Editors' ChoiceNeuroscience

The benefits of memory growing granular

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Science Translational Medicine  20 Jan 2021:
Vol. 13, Issue 577, eabg4725
DOI: 10.1126/scitranslmed.abg4725

Abstract

Brain imaging approaches show that the essential machinery for memory formation continues to develop into late childhood.

We’ve all seen a preschool soccer team attempt to score a goal—a herd of tiny competitors slowed down by oversized shin guards that move down the field as a group. Adorable. But as children gain an understanding of the game, each team member has a role that becomes more differentiated from those of his teammates, each operating more independently. Callaghan et al. recently showed that a similar phenomenon may occur during memory development, using functional magnetic resonance imaging (fMRI) in school-aged children.

The study focused on the hippocampus, a brain region known to support memory formation and recall. Early work demonstrated that associative memories cannot be created when the hippocampus has been removed or damaged. One of the special features of hippocampal memory is that information is stored in a “sparse” manner. In most other brain regions, large populations of cells appear responsive to certain cues, but in the hippocampus, memories appear to be stored in just a few cells. This is thought to allow for detailed information to be stored in non-overlapping sets of cells, creating memories that are differentiated from one another. Callaghan et al. investigated a potential fMRI-based correlate of information sparsity in the hippocampus. They looked at how activation in small sections of the hippocampus (voxels) correlated with one another over time, during and after memory encoding. If large sets of cells were operating in concert, more like the preschool soccer team, the fMRI signals would be more correlated with one another over time. In contrast, if cells were operating more independently, the fMRI signals would be less correlated with one another. The study found that the signals from voxels in the posterior hippocampus were more intercorrelated in younger children, and were less correlated by adolescence, potentially reflecting sparser and more detailed information storage. Interestingly, children who showed a sparser pattern of activity during a rest period after the learning task performed better on a later test of memory for what they had learned. Post-learning activity therefore appeared important for stabilizing the new memory.

The authors concluded that the development of more differentiated activity in the posterior hippocampus supports memory, like a soccer team winning more games as players learn different positions. It is notable that the study was cross-sectional; it is not yet clear whether hippocampal signals would sequentially differentiate over development. Furthermore, this fMRI correlate has not yet been linked with information coding at the cellular level—reverse translation with both fMRI and cellular imaging could help resolve whether these signals do reflect sparse coding. However, the approach integrating developmental neuroscience and advanced computational analyses promises to reveal many further insights about how the building blocks for cognition are assembled.

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