Though the left and right hemispheres of the human brain are known to process words and faces respectively, new research reveals that people lacking one half of their brain are still pretty good at recognizing both of these object types. According to the authors of the as-yet unpublished study, this finding provides new insights into the brain’s plasticity, suggesting a single hemisphere can rewire itself in order to take on extra tasks following major surgery.
To determine whether either hemisphere is capable of supporting both face and word recognition, the researchers recruited 40 adult volunteers who had undergone a hemispherectomy to remove one half of their brain as a child. Such procedures are conducted as a last resort in extreme cases of childhood epilepsy and can help to attenuate seizures that originate in one of the hemispheres.
Participants were shown either a colorless face or a four-letter word for three-quarters of a second, before being presented with another word or face for just 150 milliseconds. They were then asked to determine whether the two words or faces they had just seen were the same or different.
Though hemispherectomy patients performed slightly worse than a control group, average accuracy rates exceeded 80 percent for both face and word recognition. Somewhat surprisingly, the researchers found that “patients' accuracy was not dependent on the hemisphere removed. That is, the single [left hemisphere] or [right hemisphere] patients showed comparable performance on face and word recognition.”
In a second experiment, the study authors sought to limit those in the control group to the use of one hemisphere at a time, and therefore challenged all participants to recognize words and faces that appeared at the side of a screen rather than in the center. Objects viewed in the left half of the visual field are generally processed by the right hemisphere, and vice-versa.
Once again, participants who still had both halves of their brain outperformed those who had undergone a hemispherectomy, except when viewing words that were displayed on the left side of the screen. Strangely, controls were no better at recognizing these words than individuals with just one hemisphere, regardless of which one had been removed.
“Altogether, the findings indicate that a single developing hemisphere, either left or right, may be sufficiently plastic for comparable representation of faces and words,” write the authors. “This demonstrates the remarkable plasticity of the developing brain.”
These results also imply that such plasticity is age-dependent, as previous studies have shown that adults who suffer right and left ventral occipitotemporal cortex (VOTC) injuries tend to lose their ability to recognize words and faces. In one study, adults with left hemisphere lesions took 100 times longer to read words than healthy controls, while those with damage to their right hemisphere made 10 times as many mistakes on a facial recognition task than those lacking any injury.
In contrast, the researchers conclude that “the preserved hemisphere following childhood hemispherectomy, be it the [left hemisphere] or [right hemisphere], supports both face and word recognition.”
At the same time, they highlight the fact that “patients' performance was still suboptimal relative to controls. Therefore, while competition between face and word representations may persist following childhood hemispherectomy, the single hemisphere appears inefficient, relative to two hemispheres in typical development, at representing both stimulus categories.”
[H/T: New Scientist]