How Big Is Your Hippocampus? Does It Matter? Yes and No.

There is a critical link between hippocampus size and functional connectivity.

Posted Oct 29, 2015

Wikimedia/Life Sciences Database
Hippocampus in red. 
Source: Wikimedia/Life Sciences Database

The hippocampus was given its name because it resembles the shape of a seahorse. Hippocampus comes from the Greek hippos, "horse," and kampos, "sea monster." Humans have two hippocampi, one in the left hemisphere of the brain, and one in the right hemisphere. The hippocampus plays an important role in the consolidation of learning from short-term memory to long-term memory and in spatial navigation.

Fifteen years ago, neuroscientists identified that professional London cab drivers had a bigger hippocampus, which was linked to their ability to navigate 25,000 complex city streets from memory. In 2011, a follow-up study, “Acquiring “the Knowledge” of London's Layout Drives Structural Brain Changes," identified specific structural changes in the hippocampus volume of London taxi cab drivers who had "the Knowledge." Those who qualified to be London cab drivers were found to have a selective increase in gray matter (GM) volume in their posterior hippocampus.

Can Practicing Spatial Navigation Change the Size of Your Hippocampus? 

Talkandroid/Labeled for reuse
London cabbies have bigger hippocampi. 
Source: Talkandroid/Labeled for reuse

Researchers at Carnegie Mellon University (CMU) were curious to identify whether the experience of navigating London's complex system of streets caused changes to hippocampal size or, conversely, the correlation reflected the fact that only people with a larger hippocampus at the outset of the study were predisposed to succeed at becoming licensed cab drivers.

In their new study, the Carnegie Mellon researchers were able to determine that learning and practicing detailed navigation information does, in fact, cause changes in hippocampus size. But, that's only half the story. The structural changes in hippocampal size were also linked to an alteration of functional changes in terms of the hippocampus’ connectivity and its ability to communicate, or “synchronize,” with other brain regions.

The October 2015 paper, “Structural and Functional Neuroplasticity in Human Learning of Spatial Routes,” was published in the journal NeuroImage. In this study, Timothy Keller and Marcel Just discovered that brief spatial navigation training changes a person's brain tissue and improves how that changed tissue communicates with other brain areas involved in spatial navigation.

This groundbreaking discovery establishes a critical link between structural size and functional connectivity brain alterations that occur in the hippocampus during spatial learning. Importantly, this study also illustrates that the brain changes linked to spatial learning are related to how neural activity synchronizes the communication between the hippocampus and other regions necessary for navigation.

In a press release, Tim Keller, a senior research scientist in CMU's Department of Psychology and Center for Cognitive Brain Imaging (CCBI), said,  

"The hippocampus has long been known to be involved in spatial learning, but only recently has it been possible to measure changes in human brain tissues as synapses become modified during learning. Our findings provide a better understanding of what causes the hippocampal changes and how they are related to communication across a network of areas involved in learning and representing cognitive maps of the world around us."

To examine how the hippocampus changes, Keller and Just recruited 28 young adults with little experience playing action video games. For 45 minutes, the participants played a driving simulation game. One group practiced maneuvering along the same route 20 times. The control group drove for the same amount of time, but along 20 different routes.

Before and after each training session, each participant's brain was scanned using a novel brain imaging technique called diffusion-weighted imaging (DWI), which measures water molecule movement in the brain. Then, the researchers used traditional functional magnetic resonance imaging (fMRI), to analyze brain activity.

The researchers found that the group that practiced the same route over and over again increased their speed at completing the driving task more than the group that practiced on different routes. The group that practiced the same course repeatedly also improved their ability to order a sequence of random pictures taken along the route and were able to sketch a map representing the route from a bird's-eye view.

Conclusion: Hippocampus Size and Functional Connectivity Both Matter

Wikimedia/Creative Commons
The hippocampus is named after "seahorse." 
Source: Wikimedia/Creative Commons

One important conclusion of this study is that only the spatial learning group that practiced the same route repeatedly showed structural brain changes in central spatial learning areas of the hippocampus, called the left posterior dentate gyrus.

The other important finding is that particpants who improved their spatial navigation also showed increases in the synchronization of activity—or functional connectivity—between the hippocampus and other cortical areas in the network of brain regions responsible for spatial cognition and navigation.

The amount of structural and functional changes in the hippocampus were directly related to the amount of behavioral improvement each person showed on navigational tasks. In a press release, co-author Marcel Just concluded,

"The new discovery is that microscopic changes in the hippocampus are accompanied by rapid changes in the way the structure communicates with the rest of the brain. We're excited that these results show what rewiring as a result of learning might refer to. We now know, at least for this type of spatial learning, which area changes its structure and how it changes its communication with the rest of the brain."

There appears to be a growing consensus among neuroscientists that changes in gray matter brain volumes and white matter integrity work in tandem to optimize cognitive function. Gray matter houses the neurons in specific brain regions. White matter facilitates the communication between various brain regions. 

Various studies examining the benefits of physical exercise, brain health, and cognitive function have identified the important link between brain structure and functional connectivity. This recent study from Carnegie Mellon enhances our understanding of the critical link between structural and functional brain changes in the process of human learning and the importance of practice, practice, practice.

If you'd like to read more on this topic, check out my Psychology Today blog posts,

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