Surprising Synchrony

Our corpus callosum is a bundle of fibers that allows our brains’ left and right hemispheres to communicate – but even in people born without these connections, the hemispheres are still somehow able to synchronize their activity, reports a new study.

Activity in a brain with a corpus callosum (left) and one without (right). See, they're twins - just like Schwarzenegger and DeVito!

The brains of people born with a condition called agenesis of the corpus callosum (AgCC) – basically, absence of a corpus callosum – show activation patterns that are essentially the same as those of people with an intact corpus callosum. It’s a Neuroscience Mystery!

For decades, the corpus callosum’s purpose seemed straightforward enough: though certain areas of our left and right hemispheres specialize in certain tasks – the left hemisphere is more adept at retrieving specific facts, for instance; while the right is better at estimation – the two hemispheres work together to achieve most goals, and show a great deal of symmetry in their activation patterns. Since the corpus callosum is composed mainly of white matterconnective neural tissue – and severing it seems to prevent epileptic activity from spreading, it seemed obvious that this structure was crucial for communication between the two hemispheres.

But now, as the Journal of Neuroscience reports, a team led by Caltech’s J. Michael Tyszka has used fMRI scans to detect communication between the hemispheres in the brains of eight adults who lack a corpus callosum:

“This was a real surprise,” says Tyszka. “We expected to see a lot less coupling between the left and right brain in this group — after all, they are missing about 200 million connections that would normally be there. How do they manage to have normal communication between the left and right sides of the brain without the corpus callosum?”

How, indeed? Well, Tyszka’s team has a few bright ideas. The most promising one hinges on functional networkspatterns of brain connectivity that rely somewhat on anatomical (structural) networks, but are much more flexible. You might think of them as mental “software” as opposed to “hardware.”

This new research suggests that functional networks may be more flexible and adaptive than many scientists suspected:

We tested two distinct possibilities: (1) functional networks arise largely from structural connectivity constraints, and generally require direct interactions between functionally coupled regions mediated by white-matter tracts; and (2) functional networks emerge flexibly with the development of normal cognition and behavior and can be realized in multiple structural architectures.

In short, the two possibilities are that 1) functional networks mostly depend on structural ones, or 2) they arise in response to brain function, and aren’t particularly dependent on anatomical structure. The surprising functional symmetry in brains that lack a corpus callosum seems to strongly imply that #2 is closer to the truth.

Like many answers in neuroscience, this one raises a lot of new, intriguing questions – the first and most obvious among them being, How the hell are these hemispheres communicating? We don’t know yet. Just how flexible are functional networks? We don’t know yet. Might there be a link to the unusual white-matter connectivity in children with autism? Probably, but we don’t yet know what it is.

And what does this suggest about the nature of consciousness? Well, let’s see – you got a comfy chair and about three hours to spare?

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