Brain may forge some memories in waves

Although people effortlessly remember all sorts of everyday events, scientists are struggling to explain how the brain makes this possible. In two critical brain areas, such memory may hinge more on the timing than on the strength of neural activity, according to a team of neuroscientists.

As volunteers study word lists, clusters of neurons in the rhinal cortex and the hippocampus–adjacent brain areas already implicated in memory–fire synchronized electrical bursts that pave the way for remembering those words later, argue Jürgen Fell of the University of Bonn in Germany and his colleagues.

Moreover, the coordination of cell activity in the same two brain regions plummets for a fraction of a second just after participants remember a word from the list, possibly signaling an end to a coordinated neural effort, Fell’s team proposes in an article slated to appear in Nature Neuroscience.

“These are enticing data,” says neuroscientist Anthony D. Wagner of the Massachusetts Institute of Technology. “Memory may emerge when rhinal and hippocampal neurons synchronously oscillate and then desynchronize.”

Some scientists theorize that synchronized neural firing, which generates brain waves, lies at the root of perception and memory (SN: 2/20/99, p. 122). Attention has focused on gamma waves, which are the result of thousands of neurons emitting equivalent electrical pulses about 40 times a second.

Fell’s group studied gamma waves in nine epileptic adults in whom surgeons had temporarily implanted electrodes to find seizure sites. None had suffered a seizure for at least 1 day. In the investigation, the volunteers studied a word list, performed a brief distracting task, and then tried to recall words from the list.

About one-quarter second and again one-half second after viewing words that they would later remember, participants displayed bursts of gamma activity in the rhinal cortex and the hippocampus. One second after they recalled a word from the studied list, synchronized neural firing declined sharply for a fraction of a second.

No distinctive type of gamma activity occurred while participants viewed words that they later failed to recall or when they tried to recall a word in vain.

It’s unclear how the waxing and waning of gamma activity in the rhinal cortex and the hippocampus boosts memory, the researchers note. Preliminary evidence suggests that gamma waves in the hippocampus render cells more capable of receiving incoming messages, they say.

The origins of synchronized firing in the rhinal cortex and the hippocampus are also unknown, Wagner remarks. Another brain area, the prefrontal cortex, may regulate memory-related gamma activity elsewhere, in his view.

Another open question concerns the type of memory fostered by rhinal cortex-hippocampus cooperation. Fell’s team examined conscious recall, which has often been attributed to the hippocampus. In contrast, many neuroscientists suspect that the rhinal cortex supports a sense of familiarity that falls short of conscious recall of previously encountered information.