After a person memorizes a series of numbers, the numbers can be repeated in the correct order or in reverse. So, how does the brain sort a series of items in a specific order within short-term memory, or working memory?

To figure out the neural mechanisms of sequence sorting in working memory, Chinese scientists trained two macaque monkeys to perform a visuospatial delayed sequence-sorting task and conducted high-throughput electrophysiological recordings in the frontal cortex of the monkeys.

The research was carried out by a team led by Wang Liping, a researcher at the Center for Excellence in Brain Science and Intelligence Technology under the Chinese Academy of Sciences (CAS), and their findings were published in the journal Science on Friday.

Memory can be divided into short-term memory and long-term memory. Images seen and words heard in people's daily lives are first stored as short-term memory and then processed through sorting, compression, forgetting, and other processes according to needs, said Wang.

According to the research, spatial locations in each rank were encoded in separate rank subspaces, depending on rule cues. They were swapped between the subspaces by using two extra temporary subspaces in two parallel operation steps.

Tian Zhenghe, a doctorate student at the center, said the process is not a simple direct exchange. Instead, each subspace recruits a temporary subspace to store information, and after its own information is cleared, it receives the information from the other via another temporary subspace.

Sequence sorting, the mental manipulation of multiple items with different ordinal ranks in working memory, is essential for everyday activities such as planning, navigation and speech, said Peter Stern, a senior editor at Science who handles research papers related to neuroscience and brain research.

"These findings are an important step in understanding the neural mechanisms of reasoning and planning," Stern commented in the editor's summary.

Friday's edition of the journal also published the findings of another team from the center, led by the CAS academician Pu Muming, which obtained single-cell spatial transcriptomic atlases of macaque, marmoset and mouse cerebella. 

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