Concussion Can Affect How the Brain's Hemispheres Communicate
Key Points: Scientists continue to uncover the ways in which even mild brain injury influences the brain's functioning. Recent findings suggest that such injuries can have an impact on the structure that links the brain's two hemispheres—and affect how information passes between them.
Concussions, or even any notable impact on the brain or cranium, can go on to have structural effects on that precious organ, and these effects can result in functional abnormalities.
Hence, the following findings should not come as a surprise: Research at New York University (NYU) by Wegener et al. (2019) found that the transmission of information between the two hemispheres of the brain is susceptible to being detrimentally altered by even mild traumatic brain injury (TBI). This stems from the fact that the corpus callosum, which links the two hemispheres, is a structure vulnerable to damage in cases of TBI.
To find this out, the team of researchers evaluated the brains of patients with recent concussions, in comparison with healthy controls. They used MRI to measure water diffusion in the corpus callosum nerve fibers—a marker of neuronal activity and axonal integrity (Tsurugizawa, Ciobanu, & Le Bihan, 2013). This was done to gauge the injury at a microscopic level within the white matter.
Concurrent with this MRI technique, the team used an information-processing task to assess the degree of interhemispheric communication (and hence corpus callosum function).
Here's how the task worked: The participants had to focus on an “X” on a screen, after which words were flashed to the right and left of the “X.” The participants then had to say those flashed words as quickly as they could. The key variable being evaluated here was reaction time.
Researchers found that in concussion patients, there was a noticeable reaction-time lag on words flashing to the left of the “X” compared to those appearing on the right. This was indicative of some degree of compromise in information crossing over the corpus callosum.
The left-right disparity is thought to stem from the fact that language function is largely situated in the left hemisphere. The information presented on the left side of the participant’s visual field was initially transmitted to the visual cortex on the right side of the brain, after which it had to cross at the corpus callosum to reach the language center in the left hemisphere.
On the other hand, words on the right side of the participant’s visual field were already transmitted to the left side and did not need to cross the corpus callosum. In patients with an unimpacted corpus callosum (the controls), there was, as expected, no notable information processing lag time.
The results of this task corresponded with MRI findings as well. In healthy controls, water diffusion was duly detected in the corpus callosum, particularly at the splenium, which is situated between the language center in the left hemisphere and the visual cortex in the right hemisphere. On the other hand, in concussion patients, there was a disparity in these diffusion measures, which was significantly correlated with performance on the aforementioned processing task. These findings provide empirical evidence of how concussion impacts this crucial brain structure, and how it manifests functionally for these and other concussion victims.
With this said, the distinction between concussion and diffuse axonal injury should be mentioned. Both kinds of cases have been associated with injuries spanning the whole corpus callosum. However, in measures of neuronal fiber number, concussion patients have been found to have a specific vulnerability in the anterior portion of the corpus callosum connected to the frontal lobe. For patients with diffuse axonal injury, damage has more likely transpired in more diffuse regions of the corpus callosum connected with the entire frontoparietal lobes (Jang et al. 2019)
How These Findings Connect to Treatment
In any case, these most recent findings further corroborate an already intuitive idea: that the corpus callosum is susceptible and often damaged in cases of head trauma, which manifests as compromises in information processing in a left-right dependent manner.
What we should keep in mind is that concussion patients, once diagnosed, receive almost the exact same treatment. However, no one concussion patient is the same. There are many types of concussion, for which symptoms are generally dependent on the damaged brain region(s), and to what extent it (or they) were damaged. However, the corpus callosum findings seem to establish some common ground among concussion patients in general: that information processing along this conduit is at risk.