The left side of the brain controls the verbal expression of our long-term semantic memory, according to a new study led by University of Manchester psychologists.
The work is the first of its kind to assess the similarities and differences in how the left and right sides of the brain process semantic memory. The team, working with neuropsychologists at Salford Royal and The Walton Centre for neurology in Liverpool, involved 41 patients who had part of their brains removed to treat their long-standing epilepsy.
Semantic memory, or conceptual knowledge, refers to our knowledge for the meanings of words, objects, people, and emotions. There is growing, convergent evidence that semantic memory is supported by a large, distributed network of regions across the brain including the anterior temporal lobes (ATLs), bilaterally.
Anterior Temporal Lobe Surgery
The patients in the study – who now experience fewer seizures and are able to go back to work and learn to drive as a result of the surgery — had their verbal and visual semantic memory tested.
The surgery removes part of the brain that causes the seizures, but also removes tissue which researchers believe is involved in storing semantic memories. Twenty of the patients had surgery to remove part of the brain, called the anterior temporal lobe, on the right side, and 21 had surgery to remove the left anterior temporal lobe.
Summary of accuracy (A) and reaction time (B) data from the neuropsychological battery. Data for the control participants are shown in grey and data from the left and right TLE patients are shown in white and black, respectively. Accuracy data reported as percentages and correct response times are reported in milliseconds. Significant differences between the groups based on one-way between group ANOVAs are noted with asterisks; the color of the asterisk denotes the direction of the effect. Credit: Grace E Rice et al. CC-BY
To test their verbal semantic memory, the team’s assessments included testing patients’ ability to name pictures and celebrities (including Brad Pitt, Princess Di and the Queen), and their ability to match words in terms of their meaning.
And to test their visual memory, the patients were asked to identify emotions of people in photographs and say if a face was familiar to them. The test results were compared with 20 more people who did not have any neurological problems.
“Popularly, there is a lot of interest in whether there are similarities or differences between the left and right sides of the brain. Our research for the first time shows that — at least for semantic memory — both sides of the brain play an important role in visual and verbal semantic memory.
But there is a significance difference when it comes to verbal expression of this knowledge, which was effected more by surgery to the left side of the brain. Our research provides an important insight both into what effects this particular kind of epilepsy surgery has on behaviour, but also helps us to understand where in the brain memory is stored.”
The authors note that the findings reflect postsurgical behavioral performance, and no conclusions can be drawn in regards to presurgical performance. Also, whether behavioral performance has improved/declined as a result of the resection surgery can not be determined.
The research was funded by the Engineering and Physical Sciences Research Council and the Medical Research Council.