PDF of the entire series.