Distant regions of the human brain are wired together by surprisingly
few connections

Date:
March 24, 2022
Source:
PLOS
Summary:
Understanding how the brain functions, particularly how information
is processed during different activities, is difficult without
knowing how many axons are in the brain and how many connect
different functional regions. A new study shows that despite the
functional importance of connections between far-reaching regions
of the brain, the actual number of these connections is low.



FULL STORY ========================================================================== Understanding how the brain functions, particularly how information is processed during different activities, is difficult without knowing how
many axons are in the brain and how many connect different functional
regions. An approach by Burke Rosen and Eric Halgren at the University
of California, San Diego, U.S. published March 24 in the open-access journalPLOS Biology, shows that despite the functional importance of connections between far-reaching regions of the brain, the actual number
of these connections is low.


==========================================================================
In the new study, researchers combined diffusion MRI data from the
Human Connectome Project with histological cross-sections of the corpus callosum, the major tract that connects the left and right sides of
the brain. The Human Connectome maps the strength of all connections
in the brain but does not provide the actual number of axons, while
the histological cross-sections allow estimates for how many axons are
packed into a given volume. Combining the connection strengths with
the axon densities yielded estimates for the number of axons in the
cerebral cortex.

The analysis indicated that there are almost 2.5 billion long-range axons traversing the cerebral cortex. However, despite this large number, they
found that the numbers connecting different functional brain regions
were quite low.

For example, among the estimated 130 million axons in the arcuate
fasciculus tract, only about 1 to 2 million (less than 2%) directly
connected Broca's and Wernicke's areas, a connection which is necessary
for normal language ability.

The model predicts that other long connections, like those from the
hippocampus to the frontal cortex that are needed for memory retrieval,
are actually made in multiple steps. The findings will thus improve
models of cognition, especially processes that rely on connections
between distal regions of the brain.

"A major unsolved problem is how the human cortex integrates information processing by its 16 billion neurons across its surface to unify
awareness," Rosen adds. "Our finding that cortical areas are sparsely
connected implies that this integration is accomplished either via linkage
of the dense local connections or by rare, extraordinarily privileged long-range axons."

========================================================================== Story Source: Materials provided by PLOS. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Burke Q. Rosen, Eric Halgren. An estimation of the absolute
number of
axons indicates that human cortical areas are sparsely
connected. PLOS Biology, 2022; 20 (3): e3001575 DOI:
10.1371/journal.pbio.3001575 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220324154137.htm

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