Hippocampal subregions growing old together
To understand and remember our experiences, we need to think both big and small. We need to keep track of our spatial location at broad levels ("what town am I in?") all the way down to precise levels ("what part of the room am I in?"). We need to keep track of time on scales from years to fractions of a second. We need to access our memories at both a coarse grain ("what do I usually bring to the beach?") and a fine grain ("remember that time I forgot the sunscreen?").
Data from both rodents and humans has suggested that different parts of the hippocampus keep track of different levels of granularity, with posterior hippocampus focusing on the fine details and anterior hippocampus seeing the bigger picture. Iva Brunec and her co-authors recently posted a preprint showing that temporal and spatial correlations change along the long axis of the hippocampus - in anterior hippocampus all the voxels are similar to each other and change slowly over time, while in posterior hippocampus the voxels are more distinct from each other and change more quickly over time.
In their latest work, they look at how these functional properties of the hippocampus change over the course of our lives. Surprisingly, this anterior-posterior distinction actually increases with age, becoming the most dramatic in the oldest subjects in their sample.
The interaction between the two halves of the hippocampus also changes - while in young adults activity timecourses in the posterior and anterior hippocampus are uncorrelated, they start to become anti-correlated in older adults, perhaps suggesting that the complementary relationship between the two regions has started to break down. Also, their functional connectivity with the rest of the brain shifts over time, with posterior hippocampus decoupling from posterior medial regions and anterior hippocampus increasing its coupling to medial prefrontal regions.
These results raise a number of intriguing questions about the cause of these shifts, and their impacts on cognition and memory throughout the lifespan. Is this shift toward greater coupling with regions that represent coarse-grained schematic information compensating for degeneration in regions that represent details? What is the “best” balance between coarse- and fine-timescale information for processing complex stimuli like movies and narratives, and at what age is it achieved? How do these regions mature before age 18, and how do their developmental trajectories vary across people? By following the analysis approach of Iva and her colleagues on new datasets, we should hopefully be able to answer many of these questions in future studies.